Obviously, anything is possible but I think I might have a shot EGR valve.

Any time the motor is warm enough to activate the EGR valve you can step on the gas and it goes good for a second and then gets "flat" and slowly recovers. Not a stumble or anything major- just a smidge sluggish. Unplug the EGR valve and it pulls evenly with no "flat spot."

4.3L TBI V6 in my 91 Jimmy. The motor is fuel injected and I have jsut gone through a long period of fixing various small problems with the motor- vacuum leaks, bad sensors, etc. Everything has made it run better and better. I am convinced there are no other physical or sensor-related problems left with the motor. I just can't get rid of this slight flat spot. My brother has the same engine in his Blazer and reports the same kind of behavior.

The EGR "fix" just seems too damned convenient. Diacom says the EGR vacuum solenoid is operating properly and at the correct duty cycle. Our tests of the vacuum delivered to the EGR valve confirm this as well. Only leaves one thing I can see- the EGR valve is opening too fast, too far and letting in too much EGR gas. Sounds weird, I know, but that's what it seems like to me.

I'm hoping someone will say "Ah hah! I had the same problem! The fix is.............."

 

and have to be cleaned .. or in some way they don't open due to vacuum problems.

 

Welp; EGR should only activate during low-throttle, high speed situations- an example of this would be cruising at 55mph on the highway.

My EGR is busted, though. I thought it was my brakes! One day, I slammed on the brakes (a moron rolled a stop sign in front of me)- and my car stumbled & stalled & wouldn't start. I got it to start by pressing on the gas... and it would die as soon as I let off. If the car sat, it would sometimes fix itself, otherwise, I'd be playing the gas & brake at the same time to get where I was going. I thought my brake booster was shot, and I had a huge vacuum leak.

One day, as I had just limped home, and was staring at the barely-running engine, I realized "EGR!" I felt the diaphragm underneath and, sure enough, it was stuck "up". Turned out it wasn't me hitting the brakes that was killing the car- it was the inability of the EGR to close when the car cut the vacuum signal off.

Right now the EGR is shut, and a golf tee is plugging the vacuum line so I don't get a SES light. Inspection's coming up this month, so I've gotta swap it out. Maybe you can plug your EGR vacuum line (if it's not a digital unit) with a golf tee and see how the engine works.

How's the coolant temp sensor? Is the catalytic convertor getting clogged?

 

I thought about a clogged cat. Don't think that's the problem. Did the old "vacuum at idle versus 2000 RPM" test and it checks out.

CTS is telling the truth (Diacom verified).

The EGR diaphragm is getting all the way closed (it has no problems idling smoothly) but I can push it open with very slight pressure from a single finger. Testing another EGR valve I have laying around in my garage (from a different engine) the spring is MUCH stiffer than that even though the diaphragm is the same size.

Just seems like it is getting TOO MUCH EGR when it kicks in. When the valve is not in use the engine runs just fine (idle, WOT).

 

But as I was reading about your prob damon, I came acrossed Tom's comment about braking hard.. I was cruisin along behind a vette yesterday (a new V06) when the idiot decided to turn into a lot without signal warning.. my 4 disc brakes got me stopped in time but two lights later and my car starts idling real rough.. the tach was bouncing around from 6-750 or so. As I push on the throttle (while holding my brakes; stopped at a stop light, i was worried the car would die) the problem either subsides or gets burried. I pulled into a parking lot and turned the car off.. Turned it on a few minutes later, still idling rough. I go back out to the car several hours later, it was cold all evening, and the car is almost back to running normally.. like after the engine cooled off the problem went away on its own.

That situation made me think my EGR was probably dirty, or stuck like Tom was describing. I was in a high-speed (about 65), low throttle situation... and after letting my car sit the problem went away-

So my question Tom, is that assuming when i slammed on my brakes, I either plugged or jammed my EGR, how do I go about fixing it? I think I need to give it some attention since i'm worried that the problem will happen again if I'm in a similar situation.

Thanks in Advance,

James

 

Well, if it is the EGR, it wasn't the brakes that plugged/jammed it, it's either years of carbon or years of rust. If it's the EGR, it's just a wild coincidince that it decided to happen then. Unfortunately, I think you'll have to wait for it to happen again to diagnose it- the car will have to engage the EGR again (high speed, low rpm, cruising). The good news is coming to a "normal" stop will make the computer disengage the valve- and if it gets stuck, limp into a parking lot, or to the shoulder, and pop the hood. Feel underneath the EGR with the engine running (don't get you or your clothing caught on the motor, of course). If the diaphrahm is raised up "into" the valve, the EGR's stuck open. Replacement is a few rusted-to-hell bolts. What year is your car? The 80's F-bodies had vacuum operated EGR's ($40/valve), the 90-92's had digital ones (more expensive).

And Damon, yeah, I don't think you should be able to open -any- EGR valve with slight finger pressure. Maybe, once the EGR activates, and the computer tries to de-activate it, the weak spring can't slam the valve shut? I thought maybe your EGR was sticking open a bit all the time; I misread your question the first time.

[edit] Oh & James, by the way... my car stalled the same way a while before my "slammed on the brakes episode". I had figured ignition module, since when the car cooled, it worked fine again. What I found odd though was that it didn't happen again for 6 or 7 months. After diagnosing the EGR as bad, I wonder if my guess at "ignition module" was incorrect, and it was really the EGR. You might have to wait quite a while for the car to act up again- unless, of course, it's your ignition module!

 

The vacuum diaphram(i think there's a g in there somewhere) can waer out and leak vacuum.

 

No, it holds vacuum just fine. It seems like the spring inside it is getting weak. I can start to open the valve with so little vacuum that the needle on my vacuum gague doesn't even come off the Zero Peg! Most EGR valves I have seen take at least 1-2" of vacuum to start to open, 3-4" to fully open. I guess I'll jsut replace it and hope for the best.

I can see an EGR acting up after a 60+ MPH panic stop due to the high levels of manifold vacuum that might build up after the throttle plates are slammed quickly shut from elevated RPMs. Might be the "last straw" to an old crudded-up EGR valve. I have come to beleive that there are no coincidences. Just unknown causes.