Car in question is our 86 Firebird.
305-4bbl motor.
Thought I had a defective EGR so I picked one up from Dal. The new EGR valve has the same symptom. When I supply manifold vacuum to the valve it doesn't open. When I supply the same vacuum source to the old EGR valve in my hand the plunger moves. Same thing happens with the old valve on the motor and the new one in my hand.
Seems like the vacuum can't overcome some kind of resistance caused by the running motor. Car is in very good tune. Not a beater. Never seen a winter etc.
Any ideas or thing to look for? I thought the intake might be plugged in the EGR passage. Maybe I have a vaccum leak between the EGR passage and an intake port.

Thanks

Scott



------------------
************************************************** 1986 Firebird.
LG4 w/1985 TPI exhaust system including stock TPI exhaust manifolds.
Derale oil cooler, trans cooler, self-installed 3.73's & Torsen posi, Vette servo,hollowed cat, delaminating clearcoat. Have factory manual if anyone needs help.
'96 Impala SS - Black,Tint,Kill Switch, Just Purchased the Car
'93 Caprice Wagon
**************************************************

 

Look at the old EGR valve and see if it has a "N" (negative)or a "P" (positive) as the last digit in the part number.

If it has a N it should open while engine is running when applied vac and should make the engine stumble.

If it has a P than it wont let the EGR function, even with vac applied, until the ex. has enough backpressure. You can usually get it to work if someone covers the ex. tailpie and sits there for a few minutes.

------------------
ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R
and 2 POS commuters...

R.I.P. #3
"He can see air"

[This message has been edited by Engineboy (edited October 24, 2001).]

 

In essence, you have one of these, which may not be the same as the original valve:







------------------
Later,
Vader
------------------
If you want to beat the World, it might reach up and pull you down...
Adobe Acrobat Reader

 

Thanks Vader, you always have the cool diagrams....

------------------
ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R
and 2 POS commuters...

R.I.P. #3
"He can see air"

 

The Positive type valves are good for rigs that stumble from the egr opening too soon. I've had this problem on a couple of chevy trucks. With the P egr valve the valve wont open unless theres enough exhaust back pressure. Which pretty much guarantees the motor has to be revved up first.

 

You've got 'em, too. But I'll bet you didn't have your Helms manuals with you...

------------------
Later,
Vader
------------------
If you want to beat the World, it might reach up and pull you down...
Adobe Acrobat Reader

 

Yeh, all the cool manuals are at the shop not here....



------------------
ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R
and 2 POS commuters...

R.I.P. #3
"He can see air"

 

I forgot to add that I have a complete factory TPI exhaust including manifolds on the motor. Also, the cat is gutted.

Would all that reduce the backpressure too much. Should I get the other negative valve?

Thanks,

scott

 

Scott,

Get the original EGR valve for your engine. Your valve has (and NEEDS) the tapped port in the base for the diagnostic temperature sensor. I believe they are a negative backpressure valve, but it doesn't matter since the GM parts counter will have the right valve anyway.

------------------
Later,
Vader
------------------
If you want to beat the World, it might reach up and pull you down...
Adobe Acrobat Reader

 

I'm bringing this thread back from the dead because I am having the same problem.

I have 1991 305 TBI engine with the "N" EGR valve.

When I rev the engine by hand and watch the valve, nothing happens. It's as if the intake vacuum is holding the valve closed, but I don't understand why. I have this engine installed in a 1956 Chevy, so I have two of the vacuum ports blocked off (the one that would go to the air cleaner valve and the one to the canister purge solenoid). Anyone have an idea why this is happening?