Gear Jammer

Hey gang,

Rough/choppy idle on my 91 L98 auto, no lights on the DIC, car runs good once under way, bad/choppy/rough idle.

Im confident I dont have a major vacuum leak (new intake plenum gaskets with sealer, overkill I know).

Heres all Ive done:
New plugs (replaced twice)
new wires (Magnacore)
New cap and rotor
New O2
Cleaned throttle body bores and IAC bores
New IAC
New Ignition Module
New Ignition Coil
Cleaned and retorqued ECM grounds on the engine
R&R intake manifold (to eliminate rear china wall oil leak), made sure to scrub egr passways clean
New Bosch injectors from FIC
Fuel filter

Checked and double checked sensors for proper operation as best I can without a Tech-1 (MAP, throttle position, etc).

Ive troubleshot the EGR code 32 using the FSM procedures, system checks ok according to the procedure. FSM implies that a vacuum drop is expected when egine is started, but I dont see how this is normal operation.


Now heres where the EGR question comes in. When I bought this car, the DIC had a code 32 set (EGR). There was a new EGR valve in the box that came with the car but no info as to where this valve came from (GM? autozone?, etc?). So the first time I had the upper plenum off, I R&R the EGR valve. Unfortunately, I cant remember if thats when the bad idle showed up. I ve since replaced just about every other sensor and plugs (again) in an effort to get a good idle.

So, with the engine off, the EGR valve will hold a vacuum (with my Miteyvac) and I can see the diagram moving when I apply vacuum. Now with the engine running, I cant get the EGR valve to hold a vacuum?! I think this might be the root of my problem. To further isolate the EGR, I blocked the exhaust passageway (where it T's off the manifold - with a dime). Now it may be subjective, but I think my idle might have smoothed out just a tad. Another odd thing with the egr blocked, a trouble code doesnt always get set even tho I have plenty of constant speed highway as I drive around (in other words, should meet conditions for the computer to apply EGR).

I also did some research and compared the part number markings on my EGR valves:

Old one I took off had P/N 17090106 - this is a '90 - '91 production unit. Makes sense, original valve from factory.

New one I installed has P/N 17086078 -parts books indicates this P/N is for '86 - '89 applicability.

GM Corvette Parts book shows GM 17113456 for a 91 L98, which crosses to AC-Delco 214-5535, which spans all years '86 - '91. I think its safe to assume that the replacement EGR is a proper replacement part.

So, to wit:
Is it normal for the EGR not hold a vacuum with engine running, even though it does hold a vacuum with engine off?
Due to blocked exhaust passway, I would expect a code 32 everytime I get on the highway for awhile, more often than not, no code is set.

I also know a L98 idle isnt going to be glass smooth...Im puzzzled


john

kenmohr

[QUOTE=


I also know a L98 idle isnt going to be glass smooth...Im puzzzled


john[/QUOTE]





Had the same thing going on with my 89. Decided to change the sensor circled and the top slid out of the sensor. Haven't had a code since. The part is not expensive.

AirForceOne!

Have you checked the fuel pressure yet or replaced the fuel filter?

leesvet

Theres more to it than just the EGR vale as others have indicated !

that time/temp switch is a biggie...out of production for a while. Its now available again and should be looked at.

the EGR solenoid is the other. If thats not functional or the vac supply is not from "port" vacuum, it can cause odd EGR events.

The egr valve acts strange because these are neg back pressure valves...this is hard to explain so I'll just say that it helps the valve respond and react to close faster when the signal from the solenoid is stopped according to what I've read. It also makes them seem to be faulty when they are not. The port vac is part of this so the proper vac source is critical.

TA

91s don't have that part

Gear Jammer

Good catch - yes, one of the first things I did was to replace fuel filter and check the pressure. Pressure checked out fine, even tho its been coupla years since I checked it.

Heres an aside:
About a month ago I decided to re-check the pressure, so I rented a fuel pressure gauge kit from Autozone. When I started to use it, I realized somebosy prior to me had really effed the thing up, the orings were broken on the fittings, most of the fittings had stripped threads. So I took it back and decided to buy a harbor freight fuel pressure checker kit, talk a worthless POS. Leaked like a seive, I was so pissed I didnt even bother to take it back, straight to the trash.

I guess I need to try again, I would like to rule out the fuel pressure, even tho Im pretty sure its ok.

gmboileau

"So, to wit:
Is it normal for the EGR not hold a vacuum with engine running, even though it does hold a vacuum with engine off?"



This is perfectly normal for a negative back pressure EGR. When engine vacuum is high as it is at idle the the EGR stays closed. A test of this EGR is to open the valve by applying vacuum when the engine is stopped and watch as you start the engine... the EGR should immediately close.

leesvet

Thank you for expaining that....I knew the reason, I just couldn't describe how..

Gear Jammer

well, drat, I kinda thought that was how it was supposed to work, guess I need to re-assess what could be causing my bad idle.

thanks for the info.....
john

mkrail@flash.net

Quick question in regards to EGRs. If the EGR is slow to close after coming from cruse to idle will it introduce more air into the system causing a high idle??? I have been fighting a high idleing problem for sometime now and was wondering if that could be my issue.

I had an EGR code in the pass that went away so I assumed all was good.

THX