I have a 96 LT1 that is running rough at idle. When you first start it, it idles fine for about 20 seconds then starts running really rough (definite miss) and trips a P300 (Random Misfire Code). It seems to run fine when going do the road just crappy at idle. I checked for vacuum leaks and did not find any. I am about the throw in the towel on this girl.....she has been one problem after another. All this started when I was chasing O2 sensor problems on the right bank.

 

Have you checked the IAC Idle Air Control valve?

 

Took the TB off and cleaned it using Valvoline TB cleaner. I removed the IAC, cleaned it up real well. Not really sure how to test it.

 

Is there a means of checking the IAC?

 

You can ohm it out with a multimeter. Should be 40-80 ohms between A and B and between C and D. The resistance should be infinite between B and C and between A and D.

A being the lt green and blk wire
B being the lt green and wht wire
C being the lt blu and blk wire
D being the lt blue and wht wire

I hope this helps.

 

Put the car under alot of load in the lower rpm and see if it misses there to.

 

I will test it with my meter tonight.

 

Just out of curiosity, it appears that a wire I thought was suspect (oil temp lead to pan) did actually go south. Oil temp stays low and gauge does not move. Could this contribute to the rough idle problem or am I just being a little to hopeful. It seems that the system is prone to failing based on one sensor going south or off-lilne and I am still learning what is related to what.

 

Thats a hard one for me to awnser. I am an F-body guy and I know there are some differences. But I am going to have to say no it wont cause the idle issue. Its going to be like the oil level sensor. It just pops a hey dummy light if anything. What trouble shooting have you done already? That way I dont direct you at the same things.

 

The more I read, it has the be the IAC. I was incorrect about the sensor.....it is the low oil sensor. I will use my meter tonight to test the IAC.

 

I have replaced the IAC. No improvement as matter fact I have noticed the following:

1. Still running rough

2. I can hear the EVAP motor spin up from time to time when starting but not every time I start the car.

3. About the time the EVAP stops making "noise" the idle really goes to the dogs

4. It takes a little longer to start than normal

5. There is now a definate miss

6. And she is running hot - boiling over (suppose this means we are running lean - right?)


I have sprayed carb cleaner around everything and can not find an vacumn leak. The miss is inconsistant or so it seams. Anyone with any ideas would be deeply appreciated.

 

Now that you replaced it reset it computer. Then reset the IAC. You should not be over heating from an IAC change.

 

Always check the easy stuff to give you ease of mind.
Like
Are the Plugs fouled?
Do you have the right plugs?
Is there a Spark plug wire burnt, arcing?
MAF is it damaged?
Check the tips of your exhaust, is there black residue?
Try to find suspected vacuum areas and test them by removing while car is running to see if there is a change.

Just my input.

 

Found the problem......the EGR was stuck! She is much happier now.

 

Good. I am glad you got that figured out.

 

Hi,
I have a 96 corvette...and having the same problem with your car. But I had change a new EGR vale but it doesn't fix the problem....It still running rough idle....Is there any advise you can give me? Thanks

 

What trouble shooting have you done? Got get some info up.

 

My 95 has similar symptoms suddenly--idles rough after warming up but seems to drive fine otherwise. How did you determine the egr was stuck--what do you look at or test.

Also, what is the EVAP motor?

Thanks,

Doug

 

My "amateur" trouble shooting (aka guessing) procedure:

1. Spent hours with my best friend GOOGLE researching the problem.
2. Wore the search engine out on this forum. Read every post remotely related to my problem.
3. Sprayed everything down with Carb cleaner searching for vacuum leaks. Actually found a couple of hoses that were not leaking but needed to be replaced.
4. Cleaned the throttle body and replaced the IAC. No improvement.
5. Eliminated an individual cylinder problem by unplugging one plug wire at time.
6. Did the same test with injector leads for each cylinder.

** In my case I presumed it was not a fuel delivery problem as the she did not starve for fuel when driving. May be faulty logic but it worked for me. Therefore, I did not check the fuel pump (other than to hear it wind up) nor the fuel pressure in the fuel rail.

While drinking XX Guinness and ponding the problem I decided to start the car a few times and just listen. I noticed that the EVAP system would run for a few seconds at start up and the vehicle seemed to fun fine until it stopped. The idle would go to the dogs at this point. That just happened to remind me of a similar problem I had on a C1500 which I had bent the EGR valve while working on it. So...

7. I applied direct vacuum to the EGR from the manifold power brake port. Observed no movement. Manipulated the diaphragm with a screw driver and noticed it worsened.
8. Removed the EGR valve and confirmed that it was sticking. I cleaned it with Valvoline Throttle Body Cleaner till it moved smoothly and then reinstalled it. The car ran fine afterward.


Regarding the EVAP question:

Evaporative emissions from the fuel system (fuel vapors) are trapped and stored in a charcoal canister. Later, a purge valve opens allowing the vapors to be sucked into the engine and reburned. The EVAP system usually requires no maintenance. The fuel filler cap is also part of the EVAP system, and is designed to keep fuel vapors from escaping into the atmosphere. A leaky or missing fuel filler cap may cause a vehicle to fail an emissions test. Also check out this link:

http://www.search-autoparts.com/sear...58/article.pdf

 

With the engine off, I was looking and feeling at the EGR valve to see what I could move. I was able to push on something and I could push it in and it would spring back. I assume this is the diaphragm. So if it is able to move as described, does that eliminate the EGR?

Also, I understand that the canister in my 95 is located behind the passenger side wheel well. Where did you "listen" to be able to hear something running?

Thanks,

Doug