How is an EGR stuck open/closed code set?
 

I have been plagued with this EGR stuck closed code since I bought it, I know its stuck closed because it runs great otherwise. I have cleaned the EGR valve in and out with solvent, verified the operation of the pintle, and cleaned the filter that sits above it. The MIL stayed off for about a week after this was done but has returned with the same code 56. With no sensor lead to the EGR valve, how does the computer determine that the valve is either stuck open or closed? How could it possibly know? If someone could answer this id be that much closer to finding the real problem.... thanks in advance!

Re: How is an EGR stuck open/closed code set?
 

The code 56 on my '92 was just a general EGR system fault. I replaced the whole thing eventually, EGR, EGR controller, hoses, and temp sensor. I think the temp sensor was the culprit, it was very gunked-up. I'd recommend pulling that off and cleaning it, it's the easiest thing you can try...

Re: How is an EGR stuck open/closed code set?
 

Where is the temp sensor? Any pics of it?

Re: How is an EGR stuck open/closed code set?
 

I checked the online service manual, looks like ill have to yank out my solenoid and clean and check that too. I havent had a chance to check the clearance under the manifold yet, looks like it could be a pain. Other than that i've covered every base as far as the vacuum goes. Once I get the MIL off for good I can finally smog and register it (1 year driving so far w/no reggie :D).

Re: How is an EGR stuck open/closed code set?
 

Hmmm, don't have a picture but it's on the passenger side, rear corner of the engine. It's like a 10mm silver bolt with a 4" wire coming off to a connector, all located above the EGR system. It's pretty easy access to take off and at least clean...

Re: How is an EGR stuck open/closed code set?
 

Maybe ill try that too. I still dont understand how the ECU can detect a fault with the EGR valve itself. How can it know if the pintle is stuck open or closed without a sensor lead to the EGR? Knowing this will help me find the real problem with my low idle, as my only code is the 56.

Re: How is an EGR stuck open/closed code set?
 

by vacuum or some other type of pressure i would think...

Re: How is an EGR stuck open/closed code set?
 

Well today I yanked off the intake and bobbled around with the EGR solenoid..... taking it off was easy but getting it back on took years off my vertebrae. The solenoid passed the suck test with 12V applied to the terminals so its verified OK..... and when putting everything back together I noticed the vacuum line from the vacuum control valve to the EGR had disconnected, so im pretty sure my EGR code will stay away this time.

The only issue I have now is really low RPM when the car is fully warmed up.... im wondering if the TPS controls any engine management functions....since when I lightly press on the accelerator while sitting in Park, the RPMs go from low idle to 1300 rpm.... theres no in-between :confused: No other codes have shown up, and it accelerates and drives great.... its just the idle that bugs me.

Re: How is an EGR stuck open/closed code set?
 

The detection for the EGR system is based on the temperature sensor.

Re: How is an EGR stuck open/closed code set?
 

Ok heres the million dollar question..... if the EGR code is set by the temp sensor..... how does the computer distinguish between a bad EGR and a bad temp sensor?? So my code 56 could be set by the temp sensor itself?? I have pulled off the temp sensor and resistance tested it..... does anyone have the specs for resistance values (not in service manual)?? My resistance drops when put in hot water from 2.24 milli-ohms to about 0.72 milli-ohms in hot tap water. The EGR opened and closed fine when I had it out, should I suspect the EGR or the temp sensor? The hoses are all fine and the solenoid is operational....:confused:

Re: How is an EGR stuck open/closed code set?
 

It detects the change in temperature as the hot gas flows by it, this is how it knows the valve is open. If there is no temp increase when the valve is open or the temp isn't down when the valve is closed it thinks there is a problem with the valve. This is the point of the sensor. The code for the sensor is only set if there is an open or short in the circuit. Usually when There is an egr problem on a vehicle like this you would just replace the valve, but it is possible to clean it.

Edit:

I meant valve and sensor together. Your low idle could be caused by the egr being slightly stuck open. If the egr opens at idle it will stall the motor. A common test is to apply vacuum to the egr and see if it stalls while it is idling. If it does, you know the valve is operating.

Re: How is an EGR stuck open/closed code set?
 

Finally nailed this one, my code was set because one of the hard-lines going to the EGR vacuum control valve had been clogged with something. It wasnt the line with the EGR solenoid valve, but a secondary line going directly to the throttle body port. Freed the obstruction, and so far the MIL has not returned.... :banana:

Re: How is an EGR stuck open/closed code set?
 

How did you figure out it was obstructed and how did you clear the obstruction?

Re: How is an EGR stuck open/closed code set?
 

The hard-line terminates on either side of the intake manifold, so I just unplugged the vacuum hoses on both ends of the line and tried to pull air through... the vacuum diagram showed it was just a straight shot with nothing in between so I knew it should be free flowing. (The other hard-line goes to the solenoid)

I used a length of 14 gauge insulated copper wire and fed it from the throttle body end until it stopped, then I just kept pulling it and pushing it in against the obstruction until I could at least get a little air through the other end. To finish up I squirted Brakeleen down it and forced it through with compressed air, which opened it up fully.

Re: How is an EGR stuck open/closed code set?
 

Question: Does anyone know if the vacuum to the EGR valve itself is supposed to be constant or modulated? The vacuum TO the vacuum control valve is constant, but from the VCV to the EGR the vacuum modulates and is not nearly as strong.