99TransAmDude

For about 4 months now during a cold start, the idle will instantly jump up to around 1800 or so then within a few seconds drop back down to normal. I don't remember it doing this before. Warm start no problems, idle never goes above 1000.

Now the problem has gotten worse. The last few times I started my car (cold start), the idle instantly jumps up to around 2000, it still drops down to normal within a few seconds but idles terribly. I have an Aeroforce Interceptor scan gauge and it's showing multiple misfires on all cylinders. After maybe 1-2 minutes, the misfires go away and the car runs perfect. Smooth idle and good power with close to zero misfires. During a warm start, smooth idle and almost zero misfires.

I checked for vacuum leaks and cleaned the MAF and IAC. I don't think it's a fuel problem since it always runs great after 1-2 minutes. I'm thinking the IAC valve could be bad. Anyone have any ideas?

This car is my daily driver so mods are light....headers, gmmg cat back exhaust, high flow airlid and performance tune. All mods were done 3 years ago and I've had no problem to date. No header or exhaust leaks. 81k miles automatic car.

Thanks in advance for any help!

xdanny510x

Check ur cam sensor and crank sensor maybe ur maf... mine had a similar problem and i started with the maf and cam i replaced the cam sensor and ckeaned the maf and it fixed my idle issue

99TransAmDude

Did yours only act up for a few seconds on cold start or all the time? I haven't cleaned the MAF yet but I would think a bad cam sensor would cause it to run bad all the time.

I ordered a new Delphi IAC sensor. Only $50 so I figured may as well toss it in and see what happens since its' a 5 minute job. Amazon.com...if it doens't work, I can always return it. Tomorrow I'll clean the MAF, IAT, and throttle body really good as well.

I searched some other forums for other cars and one common post for an idle spike at cold startup is the intake gaskets are bad. Now these are Toyota, Mustang, Tahoe, etc forums. I've never heard of LS1 engines suffering from intake gaskets but I guess anything is possible. Any way to test for it? I don't know if spraying starting fluid would get down in there if it was an intake gasket. If I go down this road, guess it's time for the LS6 intake, change knock sensors...the works.

99TransAmDude

Found the problem...and it was a dumb one...like most car problems.

A few years ago I pulled my EGR and had it tuned out. No issues. Well the plug used in the intake was one of those rubber expansion plugs I had laying around. Not really sure what they are normally used for. It was completely popped out of the intake and just laying there on it's side so I basically had a 1" hole leak in my intake. I'm amazed the IAC was able to compensate for such a huge opening. I replaced it with a higher quality one that actually bolts down to the intake. Started the car up and it starts normal and idles better than ever.

Now you might be asking yourself...how the hell did this idiot not notice right on top of his engine the plug popped out. The answer is I have a custom installed 1970 shaker scoop so I can't see the top of my engine without removing it. I took the shaker off today to clean my MAF and there it was clear as day.

jimmyblue

iTrader: (11)

Might look first for a vacuum leak (like the little PCV boot,
that is doomed and only a matter of when) that the IAC
adds to (not knowing any better, until closed loop idle RPM
is enabled). Vacuum leaks can be flakey if they are from a
cracked soft piece (which may be more soft or less, more
gapped or less, depending on a lot of conditions).