I have a 91 Firebird 5.7 TPI, ever since i put in some new injectors i've had an off idle stumble. I replaced the EGR valve with a new one from Auto Advance so i wouldn't have to take things apart later. After checking out other possible causes i've narrowed it down to the EGR system. The reason i say this is if i disconnect the EGR vavle it accelerates smoothly and if plugged back in it immediately resumes the stumble. At first i though it might be the EGR solenoid but i put a vacuum gauge on the port going to the EGR valve when warmed up it and it goes from 0-18in as the throttle is opened, which seems normal. I reconnected the EGR valve back up and put a vacuum gauge on the plenum and watch as i gave it just a little throttle and the vacuum dropped from 19.5 to 11-12 in. very quickly (just off idle) when the EGR should be opening. With the EGR valve disonnected and the port on the solenoid plugged i repeated the test and the vacuum held pretty steady. Seems like maybe a leak at the valve? The thing is brand new. I did test it when it was off the motor using a vacuum line while my truck was running and it did open and close. I hate to get a $60 part i don't need but that's my first guess. Any info appreciated.

 

Is it one of those universal ones with the calibration washers? If it is, put the old one back on.

 

It's not universal, and the old one is gone.

 

anyone else?

 

When you bought the new EGR valve, did they ask you if it was a negative or positive back pressure valve? You might have replaced the valve with the wrong type.

Some people will say it doesn't matter what type it is. If it didn't matter, why would OEM and the aftermarket have both types?

 

No, they didn't ask. I'm starting to think they gave me the wrong part, when i put the plenum back on it was touching the top if the valve which seemed odd. It didn't think it should be so tight. I have another one coming on Monday, we'll see how that goes.

 

I've heard some people found that the hoses had a small leak. One suggestion made is to let loose some propane near the valve. If a hose has a leak, the propane will suck into the hose and cause the idle to jump some. Not sure how good this would work though.

Also, I tried replacing my EGR valve the other day. I took the intake plenum off but the two bolts holding the EGR valve down to the manifold are very hard to reach. I could only get a small box wrench in and the screws were on way too tight. Looks like you might have fully remove the runners and fuel rail. What did you remove?

 

I am able to get mine off with a small box wrench. The first time i removed it all since i was replacing the injectors. Subsequently i've just had to remove the plenum. I replaced the vacuum lines even though the hard lines seemed ok, that didn't make any difference. I pull pretty decent vacuum at idle (19-19.5 in.) so i don't think i have a leak.

 

i had the same idle problem after replacing mine. The newer valves are too tall, and cause the vacuum hose to pinch and leak. in this case, OEM is the way to go. Unfortunately, OE gets pricy for ours, 130 in my case. just got lucky and found a good one at the junkyard.

 

Well, it looks like the problem is resolved. I got another EGR valve from AutoZone, threw it on and no more hesitation. I believe Advance auto gave me the wrong part.