Hello,
I have a 1990 with Manual Trans and the EGR is not functioning. On the '90 there is a Solenoid that controls the manifold vacuum to the EGR.
When the ignition is off, the Solenoid opens, when the engine is running the solenoid appears to be on and solidly closed. Hence no vacuum to the EGR.
The ECM directly controls the ERG Solenoid. My question is, what would cause the ECM to keep the solenoid on (closed) constantly?
The check engine light is off, so I assume there are no codes.

Does ECM wait for the engine to reach a given temp? If so, which sensor does it use?
On automatics, the EGR does not function in Park on Neutral....does the manual trans version have something equivalent?
The ECM apparently looks at various sensors to determine the duty cycle of the solenoid. Which should I check out?

When the system is working, what should I typically see in terms of the duty cycle and duration of the ECM signal to the solenoid? Will a Dwell meter be of value here?

One other wierd thing. When the engine is off, and a vacuum is applied directly to the EGR valve, it holds a vacuum. When the engine is running, the EGR does not hold a vacuum. I read someting about a "bleed" function......How is this supposed to function?

 

I am trying to remember the EGR programming... IIRC, it's load based.

If the EGR solenoid isn't firing, I would check the harness and be sure that the wiring back to the ECM is good.

Have you checked codes? Or just don't see an SES? I would check to be sure the SES bulb hasn't been messed with.

 

When the engine is off, no signal from the ECM, the solenoid opens.
When the engine is on, there is signal from the ECM, and the solenoid closes. So the Solenoid and the wiring to the ECM seems good. The problem is, it stays closed as long as the engine is running, instead of switching on and off to meter the vacuum to the EGR.

 

I accessed the codes and there was one code in there; code 42. This is "Electronic Spark Timing". We cleared the code and checked to see if it had an effect upon the EGR operation. It did not, the ECM is still holding the Solenoid closed while the engine is running. We drove it for a bit and the code 42 did not come back.
So I'm still at a loss to understand why the ECM is keeping the EGR in an disabled state.

 

hm. that is telling me the ECM is wack.

you really need a datamaster type of tool to see what the EGR is being asked to do. It could be as simple as a bad solenoid.

 

Upon further checking, I found the the ECM uses the following sensors to determine how to control the Solenoid:

Coolant Temperature Sensor (CTS).
Throttle Position Sensor (TPS). (It runs great, so this can't be it)
Manifold Absolute Pressure Sensor (MAP). (Runs great)
Manifold Air Temperature Sensor (MAT).
P/N Switch. (Its a manual trans, so not this either)

Also, I have learned that the EGR function is disabled until the engine reaches "operating" temperature. So this seems to point me at the CTS or MAT. The engine has three temp sensors. There is one on the front of the engine, which I believe controls the radiator fans. There is a sendor on the right side of the block....not sure what its used for.
And the MAT is under the Plenum.
I just need to know which of these the ECM uses to determine that the engine has reached "operating" temp. You would think that if one of these were bad, that I would see a code. but no codes.

 

I *think* the head temp sensor is used by the ECM. The one on the manifold, as you said, controls the fan.

 

It appears that the temp sensor on the front of the engine is the radiator fan sensor
The sensor on the right side of the right head is the one that drives the guage and I think the computer. I have checked it and it has the approapriate resistance for the given temperature.
The One under the Plenum must be used to fine tune the mixture.

Again, I'm stuck. Everything checks out. I do have another ECM, which I can try swapping with the one in the car.

 

Swapped in the replacement ECM and the EGR solenoid came to life.
Odd that the ECM was bad in such a narrow specific way.

 

Could also have been the connection, you might want to try plugging the original ECM back in again and trying it.

Tom

 

Nice call Andy.