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With engine idling,pull a vacuum on the egr nipple..Within about 5 inchs of Merc. the Engine should begin to run rough..If it doesnt,there is either a hole in the EGR diaphragm or the valve is stuck closed from Carbon..If it does open and the engine runs rough,then the valve is ok.Check the hard plastic vacuum lines that runs from the egr to the egr solenoid ,they run behind the upper intake,they get hard and break..If those check out ,check the egr position sensor and egr solenoid..Good Luck....One of the vacuum lines at the egr solenoid should have vacuum present at all times....The egr valve will not have vacuum present at an idle,the engine has to be under a load for the computer to command the egr solenoid to open the egr valve..
One quick easy test is to jumper a ground wire over to the computer side of the EVR, the terminal with the dark green wire connected to it on a 88 (not red regardless), the motor should stumble.
If it does the problem is not vacuum or EGR related and is likely a EVP issue.
Alright, I will check all this out. I have a vacuum gauge but I do not have a vacuum pump so I cannot manually mess with the EGR.
I did have the valve off about a month ago or something and it moved freely and was virtually clean of any obstructive buildup. I put it all back together and it was fine until the last week or so and now im here.
So danr1, ground out the positive (computer) side of the EVR solenoid and it should stumble?
So danr1, ground out the positive (computer) side of the EVR solenoid and it should stumble?
No jumper a ground wire to the computer side of the EVR, the dark green wire. (the computer supplies the ground to open it)
Red wire is key "on" constant 12v+ power to the EVR.
Just take a length of wire and run it from the neg side of the battery and touch it to the terminal with the dark green wire connected to it with the motor idling, the motor should stumble.
Note: My book shows the wire as "dark green" on a 88, but it has been wrong from time to time as far as wire color.
Regardless it's the one that is not "red" whatever the color turns out to be on yours.
Sorry to revive a dead thread or hijack here, but I've been having this EXACT same problem with my '88 and was wondering if there was any resolution here. I've replaced the EGR, cleaned the EGR tube (really was not that dirty), and replaced the EVP. I tried running a jumper wire to the PCM wire on the EVR and it caused the truck to stumble. So, I guess maybe I should look at the EVP I just replaced? I'm kind of lost!