608kZ28

i have 1998 Z28 automatic. the car idles at about 800-900 rpm. but when i put it into gear the rpms drop real low to 400-500 causing the car to almost die, sometimes it does. then the rpm's go right back to 900. but when the car is warmed up it doesnt do this. im thinking a vacuum leak but hell if i can find it. any thoughts?

608kZ28

almost 20 people looked at this and nobody wants to chime in? come on guys

mike mlodecki

iTrader: (9)

any mods? had it tuned before? mine used to do that and what problem was, was fuel pressure.

Esoteric[TG]

SWAG... I'd suspect IAC actuator, but before replacing it, I'd take the IAC off of the throttle body and make sure the air bleed passages are clear. I'm pretty sure LS1 has a IAC actuator anyway.

When I managed a shop, we used to get a lot of GM vehicles with similar issues, often pulling the IAC and intake tube off of the throttle body and cleaning the orifices with throttle body cleaner would fix the issues.

As I remember it, the engine computer controls the idle by bleeding a variable amount of air past the throttle plate. If the orifices/passages get clogged with carbon then the engine computer will have trouble controlling the idle.

When the engine is cold the computer will have a harder time controlling the idle since the engine doesn't run quite as well until it warms up. Putting the transmission into gear adds load to the engine, and drags it down a bit. The computer should compensate for this, but apparently isn't for some reason.

Just a guess, hopefully you get it figured out soon!


Disclaimer: I don't currently have an F-body and its been a few years since I used to work in a shop, so the information may be inaccurate, incomplete, incomprehensible or otherwise completely wrong.

608kZ28

those passages you refer to where already checked awhile back. and cleaned

Esoteric[TG]

I assume you've tried the carb cleaner method to locate vacuum leaks. (let the engine idle and spray suspect areas around the intake/vacuum hoses and listen for change in idle)

Just a brainstorm, is your IAT sensor good (do you have a scan tool capable of looking at the data stream) Probably obvious, but I'm just trying to think what would be different from cold to warm engine. Also if you can look at the data stream, your IAC counts should ideally be in the 20-40 range (if memory serves) more than that and it can point to possible issues.

Thinking out loud...Cold engine should be in open loop mode, so O2s shouldn't be suspect, MAP sensor maybe?

You've probably already been through all this and I'll keep trying to think, but unfortunately its been a few years since my last F-body.

Hopefully someone more knowledgeable will stop by and share their wisdom.

608kZ28

O2 sensors are new so is the map and maf sensor. no scan tool just a code reader

BGMZ28

I got a 98 as well. Mine was doing just as you descripe but not all the time. I later found out it was actually my battery on its way out. One day. The car wouldnt start put a new battery in and haven't had the problem since. In my experience dying bateries do make a car act strange.

608kZ28

the battery is about a year old. give or take a few months.
car was tuned on memorial day friday. but it just started doing this about 2 months ago