1990 mitsu rough idle only in gear

i can not resolve a rough idle in gear when engine is warm problem. I’ve replaced plugs, wires, mounts, isc, o2 sensor, ecu, have no codes. any ideas?

Compression check? Which car and which motor?

I’m also curious as to how many miles are on it and what the maintenance history has been like, especially transmission servicing. Also, have you checked your tranny fluid?
And are there any other operating problems?
Can I assume this is an automatic tranny?

A rough idle when in gear on a car this age is likely to be one of the below
too high an idle
uneven compression
erratic ignition (numerous possible causes)
a bad motor mount
an injector problem
an EGR valve or solenoid not working properly
(worst case) a bad torque converter…also the least likely cause IMHO

Start with the basics. Tick off the list above before getting too concerned. Post back, we do care.

Compression checks out ok. This is a 1990 Mitsubishi Mirage exe 1.5l 4dr auto trans w A/C.

Idle was set at dealer (who could not solve this problem) at 700 rpm.
Mounts are new/replaced
Injectors have been replaced
? EGR ? Solenoid? Haven’t heard that - please expound

This is regarding my 1990 Mitsubishi Mirage 1.5l, exe, 4dr auto trans w A/C 112,000 miles- Mom bought this car new it was garage kept I now am original owner. This problem just started happened one day I came home, asked mom she said it just started happening. I have replaced the transmission and or torque converter on this car. I even replaced the entire motor but no change and got my original back. As well as: ICS (twice) ECU, injectors, plugs wires, mounts, O2 sensor, timing belt. I have also had the car smoked for vacuum leaks - non found. I have also cleaned the throttle body. No one can figure this out. Its not a minor vibration its rattling my teeth so bad I have to go to neutral at stop lights and give her a few more rpms to smooth out.

My detailed observations are: starts runs fine at a COLD -really cold start. Within about 3 mins or as soon as I hear coolant moving / thermostat opening vibrations begin and get worse as engine gets warmer. Once warm vibrations are bad in gear and mostly go away out of gear. The car is trying to run right and vibrations erratically resolve and come back - vary rarely. Once warm and vibrating, they are exacerbated or lessened by the application/use or discontinuation of brakes, headlights, a/c. even turning the steering wheel. I am quite certain it is some sensor or wire or electrical issue that only happens after things start to get warmer. Thanks guys!

I should add the car runs like a top at speed and all the vibration can be resolved by the application of a little more gas. RE previous post: ICS should be ISC Idle Speed Control more precisely ISCS Idle Speed Control Servo.

Tranny fluid has been replaced twice this year.

Compression check was good - no problem found


It’s running good when cold because it’s in open loop

Have you verified that the engine coolant temperature sensor is accurate . . . the one for the engine control module, not the one for the cluster, in case this engine has more than one

I have not. But have always wondered about that - meaning the temp gauge always registers a little above norm no matter what the conditions. And there was a bad thermostat on the car for years so it never used to warm up. Any who where is this sensor? I have thought it might be some thing not telling the engine to lean out - will check that out this week.

You have a temp sensor, EGR and crank sensor. Cranksensor is in the distributor.

So I see two engine temp sensors on the dia. but this is NOT a CA vehicle. I will have both checked out.

I am changing my observations: car idles rough in gear and / or out of gear with application of headlights or brake or a/c same type severity of vibration in both scenarios. I also noticed the temp gauge jump a little and return with application of brake during COLD start / warm up.

Let us start with the basics, last time air filter and gas filter have been serviced. As you did plugs.

One more observation. The car actually now is idling better in gear with accessories on than when in neutral. The only change has been in the ambient temp here from the teens back to the mid 30s-40s F.

I have not changed the gas filter - air filter is less than a year old. Plug and wires with in the last year.

Problem found, ruptured diaphragm in brake booster. Of course now its idling rough in park - how out of tune can it be after so many adjustments with the booster leaking? Suggestions? Thanks!


Have you messed around with the throttle cable adjustment and/or the idle speed adjustment?

If you had adjustments made while the diaphragm was leaking, all the adjustments need to be done again, now that the leak has been fixed. In short everything was adjusted to try and compensate for a vacuum leak now it needs to be put right.