I have an idle problem and I am a novice at this at best when it comes to working on cars. Every new problem I encounter, I learn something about working on cars-LT1's in specific. So, please dummy down your replies. I have never worked on/tried to find a vacuum leak.

My problem has been, I have researched and found possible causes, but not really any good instruction on how to check those causes, step by step.

I think it's some kind of vacuum leak since it runs fine as soon as I give it gas. The car is 100% bone stock...no aftermarket cam or anything (I have had the car over a year). I have no idea if it makes a difference, but my cruise control works fine and I know vacuum lines have something to do with that.

1.) My car idles rough only once my engine is fully warmed up. Does that give any clue as to where the vacuum leak would be? As opposed to idling roughly at all temps or only when cold. I pressed the EGR diaphram with my thumb and it does seem to spring back and forth okay. Does that mean it's not stuck? Just trying to eliminated places where it is least likely to be.

2.) Is there a vacuum line diagram or a place that has good pictures of EVERY SINGLE vacuum line? I Googled it, searched the forum, but that is one thing that is pretty thin. A picture is worth a 1000 words. Also, there seems to be a lot of variation on where these lines are and how they look year to year on the LT1/LT4's in Fbodies and 'vettes.

3.) I do have a vacuum kit I used to bleed my brakes. Can I use that same thing to check for vacuum leaks? How do I know I have located the leak? Logically, I would assume when I found a place that DOES hold pressure where I have the vacuum gauge hooked up, that must be where the leak is coming from since, if the leak where elsewhere, it wouldn't hold pressure, right? Am I thinking that through right?

 

Hi Stew,

Did you ever receive any help with this question? Did you solve it? I have also been looking for a vacuum line diagram to no avail.

I also own a '94 Vette and have had the same problem ... the car runs fine when cold, but the car stumbles a lot at idle when it's warm. I have owned the car for about a year and a half. It also is bone stock and I intend to keep it that way.

FYI, from my research, the following items can also cause a rough idle:

1. Clogged injector
2. Clogged or failed Idle Air Control (IAC) valve
3. Issues with the Optispark.

I've put in new injectors, checked and cleaned the IAC and put a whole new ignition system in the car. Each of those has helped the car run better, including at idle. It's a lot better than it was, and cleaning the IAC gave me a much smoother idle. However, the car still stumbles a little at idle.

I used a scan tool on the car today and from some of the readings, it looks like I have a vacuum leak.

 

This may be off-base, but another (simple) item to check are the plug-wires. Check the manual for the routing of the wires - i.e. #2 & #3 usually extend around the front, away from #7 & #1. If they're making contact they can be causing cross-fire interference, hence rough engine fire. Just a thought. It's a quick check and eliminates one possibility.