1992 Mitsubishi 3000GT Runs rough and rich when cold

WE74ST

MEMBER
1992 MITSUBISHI 3000GT
6 CYL
AWD
MANUAL
66,000 MILES

I have a '92 3000GT VR4 with about 66k miles. The car starts with no problems but when it is cold will run rich (smell of gas) and the idle will jump around from 700 rpm to about 1000 rpm. If driven when cold it will also hesitate. However, once the car is warmed up with about 10 minutes of driving it operates normally with no problems. The issue will not come back unless the car cools down for an hour or so. I just had the intake gaskets, plugs and wires, TPS and fuel filter all replaced but the problem remains. Timing belt service was done at 55k. I have not checked for vacuum leaks yet but will do that next. Any other ideas would be appreciated. Thanks.

1 Reply

CARADIODOC

MECHANIC
33,871 POSTS

Hi we74st. Welcome to the forum. I invite other replies but in the meantime, look at the vacuum hose going to the MAP sensor. If it reads lower than correct vacuum, the Engine Computer will think the engine is under greater than actual load and will command too much fuel. A failing MAP sensor can do this too, but they rarely take more than a day or two to completey fail. Once the coolant reaches around 180 degrees, the computer begins making fuel delivery corrections based on readings from the oxygen sensor. That could explain why it runs better when it's warmed up.

Your thought about a vacuum leak is on target too. When the engine is still cold, use a squirt bottle to spray water on the vacuum hoses. It sounds like you know what to look for. If the idle speed slows down, look for where the water is getting sucked in.

Caradiodoc