I'v been trying to de bug the stumble in the Vette now for a month
I don't need to go over what I'v replaced...but I will list it at the bottom. The stumble happens at 1100 rpm once the motor warms up

Last night I put in the new ICM,, and the stumble is still there this morning

Well; needless to say...I'm down to my last two ideas...IAC or EGR vavle

The IAC is showing a lower then normal voltage ...but the problem of stumble only shows up after the motor hits 138d-140d when its warming up...before that the moter is fine...after 140d shows on the read out...bam...stumble city...which tells me it is a temp related problem....

I'm also starting to think that my EGR valve is carboned up and is not feeding the right amount of exhaust gas back into the motor...maybe too much at low RPMS...which would lead to a stumble...what do you think?

Put you thinking caps on for this one
Repalced parts:
Opti
Plugs
Wires
2 O2 sensors
Temp sensor
Fuel filter
Coil
ICM

I replaced the Opti, o2 sensors and ICM, because they threw a code

 

my 87 had the same stumble around 1400rpm. disconnected and pluggged the vacuum line from the egr and the stumble dissappeared.

will change the egr system in the winter.

Paul
87 coupe

 

Oh..theres an idea

 

You don't say what year you're working on. Your screen name suggests a C3, but the fact that you replaced the opti means it must be a '92 through '96. I don't know what an ICM is. Is that an item only used on the later LTx engines?

You can pretty well forget about the IAC. All it does is follow the commands of the ECM to regulate the idle speed. The EGR IS a real possibility. The EGR isn't activated until the engine is warm, so that fits. On mine, the return spring weakened with age. The normal amount of vacuum would open the valve too far and cause a lean misfire. A good test, is to make sure it seals well when closed, then disconnect and plug the vacuum line to the EGR and see if you still have the miss.

The codes you received that caused the replacement of all those parts didn't necessarily mean that the sensors were bad. For instance, the fuel mixture could have gone lean, and probably did, as evidenced by your stumble. More than likely that would cause the O2 to set a code for a lean condition. The problem, at least in this example, isn't a defective O2, but whatever caused the lean mixture in the first place. The O2 is only guilty of communicating the true facts, as designed. Don't shoot the messenger. The trouble shooting charts in the Helms has you eliminate the sensor as a cause and test components before replacement. Proper diagnosis can save a lot of time and money. Notice, I didn't just relate my problem with my EGR and recommend replacement. I recommend a TEST of the EGR. As a matter of fact, I found a "quick fix" and still have the 23 year old EGR.

RACE ON!!!

 

...some people will get nauseous at this procedure, but this is how I check to see if my EGR is leaking:

1) remove it from manifold
2) clean off the black carbon from the protruding plunger area
3) try to blow air through it. (no air should pass if EGR is OK)
4) wipe any residual carbon from your lips.....

-No air should pass through this port
-If you can blow any amount of air through it, it is not sealing as it should when closed. Could be carbon buildup preventing good seal. Clean or Replace.
-This port should only open up and allow exhaust gases to re-enter manifold during heavy acceleration(this is when there should be enough vaccum for the actuator to retract the pintle from the opening to allow exhaust gases to be re-burned.)
-Remeber, if your EGR does not have a good seal when it is in "closed" position, disconnecting the vaccum line won't make a difference......
-An EGR valve that doesn't seal properly will allow exhaust gases back into the intake at all RPM's, and poor performance will be most noticable at idle..

 

Seems more often than not when it's EGR related it turns out to be the EGR switch.A cheap part.

 

Sorry...I didn't know everyone was not following my soap opera

I had a couple of posts before this one dealing with the LT-1 motor being a little drama queen

1994 Coupe LT-1 141,000 auto trans, well taken care of, never been over heated.

Stumbles at 1100 rpm when the motor warms up past 140d on the readout. has a slight loss of power and stumbly at 1800 rpm when under load

 

Where might/would it be located on a "94 LT-1?

 

Is he talking about the LT1's EGR solenoid? It's located on the rear of the engine, it's pretty easy to locate with the plastic fuel rail covers removed.

 

I don't believe the '92 has an EGR Temp Sw like the '85-'89 L98s. In '90, with the switch from MAF to SD intake air flow sensing, the EGR Temp Sw was eliminated from the L98s and the EGR loop was closed by sensing manifold pressure change.

Since the stumble only starts after coolant temp reaches around 140deg F we know that it begins when the ECM goes into closed loop mode. If you have access to a scan tool you may be able to see what parameters are out of line.

Or you can simply replace the suspected faulty EGR valve. Of course by now you have checked all vacuum lines for cracks or broken parts.

 

BASSACKWARDS!

During heavy acceleration, the manifold vacuum drops. There is much less vacuum at advanced throttle settings than at a steady cruise. During WOT, the heaviest acceleration, the ECM blocks the vacuum (which is zero or close) from reaching the EGR valve. There is no EGR at WOT.

RACE ON!!!

 

There are NO vacuum leaks

 

Reread the second paragraph in my post #4. Your symptoms, as you describe them, fit this situation, perfectly.

RACE ON!!!

 

"You can pretty well forget about the IAC. All it does is follow the commands of the ECM to regulate the idle speed. The EGR IS a real possibility. The EGR isn't activated until the engine is warm, so that fits. On mine, the return spring weakened with age. The normal amount of vacuum would open the valve too far and cause a lean misfire. A good test, is to make sure it seals well when closed, then disconnect and plug the vacuum line to the EGR and see if you still have the miss."

Yes..I thought that sounded like mine the moment I read it

I'll play with it tonight and let you know what happens

 

Just checkin' to see if folks are paying attention!! ....My Bad....

CFI-EFI
I always thought that during moderate to heavy accleration, that this is the time when chamber combustion is at it's worst efficiency, and thusly, the EGR re-directs some of this exhaust(containing some unburnt fuel molecules) back into the intake?? Man, please set my BASS straight !!

 

OK..I just pulled the egr vavle off
LOTS OF CARBON!!!...pluss it won't hold a vacuum and I'm able to push the plunger back with my finger...I guess its used up

I reinstalled it and plugged up the vacuum line to it...we'll see what happens in the AM..I'm praying very hard that the stumble dissapears

 

Still Bassackwards. It is the efficiency of the combustion that the EGR is designed to kill. By introducing inert gases, combustion temperatures are REDUCED. We all know that an engine is "heat machine". The more energy from the gasoline that we can convert to heat, and harness, the more power we have.

The EGR valve is vacuum actuated. The wider the throttle is opened, the lower the vacuum signal available, to pull the EGR open. At WOT, the EGR blocks what is left of the vacuum, which is virtually zero, from reaching the EGR valve.

RACE ON!!!

 

whether you have a good or bad EGR, you should be able to push the plunger in with your finger. What is important is that when the plunger is closed(ie at rest, with no vacuum), no air should move through the round cylinder.

Your Friend,
Bassackwards

 

I can't add anything to help your situation, 69Mako, but I gotta admire MikeC4's humor and humility! Then again, Mike, didn't you just go thru some major diagnostic issues with your car? Hope it's solved. Both of you.

 

If you can push the egr valve diaphragm and it moves up and down,its usually OK.If its stuck up or down and wont budge its usually broken.

Sometimes a simple test can tell you if its the egr...get some rubber vacuum caps and block off the egr and the vacuum line.Then let the car run up to those temps and see what happens.If it runs alot better theres a good bet the egr isnt working right.However,keep in mind when u block the egr off and if you plan to drive drive the car and go part throttle it can ping big time.