EGR troubles..

I've been having problems with my EGR system on my '88 K2500. 350TBI. this is what I've found out. the EGR valve itself is fine... disconnecting the vacuum line and pluging it into the vent on the EGR
solenoid stops the surging. moving up the line, the EGR solenoid is a brand new NAPA Echlin piece that doesn't leak vacuum when the metripack connector is disconnected. back probing the metripack shows an intermittant spike of up to full system voltage (on my digital VOM... my analog is buried in storage) when you blip the throttle in park. I've not hooked up the MT-2500 to it to see if the P/N switch is registering in park or not.
my question is: other than a bad output transistor in the ECM or a wiring problem, can anything else cause the spike? (which opens the solenoid, which opens up the EGR valve, which causes jerking and stumbling)
TIA, Bret
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Just a thought, could a bad TPS cause this? If there's a bad spot and it suddenly signals full throttle to the ECM (or idle, I can't remember when the EGR opens) maybe that could cause your spike. A shot in the dark but what the hell.
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WOT is when the EGR opens.
GMC Gremlin

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Well, I was clarifying that it shouldn't be open at idle.
GMC Gremlin

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Big difference between should and shouldn't too :-)

Isn't it open at WOT as well?
GMC Gremlin
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wrote:

hooked up, it sounds like I've got about 750HP under the hood w/ the way the idle lopes and stumbles.
-Bret
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Bret A simple way to test egr open or closed at idle: disconnect vacume line between egr controlling solenoid and egr valve(assuming your ride is so equiped) , then run the engine.
If idle has smoothed out, the controlling solenoid would appear faulty.You could prove this out by sticking vacume gage on the downstream side of the solenoid.Should be no vacume. Then, with engine still running, connect a vacume source directly to egr valve, if idle becomes lumpy with vacume applied and smooths out when released, it would prove egr valve is serviceable.
Further, I have proven that the egr is controlled open under most part throttle settings and will close at WOT by connecting (tee in) vac gage between control solenoid and egr valve. If I'm not mistaken, at WOT, ecu control goes open loop therefore closing egr.
If you leave the vac line between the solenoid off , go for a drive, chances are the ecu will set the egr code because map sensor has not sent the correct value signal back to the ecu. Ecu is looking for set values based on CTS, TPS, MAP, Knock and VSS(again assuming all sensors are working properly).
Doc feel free to correct me...
Respectfully submitted Rolf
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Dunno if that was sarcasm or not, but I agree with everything you said. If it's loping w/ EGR attached and removing the vacuum source from EGR stops the loping, the problem is either the EGR solenoid OR the signal being sent to the solenoid. Even if the solenoid is ok, it's still going to open when it shouldn't if the ECM tells it to.
I'd hook up a vacuum gauge directly to the EGR solenoid when it's being symptomatic and see what the hell it's doing at idle. If there is vacuum to the EGR line at idle, I'd then check to see if the ECM was telling it to do so by backprobing the signal wire and checking the voltage.
Doc

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did you guys miss my OP? back probing the metripak gives intermittant voltage spikes when the engine is revved. that's what's opening up the solenoid and causing the lope.
-Bret
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No I read it. What is the voltage at idle? Even if it spikes when revved, it <should> close as soon as the voltage is no longer present.
Does driving the truck with the solenoid electrical connector unplugged make the problem go away?
Doc

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the voltage most of the time at idle is 0V... revving the engine gives a semi-repeatable voltage spike between 6 and 14V. I've not caught it w/ the VOM while the idle is messed up (it usually clears itself before I can get out, get the hood open, unplug the connector and measure it.)
it's acts up the worst when the cruising with a very light foot at low rpms (<1krpms) w/ the connector plugged in)
yes, unplugging the connector makes the problem go away.
the original solenoid leaked about 10 in/mg all the time.. new one doesn't leak. warm idle was 21 in/mg when I last checked it (same time I checked out the old solenoid) I'd go check it out now, but I've spent enough time out in the rain today.
-Bret
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