mlm0

1989, automatic, stock, 50,000 miles

At idle, is it possible for egr valve to not closed completely or not seat properly causing a rough idle without throwing a code. I know egr is not getting any vaccaum at idle, which is correct. With the engine running, I can apply vaccaum and cause the engine to idle worse

I have had the car three years now and it continues to have this same problem. It does not stall or die, and runs great other then idle correctly I have replaced EVERYTHING. I am down to maybe a leaking intake or exhaust valve or egr problem. I had injectors, egr, FPR replaced just after I bought car 3 years ago, but that didn't fix problem. I don't know if the shop checked the manifold for blockage. I set idle a little higher by programming chip and it helped some I have thrown enough money at it, and now need to get serious. Could it be a bad distributer? I replaced all normal tune up parts, and I have a replacement used distributer, that I could replace it with if I need to

Any thoughts?

beachfreak

I'm following this thread because I have a similar idle issue on my 94. I realize it's an l98, not lt1, but always wanting to learn.

DanielRicany

Yes, it can be leaking into the intake without setting a code. The code is set when the ECM commands the EGR solenoid to open and it does not see an increase in temperature on the MAT sensor.

aklim

Before we throw parts at it, start with the neglected. Have you taken the IAC off, cleaned the pintle with a gentle spray of brake cleaner? Have you taken the throttle body off, removed the IAC housing and cleaned the housing? Have you cleaned all the throttle body passages off? After all that, install with new gaskets.

Are you sure there are no hose leaks? ASSUMING that when you replaced the EGR and fuel injectors, you also replaced the hoses, can you be sure there are no leaks?

Scan the car and see what the IAC counts are. It should idle about 20-30 counts. This is with motor only running and no ascessories. Take each hose off at the manifold and the IAC counts should drop to 0 when you do and when you plug it up with your thumb, it should go back to 20-30 counts.

If you can't even get that target count, I would suspect a leak somewhere.

This is ASSUMING you set the IAC by counts and not using the FSM method which is invalid since you have changed command idle.

mlm0

I have already done all that you suggested many times, using my mt2500 scanner. Cleaned and replaced gaskets on TB, replace AIC several times, adjusted min idle many times, both by FSM and with use of scanner. Checked for vacuum leaks every way you can think of. The one thing I do need to do is disconnect AIC and then disconnect one plug wire at a time to see if it makes a difference, but it is not missing in a constant pattern so I don't know if that will help

Thanks for your suggestions

aklim

So how many IAC counts do you get with all accessories off and motor running at operating temp? Are you able to hold about 20-30 counts? Stable is when you rev it and it settles to about 20-30 counts. Before I do that, I want to make sure the harmonic balancer is ok and my timing is spot on.

mlm0

Getting about 22 counts and holds steady. Didn't try it to see if it always returns to same counts. I timed it to 6 degrees BTC, but it does seem to wander just a tiny amount when I watch it with timing light, but thought maybe it's just because of the miss. Is there a way to eliminate a leaking valve, such as one with weak or broken spring. I'm not very experienced at engine repair so I only know what I read and learn on this forum. The miss is not so bad that it affects drivability, but it bugs me to the point I put car in neutral at stop lights. Car uses no oil and has only 50,000 miles

desertmike1

What were you saying: I couldn't pay attention!!

desertmike1

Yes, an EGR can leak Exhaust gas's (Commanded) into the intake, if it does the EGR supply tube should heat up over time..

aklim

If it is leaking, I would think that the IAC counts will be lower since the ECM isn't commanding the IAC to open as much to allow more air to get the desired RPM. Is your TPS now set at 0.54V?

Who programmed the chip and what have they told you?

mlm0

TPS is right on the money. .54 volts

aklim

Have you asked the programmer what is going on. Seeing as how he wrote the program, he might have an idea what is going on.

mlm0

I did it. All I changed was fan temp setting and idle speed. It has been missing long before I changed these settings

aklim

So you have recently checked fuel pressure even at WOT and made sure it holds pressure after shutdown? If the fuel is good and the spark is good with a blue flame and not erratic we might have to ask about the size of the injectors.

jv9999

What's the manifold vacuum at idle? Most hard to diagnose TPI problems that I've seen have turned out to be vacuum leaks.

If you want to rule out a sticky EGR, just make a sheet metal plate to install between the exhaust manifold and the EGR pipe.

mlm0

I keep thinking vacuum leak also but can't fine one. Also if vacuum leak, wouldn't it raise idle. My problem seems to be idle drops a little lower when it misses

aklim

Not necessarily. The ECM will try to compensate to keep the commanded idle till it can do it no more.

mlm0

]Vacuum is 17 and holds steady mostly, but does fluctuate in sync with miss

beachfreak

Here is an article I found that is a pretty helpful write up on C4 EGR's

http://tech.corvettecentral.com/2012...egr-diagnosis/

I had my engine idling, and used my hand to move the EGR diaphragm, it definitely affected the idle. Does that mean it is working properly and I should look elsewhere for causes?

(I don't mean to hijack thread)

jv9999

Forcing the EGR open will cause the idle to falter. Doesn't necessarily mean it's not leaking a little.

I was helping a friend with a similar problem a while back. We tried the propane and car cleaner trick to no avail. I finally sprayed water on the intake and you could hear it sucking around the old dry rubber brake booster connector.

17" is low, it should be ~20" and steady. I assume you've checked your static timing?