My 1991 Vette is running flawlessly, but the "Service Engine Soon" light comes on after 10 minutes of highway driving. Does anyone know of a meter or tool I can get to pull down the codes? Most meters I see are for call 1996 or newer. Thanks.

 

A paper clip works good look on youtube to see how.

 

Check these links:

https://www.corvetteforum.com/forums...post1574918379
http://corvettephotographs.com/c4vettes/codes.htm

https://www.corvetteforum.com/forums...post1576116036

 

Thanks.

 

Thanks.

 

I've seen this happen more than once, especially the "10 minutes of driving" thing. if it just started for no reason, check your gas cap -

 

I have a '91 too and I get a similar Service Engine Soon light. It usually is a Code 32 - EGR failure.

My mechanic told me it could be carbon buildup on the EGR valve and I could pour some Seafoam into the gas tank to see if that would knock some of it loose.

 

My 86 would throw this code when the EGR temp sensor was going bad.

 

The EGR is energized and the test done after about 5 minutes of steady cruising.

The ECM turns on the EGR solenoid, which applies vacuum to the EGR valve. The test is done after that, either by the EGR temperature switch or a MAP change.

 

WOW! And if people poured Seafoam into their mouths and gargled, it would cure all dental problems, give fresh breath and whiter teeth.

Fuel in the gas tank goes to the injectors and into the combustion chamber. I suppose there might be an open chamber with fuel and the Seafoam evaporates into the intake manifold and it reacts with the carbon build up and solves the problem too.

 

Let me fix this one too, Cliff.

The EGR solenoid is never "energized". It is PWM'd. (Perhaps you mean PWM, but "energized" seems like ON (100%DC) to me).
- MAP has nothing to do with the EGR diagnostic in the L98. (I'm not aware of any GM OBDI EGR diagnostic that uses MAP. Usually INT). I'm not aware of any GM that uses the EGR diagnostic switch other than the L98 TPI engine. Don't know if the 305 TPI F-car uses the switch or not.
+ MAP has nothing to do with the EGR diagnostic in the L98. (I'm not aware of any GM OBDI EGR diagnostic that uses MAP. Usually INT).

From the FSM y'all love so much:
- 1985-1989:
"Code 32 means that the EGR diagnostic switch was closed during start-up or was not detected closed under the following conditions

• Coolant temperature greater than 176F
• EGR Duty Cycle commanded by the ECM is greater than 27%
• TPS less than wide open throttle, but not at idle.
• Code 21, 22, 33, or 34 not present.
• All conditions above must be met for about 4 minutes."

Note: The duty cycle is calculated by the ECM based on information from the coolant and mass airflow sensors, and engine RPM. In the 85-89 the EGR diagnostic is completely dependent on the switch being closed when it is expected to be closed. No other parameters like air flow or INT are considered. Only the switch.

1990-1991:
"The ECM monitors Integrator during a Code 32 test and must detect a change to pass the test. If INT is at a fixed value (lean or rich) when a Code 32 is ran, a Code 32 may be set.

The ECM will check EGR operation when:

• Vehicle speed is above 30 MPH.
• MAP is between 15 and 70 kPa.
• TPS is between 8% and 30%,
• No more than .4% change in TPS."

 

I don't suppose you are aware of a good way to bench test the EGR itself ???

 

Maybe Op will post code that is present

 

PWM is not a good description of what happens. There is no way the vacuum could respond to varying-width pulses.

The ECM turns on the EGR solenoid timer which then generates an interrupt when it times out. The ECM immediately resets the timer, which turns the EGR solenoid on again. What you end up with is a signal that is continuously on except for VERY brief pulses when it times out and turns off.

It looks like this:

 

That is a good description of exactly what Pulse Width Modulation IS. A "timer" that continuously resets. The signal is either High or Low. But the On (or Off) period (the "pulse") is modulated or changed with respect to the other. What you describe (long On, brief Off) is a High Duty cycle. A short On, long Off is Low DC. Half On, half Off is a 50% DC.

You're right, the "vacuum" itself can't respond to PWM, but the VALVE will because it has a spring in it.

Keep in mind that "vacuum" is an absence of atmospheric pressure. Atmospheric pressure is ~15psi at Seal Level. By applying vacuum to one side of the diaphragm, there's 15psi pushing on the other side. Let's say that at the half-way point in the plunger's travel the spring is exerting 7.5 psi to close it. We want to command it to open half-way. Applying 50% of 15psi is 7.5psi which balances the force of the spring at the half-way open point. So we want a 50% DC in this example, with this spring force.

The solenoid has 3 ports. When energized, the diaphragm is connected to vacuum which overcomes spring pressure to open it. When not energized, the diaphragm is vented to atmosphere, and the spring pressure closes it. The further the valve is open, the higher the spring pressure trying to close it. There is a balancing act between vacuum opening the valve, and the spring trying to close it. EGR valves are seldom "full open" or drivability would be severely impacted. So indeed the valve position DOES respond to differences between vacuum and spring pressure by varying the DC of the solenoid. Read the conditions for DTC 32 on a 85-89. One of the conditions is 'EGR commanded is >27%". How can that be if it is either 0% or 100% like you posit it is?

PWM EGR control is crude because the vacuum present is subject to variables and the spring pressure isn't, so there is no assurance that the actual valve position will be the calculated (desired) position. Later EGR systems use feedback potentiometers within the EGR valve to transmit actual pintle position to the ECM for more accurate control over actual position of valve opening. I know the L35 CPI V6 which was introduced in 1992 has linier EGR. I believe the 93-up LT5 has linier EGR with feedback. By the mid 90's all GM engines incorporated linier EGR feedback systems. The earlier PWM'd vacuum valves weren't "accurate" enough.

The exact same PWM'd solenoid principle is used to control Turbo Boost pressure in the GMC Syclone and Typhoon. I can tell you from 20 years experience with the SyTy that the wastegate responds quite well to the PWM'd solenoid. At 100%DC (signal blocked) boost exceeds 30psi. With the solenoid unplugged (0% DC, full signal) the boost is 8psi because of the spring setting in the waste gate. SyTy max "Target Boost" is 15psi, and the DC changes constantly to maintain the target boost. PWM solenoids work quite effectively at controlling the signal to spring-actuated diaphragms like those contained in waste gates and EGR valves to obtain partial opening.

You need to advance your knowledge on how various systems actually work, rather than speculating how they work. To put it bluntly, you're frequently wrong.

 

Not everyone bothers to memorize every intimate detail of every system. Instead, somtimes they just remember where the shop manual is. This is a very practical approach to high volume repair, and practical is never wrong.
A solution for 90% of a problem right now is far and away better than waiting around for a 100% solution somtime in the future.

 

Not understanding how a system functions has up to a 100% chance of an incorrect diagnosis, replacement of good parts (which usually cost money), or wasting time because you're drawing incorrect conclusions based on incorrect assumptions of how it really works.

Posting incorrect information can cause peeps to waste time, waste money, and still not fix their car.

If someone asks a question, or describes a problem and you're unsure of how it is supposed to function, get out the f-ing shop manual yourself, or hit Google, figure it out, THEN try to diagnose it for him.
A: By conducting the research you learn it (or refresh your memory) yourself.
B: He, and others view accurate information, and are not led astray. They learn it too.

 

I agree with everything you just said. Unfortunately it seems your intent on diminishing the contributions of someone who has enough practical working knowledge to get the job done 9 out of 10 times. This is a mistake. Plus he is close enough on this egr issue. Your own post admits his description of PWM solenoid operation is correct. Besides that, there are plenty of keyboard mechanics masquerading as experts; why aren't you fact checking them ? You can start with me.