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The Lowdown on Code 32 (EGR)
Ok, here is the deal, your cruisin’ down the road at 70 Mph, you got the T-Tops out and the stereo blaring. Life is grand. Then out of nowhere comes that annoying bright orange “Service Engine Soon” light. You are shocked. You pull over to the nearest gas station to do a quick inspection of under the hood and find nothing. You take off again and the light is off. Whats going on? You go home and check for codes and what pops up but the notorious code 32. The purpose of this article is to teach you the theory, operation and diagnostics of EGR in hopes that you will not be needlessly swapping parts. I will try to not make this a boring science class or a useless lecture but an informative tool.
EGR Theory. EGR serves one purpose and one purpose only. That purpose is to
reduce Oxides of Nitrogen (NOx). Under normal combustion, Nitrogen(N2)
Oxygen (O2) in the air and Hydrocarbons (HC) in the fuel combind into water
(H2O) Carbon dioxide (CO2) and the Nitrogen remains unchanged. Under very hot
combustion temperatures, the Nitrogen reacts with the other two byproducts and
forms Nitrogen oxide (NO). After being released into the atmosphere, it picks up
another Oxygen and becomes Nitrogen dioxide (NO2). In the presence of sunlight,
it combines with other compounds like Hydrocarbons and forms Smog. Since
exhaust gas is inert (very stable) it doesn’t burn again. So by being introduced into
the combustion chamber, it will lower combustion chamber temps enough so that
the Nitrogen doesn’t react with the other compounds and is passed unchanged out
the tailpipe thus not contributing to smog. Now, since exhaust gas doesn’t burn, it
doesn’t exactly help with combustion. At higher RPM’s, this really isn’t noticable,
but at idle, the reintroduction of exhaust gas will cause a very rough idle and can
cause stalling if to much is introduced into the combustion chamber.
EGR Operation. Like mentioned before, EGR flow is good at higher RPMs, but not
good at idle, so some sort of control needs to be place on the EGR system. Earlier
systems used ported vacuum straight to the EGR valve. At idle, the throttle blades
are closed, so there isn’t any ported vacuum. When the throttle is opened, ported
vacuum starts to build. The more throttle, the more ported vacuum. This is how
vacuum advance distributers work. As throttle is increased, the EGR valve opened
further. Once the throttle is closed, ported vacuum is lost and the valve closed.
Most Third Gen. F-bodies use the basic diaphram EGR valve, but instead of relying
on ported vacuum, it relies on vacuum that is allowed to pass through a solenoid.
The solenoid is controlled by the ECM. When conditions are right (engine temp,
TPS position, RPM, etc...) the ECM will ground out the solenoid. The solenoid is a
Pulse Width Modulated (PWM) meaning that the amount of vacuum is controled by
the computer rapidly switching on and off to ground. The more vacuum the ECM
wants to go to the EGR valve, the more time it lets the solenoid remains grounded,
measured in Duty Cycle. With a scantool, you can command the EGR solenoid to
say 50% Duty Cycle and actualy feel the pulses if you put your finder over the
vacuum port of the solenoid. With 20" of vacuum, a solenoid thats pulsed on a 50%
Duty cycle (50% on, 50% off) will average about 10" or vacuum on the solenoid outlet.
EGR Diagnostics. Now that you understand what EGR does and how it does it, lets
see why sometimes it doesn’t do it. Since exhaust gas if being introduced into the
intake manifold, things like manifold vacuum and airflow reduce slightly. Speed
density computers can recognize EGR flow by looking for a drop in vacuum via the
MAP sensor. MAF systems use either a temperature switch mounted
in the base of the valve or a
diagnostic vacuum switch. Since exhaust gas is hotter than
fresh air, it can pick up EGR flow by
sudden increases in temp when the valve is open. The vacuum switch monitors vacuum going from
the solenoid to the valve. In theory, if the valve is getting vacuum, then the system "should" be
working. Highway speeds is when the most EGR flow is
commanded. When cruising on the highway, the computer will pulse the EGR solenoid so that
vacuum will pass and go to the EGR valve. After commanding the solenoid on, it will look
for signs that the EGR valve opened by one of the means mentioned above. If the ECM does not
see the change its looking for, this is when the light comes on. What are some possible causes of no
exhaust flow? Well, first, make sure that the solenoid is getting vacuum. A plugged or broken
vacuum line will make everything else inoperative. If vacuum isn’t getting to the solenoid, it surely
will not get to the EGR valve. Once this is established, make sure the ECM has
control over the solenoid. Visually make sure that the connector is firmly seated
into the solenoid and that the terminals inside the connector are not corroded or
damaged. Usually you can rev the engine while it is in closed loop and it will be
enough for the ECM to command EGR so you will be able to feel vacuum at the
solenoid. If you feel vacuum, than the ECM has control over the solenoid and
adequate vacuum is reaching it. If you do not feel vacuum, you may need to drive
the vehicle with a vacuum gauge hooked up to the solenoid. If you are driving in
closed loop, you should see some vacuum. If you don’t the solenoid is probably
bad, or the ECM is not controlling it (bad ECM grounds or ECM) providing you
had vacuum going to the solenoid. If all is OK so far, inspect the vacuum line
going to the EGR valve for plugging or being broken. It is not uncommon for
previous owners to plug these vacuum lines with sticks, BBs, screws, or anything
else to try and cure a “rough idle”. The lines can also become plugged with carbon
deposits over time. If it is clear and free from defects, check out the valve itself.
Make sure it isn’t seized by manually lifting up on the diagphram. If it moves
freely, put a vacuum on it. It should move and stay there (hold a vacuum). If it
doesn’t move, try lifting a little on the diagphram (it may be a positive backpressure
vavle, lifting on it some will act as the backpressure). If it still doesn’t move, or
wont hold vacuum, the valve is bad. If everything still checks out OK, then the only
other thing that will limit exhaust gas flow is plugged up passages in the intake
manifold and cylinder head. This is a common problem with the V6’s and will
leave many technicians scratching their head because the passages can’t be seen. If
all else checks out, get a rifle cleaning brush and a shop vac and start cleaning. You
will be surprised the amount of carbon chunks that will come out of there.
Other notes: Most people like to disable the EGR because they claim that hurts
performance. In actuality, disabling the EGR can hurt performance. Here is why.
As we already know, at certain thorttle postitions and RPMs, the ECM will
command EGR operation. This is to cool combustion chamber temps under load.
Well, with cooler combustion chamber temps, we can further fuel economy by
advancing the timing. We know that to much timing will cause “pinging”. But
when we keep the combustion temps down, the timing can be advanced without the
“pinging” effect. At highway speeds, the ECM commands EGR operation and will
advance timing accordingly. With a blocked of EGR, the computer thinks it is
flowing when it is not and will advance timing. Now that the combustion chamber
temps are much hotter, the advanced timing is no longer a good idea and detonation
occurs. Since detonation can severly damage an engine, knock sensors are used.
When the knock sensor detects detonation, it will retard timing. It takes more to
stop detonation that it does to cause it and this is where it hurts performance. For
example, at highway speeds, your total advance may be, lets say 30* BTDC. If the
computer advances it one more degree to 31* and it detects detonation, it can’t just
go back to 30* to stop it, it must retard timing to like 25* to try and stop it, and if it
still occurs it will further retard timing. If the EGR was working properly, the
temps would have stayed cool enough to operate at 31* with no problems.
Many items were forgotten about when this article was written.
Part II includes these updates.
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