Engine idle rough or stalls on cold start, Please help!! Options

[mechedjun]

Hey guys, I got a 1994 Celica GT 2.2L with an idle issue. I know this issue has probably been beat to heck but I can't seem to get a straight forward answer. I'm looking for someone who's gone through the same issue and hopefully can advise with their fix. So here's the situation.

During the cold-ish (say 30 Degrees and up) days, the engine will start up but immediately drop to an low idle and ALMOST stall. It'll continue to idle rough for about 5-10 seconds then it'll run normally. On really cold days (say under 30 degrees), the engine will stall. On these days I would have to hold the throttle to approximately 2000rpm for about 10 seconds and the engine will run normally. It ONLY does it on cold days and ONLY on the first start of the day. Once the engine has run a bit, it doesn't matter if I leave it (in the same cold temperature) for hours, it'll start up normally (as long as the start is within the same day).

Here are the things I've already done.

1. Changed coolant temperature sensor
2. Cleaned throttle body
3. Checked for vacuum leaks (none were found)
4. Checked MAP sensor (reads at correct voltage per pressure)
5. Changed valve cover gasket including re-sealing half-moon
6. Changed spark plugs
7. Changed spark plug wires
8. Checked EGR Valve (Opened correctly at 10psi vacuum)

So basically I'm pretty much stumped. Here are the things that I haven't tried but pretty sure isn't the issue.

1. Fuel injectors
2. Cylinder compression leak

Reason: The car recently passed emissions with flying colors. No excessive burning or signs of blow-by were detected during the emission, suggesting fuel isn't passing past the piston rings. I get average 25-26mpg, bang on where I should be, suggesting combustion ratio (Air/Fuel Ratio) isn't offset. No signs of power loss, even taking off from the cold start (after the engine idles normally).

There are two other symptoms with the car that may or may not be related.

1. Once a day, when the car comes to a stop (at the red light for example), the engine will "jerk" once with idle dipping very slightly for fraction of a section. Looks like minor knock to me. Weird thing is that this "knock" happens after significant time passes from the cold start but the time difference is random. Sometimes it'll do it 15 minutes after it set off in the morning or after hours of driving. Further more, it ONLY does it ONCE a day. Not sure if its related.

2. The coolant temperature gauge in the car reads a little..."funny". Not sure if its supposed to do this so let me know if what I'm going through is normal. On higher temperature days (50 degrees or more) the gauge will read normally (needle is pointing slightly lower than the middle). However, on colder day (depending on how cold it is) the gauge will read low as almost C, even after I've driven 6 hours non-stop. Granted I was traveling to New Hampshire and the outside temperature was 25 degrees with me traveling through it at 70-75 mph, gauge reading close to C just doesn't seem normal. Weird.

Anyway, I've been yakking too long. Thanks for reading and let me know if anyone has any thoughts.

-Dave

This post has been edited by mechedjun: Mar 6, 2012 - 6:56 PM

[Syaoran]

IAC Valve.