Mechanics

ROUGH IDLE IN GEAR THROTTLE POSITION SENSOR OR IDLE SPEED

1990 Mitsubishi Mirage • 100,000 miles

I have a rough idle only in gear and wandering / surging rpms. Ive replaced motor and trans mounts, idle speed control, spark plugs and ecm/ecu but no joy. Is it the throttle position sensor? Or transmission? I do experience some slip and odd shifts but only when all sensors are connected otherwise she shifts smoothly. I have no CEL.
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Mhooper87
April 16, 2012.



I know. The idle surge has resolved / is gone. Suggestions to read RPMs? I have no tach installed


Tiny
Mhooper87
Aug 14, 2012.
You lost me there. I am confused as to the problem you are facing now. You mentioned surging is gone but then you have intermittent surging which you are only feeling because you don't have a tachometer. For accuracy, you would need to install a tachometer or at least attaceh one for testing. There are clip-on tachometers availble at parts outlets.

Btw 3 months though not too long is enough for engine mounts of inferior quality to fail. One thing that can cause this is a bent center support. When this support is pushed upwards, it would cause vibrations as the front and rear mount would be compressed. Have a visual check and ensure they are not compressed. Removing the center bolt of the front mount would allow you to see if front and rear are aligned.


KHLow2008
Aug 14, 2012.
Your last comment might be the answer. Even though the mounts were bought from RockAuto less than a year ago the vibration never resolved. I can see the engine "jump/move" more than an inch when in gear and out. Does this sound like a bent center support? I have not inspected it lately but will tomorrow. It did take some muscling to get the front mount in and adjusting to get it to line up.

This problem manifested itself one day years ago. It just appeared. It was mom's car perhaps she did hit something or run over something.

The only other weird part to this is that when the engine is cold there is no vibration in gear. But as soon as it reaches operating temp the vibration is there when in gear. I am also noticing a tough start in the morning, the engine idles very low and then the ISC kicks in and rpms come up - but not right at start up.

Thanks for your continued advice/support.


Tiny
Mhooper87
Apr 22, 2013.
The only other weird part to this is that when the engine is cold there is no vibration in gear. But as soon as it reaches operating temp the vibration is there when in gear. I am also noticing a tough start in the morning, the engine idles very low and then the ISC kicks in and rpms come up - but not right at start up.

Yes, I believe the center support is damaged since you need to force the front mount in.

When cold, the engine would be running at a higher rpm due to the cold start feature so vibrations might be minimal but when idling speed drops, the vibrations would increase.

During cold start if the rpm does not increase immediately, it could be due to a dirty ISC. The other thing would be the valve clearances. If they are too tight, the engine would idle slightly rough and that might cause the vibrations and the
low idle at initial startup.


KHLow2008
Apr 26, 2013.
Please see the attached video pics. I can not tell if its "bent" or not but obviously took some damage. Re Center Support - is that, scratched, rusty bar running font to back it? The front of the car is to the right of the screen.

I suspect the ISC is dirty / other since I have pulled it, connected, unconnected, reconnected it many times. Suggestions on cleaning it?

I did have a guy adjust the valves - I believe he got creative on setting them to resolve this problem-to no avail-do you suggest another valve adjustment/job? The car does run rough (quite) at cold start up, and will stall if I drop it in to gear, if the ISC has not engaged.

Would additional video help nail this down? Im pretty sure we've got it ID'd but would like to be quite sure before I drop it off for another attempt to resolve.

Thanks again.


Tiny
Mhooper87
Apr 26, 2013.
I can't determine anything from the pictures so am not able to make any comments.


KHLow2008
Apr 27, 2013.
Well Im stumped. I replaced the center brace, got the ECU rebuilt, replaced all fuel injectors, timing belt, had valves looked at and anything else I could think of. The good news is the ECU rebuild fixed the ISC fail/sporadic behavior. The rpms are high at cold start up and adjusts as the AC compressor kicks in and out and temp comes up and I have no vibration in gear. But as soon as the engine is warm the vibration in gear is back and is exacerbated as AC, lights, brakes or steering are engaged at normal op temp. It idles smooth as silk out of gear and warm but starts vibrating in-gear under any kind of load but only in gear.

Ive notice the head lights dim as AC is put on when in gear. Idle has been set with code scanner (no code - no check eng light) to factory specs.

Could it be a sensor or switch in the gear selection housing? Or the alternator? It happens in every gear and I mean as soon as I shift into a gear (auto trans). Sometimes it smooths out a little - usually when the AC compressor kicks in or if I put it on a high setting (more blower) and rpms come up.

Engine temp sensor checked out ok. The only other thing Ive noticed is despite a new thermostat the temp gage still shows the normal op temp running a little above the normal hash mark. I am at a loss for what to do next. Thanks again.


Tiny
Mhooper87
Aug 21, 2013.
I believe the engine mounts are bad. Check the ones at either side of engine compartment, timing belt and transmission


KHLow2008
Aug 22, 2013.
That (mounts) was the first thing I did in April of last year. I just replaced two more this week with no improvement.


Tiny
Mhooper87
Aug 22, 2013.
Which 2 were recently replaced and were the mounts allowed to settle down correctly before they were tightened?

Did you check if the front and rear were installed in the correct direction?


KHLow2008
Aug 22, 2013.
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