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Understanding Emissions Controls And How to Reconnect Them
In this section I will discuss what the emissions controls do on most any engine and how to reconnect them on Jeeps specifically.
First, review the following photos of a typical GM TBI. Note the names of the different ports. These ports are vacuum ports to operate the various emissions control devices. There are two types of general purpose vacuum ports. They are called manifold vacuum and ported vacuum. Manifold vacuum ports have engine vacuum signals on them at all times. Ported vacuum ports only have engine vacuum on them when the throttle plates have been opened ever so slightly. So at idle, when the throttle plates are closed, you won't have a vacuum signal on the ported vacuum port, but you will once the throttle is opened. Once the throttle is opened, ported vacuum ports act just like manifold vacuum ports.
TBI Photos:
FRONT
PASSENGER SIDE
BACK
Jeep Motorcraft 2150 2bbl Carburetor Photos:
FRONT
BACK
Jeep Grand Wagoneer Emissions Controls Vacuum Diagram:
Understanding Emissions Control Systems
For a quick summary on how to reconnect your emissions control devices without understanding why, go to the summary at the bottom of this page.
Since most emission control systems perform the same function, regardless of which vehicle they are on, I will use the Jeep emissions diagram above to discuss the emissions systems on just about any vehicle. The only difference between a modern computer controlled engine is that the ECM decides when to engage the emissions control devices, whereas on a non-computer engine, temperature switches and manifold and ported vacuum signals control the emissions devices. There are 6 separate emissions controls systems in the Jeep diagram above. Going clockwise around the engine starting at the back, they are the EGR system, the Air Pump system, the Spark control system, the Thermostatically controlled air cleaner system, the Evaporative emission control system, and the Positive Crankcase Ventilation (PCV) system. I'll start with EGR since its the easiest to understand.
EGR - EGR stands for exhaust gas recirculation. Its purpose is to reduce oxides of nitrogen pollution which causes smog. Oxides of nitrogen are produced when combustion temperatures are high. By allowing some of the exhaust gas (mostly carbon dioxide and water vapor) to be recirculated back into the engine, the inert exhaust gas cools the combustion process (relative to no EGR) by taking up space in the combustion chamber and making combustion less efficient. It also slows down combustion, which will have an effect on the ignition timing as I will discuss later. Typically, about 10% of the combustion chamber volume is filled with exhaust gas when the EGR valve is full open. There are two problems with EGR. One, since exhaust gases don't burn, they reduce the power capability of the engine, so you don't want EGR at full throttle. Two, at idle, there is very little air entering the engine. Any dilution of the fresh air charge with exhaust gas at idle will make the engine stumble and maybe not run at all. At idle, combustion temperatures are lowered in another way, which I will discuss later. Another EGR control issue is that on a cold engine, no EGR is needed because combustion won't be hot anyway. So, we want EGR only at part throttle on a hot engine when the engine can handle it, but not at idle or at full throttle. To see how the stock emissions system on the Jeep accomplishes this control strategy, refer back to the Jeep diagram. The EGR valve is on the upper back of the engine in the diagram. There is a vacuum line that leaves the EGR at the bottom of the valve. This line goes through a delay valve (all this does is slow down the vacuum signals so that the valve doesn't slam shut or open quickly) and then to a TVS. This is a vacuum switch located in the air cleaner housing responding to incoming air temperature. It prevents the EGR from opening when the air entering the engine is too hot. If the air is too hot, mixing EGR gases with it can make the air even hotter, which can cause detonation (detonation is abnormal combustion; most people call it pinging or knock). Next, the vacuum line goes from the TVS valve to a dual CTO (coolant temperature overide). This is a vacuum switch that operates in response to engine coolant temperature. This valve is installed in the coolant line leaving the back left of the intake manifold. This is called a dual CTO because it is serving two different systems, the EGR system and the air pump system. Looking at the dual CTO, there are five ports, two on the top and three on the bottom. The two on the top control the EGR valve and the three on the bottom control the air pump. Getting back to the EGR system, follow the line leaving the dual CTO from the top left. This line goes to the front of the carburetor to a port called E. This is a ported vacuum port, and remember ported vacuum only has vacuum on it when the engine is NOT idling. So, if the engine is idling, there will be no vacuum getting to the EGR valve. This takes care of no EGR at idle. Also, when an engine is at full throttle, both ported and manifold vacuum go to zero, so this takes care of no EGR at full throttle. So lets say you are at part throttle, and there is vacuum on port E, but the engine is cold. When the vacuum signal gets to the dual CTO, it will be stopped there until the engine warms up, so this takes care of no EGR when the engine is cold. We have now satisfied all of our EGR control strategy. But the question remains of how to connect your EGR up to a GM throttle body. There are two ways, 1) you can leave all the devices in the diagram in their stock locations, and hook the vacuum line that was at carburetor port E to the ported vacuum port at the front bottom of the GM TBI. This is the cheap way, but will cause your system to be slightly out of tune under some circumstances, although the O2 sensor will correct the problem in a short time; 2) another way is to use a GM EGR vacuum solenoid and let the ECM completely take control of the EGR valve. This will eliminate the need for the dual CTO and the TVS valve. Also, the ECM will correct the mixture in anticipation of opening the EGR valve, so it won't have to correct the mixture with the O2 sensor after the fact. For customers that go with method 2), you'll have to pay me extra for the EGR vacuum solenoid and the special connector that goes with it.
If you would prefer to not reconnect the EGR valve, let me explain what will happen. First, it's illegal, so hook it back up as in method 1 or 2 above, or you could be fined thousands of dollars. Second, since the EGR gas takes up space in the combustion chamber, it actually improves your gas mileage about 5% because less fuel is needed to go with less air. Third, if you don't have it working, but the spark timing is set up as if it were, you will get detontation. I will go into more detail about this in the spark control section.
Air Pump System - The air pump system pumps fresh air under pressure into the exhaust manifolds during certain operating conditions, and into the catalytic convertor during other operating conditions. The purpose of the air is to help burn off excess fuel when the engine is cold, and also to help the catalytic convertor warm up faster. Unbeknownst to most people who work on their cars, the air pump itself does not reduce the performance of the engine in any way, but it is usually the first thing a shadetree mechanic tears out, probably because of all the air hoses that are running everywhere. When the engine is cold, it needs excess fuel to crank and stay running. Cold gasoline doesn't mix as well with the cold air, so the fuel control system has to dump in extra fuel to make up for what didn't get mixed with the air. Well, because carburetor choke systems are not very precise ways to meter excess fuel, you get a lot of excess gasoline that doesn't burn completely and comes out of the engine exhaust as unburned hydrocarbons. If you add fresh air to these hot hydrocarbons in the exhaust manifold, they will continue burning in the exhaust pipes, and be converted to carbon dioxide and water. Not only does the fresh air clean up the hydrocarbons, but the continued combustion in the exhaust pipe warms the catalytic convertor quickly. Once the engine warms up and the choke function is no longer working, the fresh air is pumped directly to the catalytic convertor, where it helps the convertor to breakdown any residual hydrocarbons. However, there are conditions where you want to dump the air to the atmosphere to prevent damage to the catalytic convertor or to prevent an explosion in the exhaust pipe. When you all of a sudden decelerate, usually the mixture will go rich because the engine needs no power from the fuel at all. This rich mixture doesn't burn completely and the unburned gas can be discharged into the exhaust manifold. If fresh air is injected under this condition, you can get a loud backfire in the exhaust pipe, which is potentially damaging. Another situation is when you are running too rich due to a carburetor malfunction. If the fresh air is injected under this condition, the catalytic convertor can overheat and be permanently damaged, i.e. it will do a China syndrome on you. Finally, at full throttle, carburetors are intentionally jetted rich for maximum power, so again, you don't want to inject fresh air into the exhaust manifolds due to the rich mixture possibly overheating the convertor. So the air pump control strategy is to inject air into the exhaust manifolds when the engine is cold, but send it to the catalytic convertor when the engine is warm. But we want to divert the air to the atmosphere when decelerating, or at full throttle. Unfortunately, non computer carburetor equipped engines have no way of knowing if they are running rich, and will melt the catalytic convertor if the car owner doesn't smell the foul odor and do something about it first. To see how the stock Jeep emission control system controls the air pump, refer to the Jeep emissions diagram above. First, find the air pump in the upper right corner of the diagram. Note the air hose going from the air pump to the air control valve. Since the air pump is belt driven, it can't be turned off, so the air has to be diverted to the atmosphere when it is not needed. The air control valve does exactly that. The air control valve is really two valves in one. The first half of the valve either lets the air continue to the second half, or diverts the air to atmosphere. Note the black vacuum line coming off the right side of the air valve and going through a delay valve, and then continuing to a manifold vacuum fitting (the hexagon with the letter "M" inside; ignore the two lines teed off the side of this one for now). This vacuum line serves one purpose. It diverts the air pump air to atmosphere when the engine is at full throttle. This is because engine vacuum goes to zero at full throttle and there is no vacuum to hold the valve closed. At any other time, the air passes on to the second half of the valve.
If the air makes it to the second half of the air control valve, it can go in only two directions. If you follow the air hose leaving the top of the valve, it goes up and over to two air pump (A/P) check valves. These are check valves located at each exhaust manifold. Their purpose is to prevent exhaust gases from entering the air pump system when the air pump is diverting air to atmosphere or when there is an exhaust backfire. Going back to the air valve, follow the other hose leaving the air control valve on the left side. This hose goes to another A/P check valve, but this one says down. This check valve leads to the catalytic convertor. So the second half of the air control valve either sends the air to the exhaust manifolds, or to the catalytic convertor. But when does it do one or the other. Well, follow the black vacuum line leaving the bottom of the second half of the air control valve all the way to the back of the engine where it connects to the bottom of the dual CTO. There are three ports on the dual CTO. The rightmost one is connected to engine vacuum. The center one is connected to the air control valve, and the leftmost one is vented to atmosphere. So, when the engine is cold, engine vacuum is applied to the air control valve through the dual CTO, which routes the air pump air to the exhaust manifolds, but when the engine warms up, the dual CTO vents the air valve to atmosphere and the air valve routes the air pump air to the catalytic convertor. To reconnect the air pump control system after installing a GM TBI, there is really nothing to do. All of the vacuum lines are coming off ports away from the carburetor, so if you didn't disturb the lines for the air pump system, there is nothing to reconnect. If you did distrub the vacuum lines, just follow the diagram and my explanation and try to hook them back up. There is one concern about leaving the air pump system operational when adding a GM EFI system to your Jeep. Since the oxygen sensor responds to residual oxygen in your exhaust manifolds, fresh air pumped into the exhaust manifolds when the ECM is reading the O2 sensor will cause the ECM to add a lot more fuel to "use up" the excess oxygen it thinks is coming out of your engine. To alleviate this problem, I have to reprogram the EPROM such that the ECM waits longer than usual and lets the engine warm up until the dual CTO redirects the air pump air to the catalytic convertor. Since the convertor is downstream from the O2 sensor (hopefully), air injected there won't effect the O2 sensor reading. In a factory computer controlled engine, the ECM knows to not read the O2 sensor until after it has rerouted the air pump air to the catalytic convertor. On a GM EFI retrofitted engine, I have to program in a hotter temperature which is above the dual CTO switching temperature before the ECM will read the O2 sensor. Another option is to let the ECM control the air control valve with a vacuum solenoid. Again, this adds extra cost onto my custom built kits.
Spark Control System - Basically, the spark control system consists of two devices, the vacuum and centrifugal advance mechanisms inside the distributor. The centrifugal advance is completely dependent on RPM of the engine, so there are no on the fly adjustments that can be done by emissions control devices. You could swap out springs inside the distributor if you wanted to have the centrifugal advance come in sooner or later, but with a GM TBI kit retrofit, swapping springs is not necessary because the EPROM contains a table that increases spark advance based on RPM. This table is completely adjustable, so you can "swap out springs" in software so to speak. This is a lot easier and more reliable than the mechanical springs.
The vacuum advance mechanism is a different story. There are 3 different devices controlling the vacuum advance on a Jeep distributor, the Non-linear valve, the Spark CTO, and the HDC CTO. Lets follow the vacuum hoses to understand what is going on. It's probably easier to go backwards to see how the vacuum signals end up at the distributor. First, follow the vacuum line from the distributor (location "C") back to the middle port on the HDC CTO. This means that the other two ports on the HDC CTO are incoming lines, and the center port is an outgoing line. The HDC CTO switches vacuum to the center port (distributor) depending on the coolant temperature. The HDC CTO is located in the thermostat housing after the thermostat, so it is a high temperature overide. If you follow the bottom port (in the diagram its the highest one; on the CTO its the one closest to the engine), it is teed into an engine manifold vacuum source (hexagon with the M in it), so the HDC CTO switches the distributor vacuum direct to engine vacuum when the engine is very hot. I have found that this temperature is 220 degrees F. The purpose of the HDC CTO is to advance the timing at idle to the maximum vacuum advance it can have. This helps to cool the engine by making it more efficient. It also increases nitrogen oxides, but since this is sort of an emergency condition, the pollution is tolerated. So the HDC CTO is only used to switch the distributor vacuum to full engine vacuum when the engine is very hot. Any other time, the vacuum comes from another source.
Now, follow the other HDC CTO port (the top one; bottom in the diagram) and you will see that it goes to the middle port of the spark CTO. We know that the middle port on a 3 way CTO is an outgoing port, so lets see what the other two are coming from. The bottom port goes to the carburetor at port S. (There is also a tee in this line that goes to the non-linear valve.) The top port goes to the non-linear valve. I have tested the spark CTO and found that below 135 degrees, the vacuum is directed from the top to the center port and above 135 degrees the vacuum is directed from the bottom port to the center port. So when the engine is below 135 degrees F, the vacuum that operates the distributor comes from the non-linear valve. Above 135 degrees F, the vacuum comes from a carburetor ported vacuum port. Since I have already discussed how ported vacuum works, we now know how the vacuum advance works between 135 F and 220 F. When idling, there is no vacuum advance. When the gas pedal is pressed ported vacuum is applied to the distributor and the vacuum motor advances the timing according to the vacuum motor spark timing curve. But what about when the engine is below 135 degrees F? This is when the non-linear valve comes into play. I have tested the function of this valve. It provides about 8-9 in HG. of vacuum at its outlet port (the one going to the spark CTO) at idle when there is engine vacuum on the middle port and no vacuum on the bottom port. But as soon as the gas is presses and port vacuum is applied to the bottom port, the vacuum at the outlet immediately jumps to whatever the engine vacuum is. So the function of this valve is to send about half the engine vacuum to the distributor when it is cold and idling, but as soon as the gas is pressed, the vacuum at the distributor tracks engine vacuum.
What does all this mean? Well, it means that the vacuum advance depends on engine temperature and is controlled differently depending on what range of temperature you are operating in. Below 135 F, you get more vacuum advance at idle than when warmed up, but not full engine vacuum advance. Above 135 F but below 220 F, you get no vacuum advance at idle because there is no vacuum on the ported vacuum port S at the carburetor. And above 220 F, you get full engine vacuum advance. The big question is why is all this necessary? Well, when an engine is cold, you want a lot of advance to help the engine run well, plus you get more power with a lot of advance on a cold engine. When the engine is fully warmed up, you also run better with about 30 degrees of advance at idle, but this creates hot combustion temperatures. So to limit the creation of nitrogen oxide pollution, Jeep engineers took away vacuum advance at idle on a warmed up engine. But this makes the engine run hotter because it is less thermally efficient. In other words, late spark timing makes the engine run hotter. So, the HDC CTO puts the vacuum advance at its maximum when the engine is running hot to help cool it down.
One more comment about spark timing. When EGR is engaged, the combustion process is slowed down. So factory spark timing curves are advanced 3-6 degrees at part throttle operation (beyond what they could normally be without EGR) to regain some efficiency due to the slower combustion. Also, on non-computer controlled vehicles with carburetors, the carbs are jetted lean (10% less fuel) at part throttle since the system was designed for the EGR valve to be operational during all part throttle operation. If the EGR valve fails to open, the mixture will be lean since you will now have more air in the combustion chamber. Now, combine this lean mixture with timing that is advanced too far for a non-EGR situation, and you have a combination that will produce engine damaging detonation. Drive the vehicle like this with detonation for very long and you can kiss the engine goodbye. This scenario affects programming an EFI system in the following ways. When I program a spark table for an engine, I start by looking at the factory curves for the centrifugal and vacuum advance. But I have to keep in mind that these curves are setup for EGR operation at part throttle. So I have to take out 3-6 degrees of timing at all part throttle load conditions. Needless to say, this involves some educated guessing and luck to get right the first time. If the engine is one that I haven't done before, a knock sensor will probably be required to zero the table in.
Without going into a lot of detail, I have duplicated all these scenarios in my TBI kit EPROM chips. Below 135 degrees, I have about 30 degrees of advance applied at idle. Above that temp, the timing drops down to about 10-15 degrees. Above 220 F, I add about 20 degrees for a total of 36 degrees of advance on a hot engine at idle. When you press the gas, the timing follows the normal advance that the vacuum motor would give you. For this reason, both CTOs and the non-linear valve can be removed when my kits are installed and the spark timing will still follow the factory timing curve.
Thermostatically Controlled Air Cleaner System - The sole purpose of the thermostatically controlled air cleaner system is to help the engine warm up faster and to help the fuel vaporize easier for cleaner burning when cold. If you look at the lower right of the diagram on the air cleaner circle, you will see two vacuum motors called TAC Vac Mtr and Evap Sys Vac Mtr. The Evap Sys Vac Mtr will be discussed in the next section since it is a part of the evaporative emission control system.
The TAC Vac Mtr directs heated air into the air cleaner housing when the engine is cold and warming up. The heated air comes from a metal shield that surrounds the passenger side exhaust manifold. Of course, when the engine first starts, there is no heated air, but as the engine warms up, the exhaust manifold heats up and the air around it is heated as well. Once the air coming into the air cleaner is sufficiently warm, the TAC Vac Mtr moves to close off the heated air and takes in air from the outside of the engine bay through the intake hose on the air cleaner. There is a temperature switch that controls the temperature at which the changeover from heated to outside air takes place. This temperature switch is called the Tac Temp Sen and is shown on the lower left of the air cleaner circle. There is also a check valve and a time delay device in the vacuum line to the Tac Vac Mtr. These should be left alone. It is recommended that this system be left just as it is when converting to a TBI system. Although I don't think it helps much since the gasoline is sprayed into the engine with a TBI injector and vaporizes much better than with a carb, I don't think it hurts either, so you might as well keep it as is.
Evaporative Emission Control System - The purpose of the evaporative emission control system is to reduce the emissions from evaporating gasoline when the engine is both running and when the vehicle is not being used. The heart of this system is the charcoal canister, which is shown at the lower right/center of the diagram above. This canister is filled with activated charcoal and traps gasoline vapors inside until they are purged (sucked) from the canister by the engine. The purge sequence is controlled by the dual CTO at the back of the engine and is essentially tied in with the EGR signal. In other words, anytime the EGR valve is activated the canister purge is also activated. This is evident from the vacuum lines being teed together at the dual CTO. The purge control vacuum line goes to a valve on the top of the canister. This valve is normally closed until the purge vacuum signal is applied. Then the canister is purged by engine vacuum on the Can Purge hose (second from the bottom). The canister purge line ties in to the PCV line at the back of the carb where fumes are sucked into the engine and burned with the PCV fumes. There are two lines that connect to the top of the canister in the diagram. One line comes from the gas tank and allows gas tank vapors to be captured in the charcoal canister. The other line comes from the carburetor bowl vent and captures vapors that are released from the carburetor. Since a TBI has no bowl, the bowl vent is no longer needed, which is why my instructions below describe how to remove the bowl vent lines and the bowl vent solenoid. What this basically means is that you can cap off the top port of the charcoal canister since it is no longer needed. The canister is still used to trap gas tank vapors though, so leave it in place.
Another device in the evaporative emissions control system is the vacuum motor in the air cleaner inlet. This motor closes off the air cleaner inlet when the engine is not running to help trap the vapors from the carb from escaping outside the vehicle. However, it also keeps the engine from getting any air at all if it malfunctions, so I recommend that this device be permanently disabled. There is an elongated control arm that moves the trap door inside the air cleaner. You can pinch this control arm down to where the trap door cannot close no matter what the vacuum motor does. I recommend that you do this. Since a TBI does not allow vapors to escape when the engine is not running, the Evap Sys Vac Mtr is no longer needed.
Positive Crankcase Ventilation (PCV) System - The PCV system uses engine vacuum to pull fumes from inside the engine and burn them when the engine is running. The PCV valve is a check valve that prevents a backfire from igniting the fumes inside the engine. Without a PCV valve, you risk an explosion inside your engine if the fumes inside the engine ignite. The fumes inside the engine come from what is called blowby. Blowby is the explosive gasoline and air mixture that gets past the piston rings inside the cylinders. Blowby is worse on very old worn engines, but occurs on all engines. The purpose of the PCV system is to basically recycle blowby and send it back to the combustion side of the piston for burning. In the old days, there was a crankcase vent tube that extended down under the car. The air rushing by the vent tube would pull the fumes out of the engine and just release them to the atmosphere. The PCV system was one of the first emissions control systems to come along. There is really not much to the PCV system and it most definitely should be left alone and intact. The PCV valve is very important to this system and should be installed. An engine without a properly working PCV system will not idle correctly because some of the air used by the engine when idling comes from the PCV valve. The PCV valve acts as a restriction to limit the amount of air that the engine recieves when idling. There are no changes needed to the PCV system when installing a TBI EFI system. However, the PCV port on a GM TBI is on the front of the TBI, so you will need about a 15 inch long 3/8" PCV hose to reach around to the front. This concludes the emissions controls system detailed descriptions. General instructions on how to reconnect the emissions control systems on a Jeep TBI conversion follow.
SUMMARY OF THIS PAGE
To reconnect the EGR, Air pump, and charcoal canister devices, reconnect the vacuum line that was on the carburetor EGR port (port E on carb) to ported vacuum on the GM throttle body (see photos).
Find tag "A", and remove the carburetor bowl vent hose from the canister back to the carb location. You will need to remove the vapor separator, and the bowl vent valve with this line. You can just leave the bowl vent valve wire laying on the engine. Using rubber caps, cap off the charcoal canister bowl vent port (tag "A"), and the branch on the tee (tag "B") in the purge line.
Next, find tag "C" at the distributor. Pull off and remove the hose at the distributor's vacuum motor. You can put a rubber cap over the vacuum motor inlet if you want. At tag "D", remove the hose and cap off the manifold port at the manifold vacuum fitting. This was the carb's power valve hose and is no longer needed. Also at tag "D", remove the two tees so that the other line from the port at "D" goes straight to the air pump valve with no tees. Leave the time delay in the vacuum line though. At tag "E", remove the vacuum line that used to be connected to the carb at port S. Then proceed to remove all the vacuum hoses connected to the spark CTO, the HDC CTO and the non-linear valve. At tag "F", remove the spark CTO and use the hole in the manifold to install the EFI system coolant sensor. You can also remove the HDC CTO, but its best to just leave it in place as a plug. All the functions of the 2 CTOs and the Non-linear valve have been dupicated in my GM TBI kit EPROM chip. Finally, using a new 3/8" rubber fuel hose, connect the PCV valve to the PCV port on the front of the GM TBI. You'll need a new hose because the old one won't reach around to the front of the GM TBI. Finally, pinch off the Evap Sys Vac Mtr control arm as shown in the photo above to keep the air cleaner inlet open at all times.
This completes changes to the emission controls systems to allow a GM TBI EFI system to work on your Jeep vehicle and keep the vehicle emissions systems functioning as the factory intended them to. It will be your responsibility however to verify the legality of these changes with the emissions inspection agency in your area prior to performing these changes on your vehicle. I have also designed these changes to be completely reversible, so save any parts that were removed per these instructions. Other manufacturer's emissions controls systems are similar, so contact me for the changes required for any other vehicle. I will need to program the chip to duplicate the function of any devices you want to remove.
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