Topic: EGR vacuum woes

[gelande-strasse]

Hi all: I've posted this problem on a couple different forums, but my problem persists despite all the advice I've been given.

The jeep is a 1990 YJ, 2.5l TBI.  The issue I'm having is that I have vacuum at idle from the EGR solenoid output. 

Here's a list of my troubleshooting:
-I followed the FSM diagnostic procedure: the EGR is functioning as it should, and I have vacuum at the solenoid input at idle and output at idle.
-Checked the vac lines: routed correctly and replaced as needed, no change.
-The solenoid has battery voltage at both he blue and red wires.
-Jumpered the blue wire to ground: no change.
-Replaced CTS: no change.
-Replaced the solenoid; no change. 
-Tested MAT sensor: voltages are correct, but the sensor doesn't hold vacuum...I ordered the part just in case it is supposed to hold vacuum.

At this point, I'm not sure whether I have a solenoid or ECU issue, or whether the jeep is stuck in closed loop, or some mode that would prevent the solenoid from closing.  Any help would be appreciated.

[chardrc]

if i recall correctly the solenoid needs power to cut off the vacuum to the egr valve (turn off egr). the solenoid should only have 2 wires so it is odd that both wires are at battery voltage (if you are reading the voltage with the solenoid disconnected). i would check the wires to the solenoid and make sure the one to the ecu is not broken somewhere. I'm guessing the positive side is always hot and the ecu grounds the second wire. in the case that the wire to the ecu lost connection and the solenoid is plugged in both sides would be sitting at 12 volts since no current is present for a voltage drop to occur across the solenoid.

[gelande-strasse]

I got the battery voltage reading back probing the sensor.  I think the jeep was off as well.  Reading your post makes the circuit clearer though, and I'll be sure to check continuity of the ECU wire.  Thanks!

[gelande-strasse]

I checked the ECU side wire from the solenoid back to the ECU with a multimeter and it seems solid.  I jumpered the solenoid, and the solenoid works as it should (i.e. when the ECU side is grounded, the solenoid closes and vacuum to the EGR is cut off).  I think I've narrowed it down to either 1) there is an incomplete ECU ground somewhere, or 2) the ECU is toast (I doubt this is the case).

 Are there other ECU ground locations besides the one near the dipstick?

[chardrc]

Pins 1,2, and 10 should all be grounds.  The wiring diagram i have show all of them coming together to the "ENG. GND." so i guess there is just the one grounding point but thats not to say one of the connections between the ecu and engine block haven't gone bad or been tampered with.

The egr solenoid should be energized when the engine is not warmed up, is idling (closed throttle switch closed), full throttle (wot switch closed), or under "heavy acceleration". have you tested the continuity of the closed throttle switch in the idle speed actuator? or you could try closing the wide open throttle switch and see if that energizes the egr solenoid although the computer may get confused since the tps wont be near full throttle voltage. i would basically go through each situation where the egr should be turned off adn see if any of them are triggering the ecu to power the solenoid.

[gelande-strasse]

The closed throttle switch works and the WOT switch works.  Tripping the WOT switch at idle doesn't trigger the solenoid. 

I'm starting to think my ECU is bad since I've tested and/or replaced all of the sensors that could affect the EGR solenoid.  I found some corrosion on the ECU PCB, but cleaned it up and everything that was affected seems to flow as it should.  All the PCB grounds seem okay, and all of the ECU harness pins which should be going to ground are doing so.  I'm hoping that some 25 year old component in the ECU is fried and a new ECU will get this jeep running normally.

[chardrc]

I think some TBI guys have taken the ecu apart and found some cacitors gone bad and rebuilt the ecu. Forget where I found it but Google should beagle to find

[gelande-strasse]

By way of update, I replaced the ECU with a reman unit, and the solenoid now works like it should (grounded at idle).  The lower solenoid line which goes to the EGR tees to the charcoal canister purge valve, and the purge valve in my charcoal canister is leaking.  So at least I know the problem now, but I can't seem to find a new replacement or a used working canister.  Does anyone know if a new part is made that would interchange?

[dwtaylorpdx]

The canister on my 94 is a GM/Delphi part, looks the same as the one on a S10..