Ok, here's the skinny. Yesterday morning i got in my car to go to work. I started it up and it revved to like 4-4.5k RPM and then it just dropped back down and died . I thought maybe i had my foot on the gas pedal a little on accident while i tried starting. I tried again and the same thing happened.
On the third try, i feathered the gas pedal a little to keep it running and i backed into the street and it died again. On the 4th try, i was able to take off and it did fine the whole way to work. 9 hours later at quitting time, the same thing happened. When i managed to get going, it did fine. This morning, the same thing, only it didn't really get better on my way to work. It kept idling high in between gears and when i'd come to a red light, it'd idle around 2K. When i got to work (8 miles away from home) and i pulled into the parking spot, it finally dropped to around 7-800 rpm, but it kinda sounded like it had a little bit of a miss. Any ideas as to what it could be? This is the first time it's happened with me.

By the way, it's a 99 SS M6 with about 125k miles on it with full exhaust (long tubes, custom y, magnaflow w/no cats). I also have the EGR and AIR removed. It currently doesn't have any codes thrown, but occasionally it'll throw some O2 codes, so i'm thinking those will need to be changed pretty soon.

 

Also, i'm sure the spark plugs are due to be changes (close to 60K miles on them) but could that cause it to act that way? I could see the miss due to the plugs being old, but the high idle seems like a vacuum leak maybe. I've got the SLP cold flo pack with the smooth bellow on it, so i'm not sure where a vacuum leak could come from.

 

i 'm just asking you to check on the back of the intake manifold near the firewall. the plug in sensor , after engine work a wire broke and threw off my manifold pressure. i was throwing a code plus had about 1/2 power. 2nd as silly as it sounds temporarily remove the air filter and see if it runs better. just my worthless 2 cents. good luck ps let us know any recent engine work you've done.

 

If the trip odo is resetting on start up check for a battery low voltage problem. Low voltage will also cause the PCM to have to relearn the idle. Just a thought.

 

Dano, I'll check the sensor when i get off work. But it's not at 1/2 power, when i'm going down the road the power is there. It just seems to be doing it when it's cold and idling. I'll check the filter too, i'm sure it could use some cleaning...

2001NMBZ28, i understand the pcm having to relearn the idle, and i don't think that's the case because of how it's starting and idling. I did have that issue before because i had let my battery die and never got it fully charged, but when it would relearn the idle, it was nothing like it's acting now. It would start fine, idle around 1800-2k for a couple minutes and be fine. This is definitely different.

Thanks for the feedback by the way!! I haven't had anything done in a long time to the engine (or car) for that matter. I'm wondering if a sensor could be going bad, but just isn't throwing a code yet. Could the O2 sensors be causing that though? Only thing, i didn't think they really started working till the car was warm...

 

Check your PCV hoses. They have a tendency to crack with age. Especially at the bend just at the rear of the drivers side tappet cover.

 

Ok, i'll check that too. I felt behind the intake yesterday and couldn't really tell anything. Another thing now, when i'm stopped and letting it idle, it will sometimes fall to about 400 rpm and stumble like it's going to die. I'm wondering if it may be a fuel problem? What are some symptoms of a fuel pump going out, and would that throw a code? I still don't have any codes, but i do have a Predator programmer so i could check it out real time, but i'm not sure what i'd need to look for.

I'll make sure that the PCV hoses are good first and go from there. Thanks for the feedback!

 

Well, i finally figured out what it was- vacuum leak alright. As i finally got to open the hood and listen while the car was running, i found that the expansion plug i have in the top of my intake had worked its way out. It was still sitting right there on the hole but when i went to touch it, it wasn't even in the hole. So i rammed it back in there, tightened it up and it's good to go now!

I like these easy, inexpensive fixes!!!! I guess after 5 years it just gradually worked loose. Not bad i suppose for a cheap part...

 

If you ever have a very high idle again, you can use starting fluid to locate the vacuum leak. I suppose you can use other sprayed fluids but starting fluid is cheap, dissipates and does no harm when ingested through the leak.

I have a 2-2.5k rpm idle that sometimes falls to the point the engine stumbles or wants to stall. I located the leak in the manner above. In my situation it is either a broken o-ring on an injector, a torn intake gasket or a crack in the manifold at the #1 cylinder.

 

That doesn't sound like a good thing at all (your situation, that is)! I think i may have to get a new expansion plug because it seemed to do it again this morning on my way to work, so i have to check it here in a few hours to see if it's loose again. If so, i'll be cleaning out the hole tonight and putting in a brand new one. Hey, a couple dollar part that's lasted about 6 years, i can't complain!

 

What intake manifold do you have there? and is the problem solved now?

 

You still there cocheese? What's the skinny, is it fixed? I'm still curious about your manifold. The OEM part doesn't have an "expansion plug" - at least mine doesn't...

 

He probably removed the EGR and that's the reason for the intake plug, 01 and 02 don't have it. Originally I thought something else was the problem, because if the plug is letting in unmetered air it should throw a code.

 

Well that helps to explain it. An expansion plug on a stock intake is a new one for me. That's why I asked what intake he had. I'm aware of an aftermarket intake that has pressure release panes for nitrous application.

 

Yeah 98-00 f-bodies came with an ERG system where the pipe entered the top of the intake just behind the throttle body. Folks that remove that system use many different types of plugs to stop up that hole.