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.



New trans and valve adjustment did not solve my problem. Idle is only rough now when engine warms up. Mechanic retarded (?) Timing and lowered idle. I am noticing ping and knock I did not have before.

Tiny
Mhooper87
May 16, 2012.
Check the throttle position sensor adjustment. Have a cpompression test done.

KHLow2008
May 16, 2012.
Compression test passed. Throttle position sensor adjusted. Idle/timing set to factory specs by Mitsu. Still rough idle. Now throwing CEL code for O2 sensor. Replacing today. Car still has rough idle in gear when warm and is exacerbated with any load (electrical) on engine (lights, brake, a/c). Also notice no high/warm up rpms. Rpms never change despite engine load or temp. Also with a/c and warm engine will idle so low and rough feels like pending stall. Thanks.

Tiny
Mhooper87
Jun 22, 2012.
Does/how the new ecu sync with new ISC? Is it necessary?
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Tiny
Mhooper87
Jun 22, 2012.
ECU would self learn after some driving time. What were the compression test results? Any vast differences among the cylinders?

If the cold start system is not working, I believe the ISC is stuck. You might need to recheck the ISC and if necessary replace it. The other thing would be the MAP sensor. It often fails and causes the symptoms described.

KHLow2008
Jun 22, 2012.
Dont have specifics on compression test but nothing significant was found out.

The ICS is new from Advance Auto - but I think you're right - how can I check it? MAP sensor? Please let me know how to locate and test if possible. I am noticing low idle rpms when the engine is cold and higher rpms when it is hot? Any ideas on that?

O2 sensor replaced some slight improvement but still shaking in gear when stopped, or under electrical load (A/C, lights)

Someone mentioned engine harmonic balancer?

Thanks again. Going on 4 months with this.

Tiny
Mhooper87
Jul 18, 2012.
What is the idling speed when stopped and in gear? It could be due to bad engine mounts.

Low rpm when cold and increases when hot indicates the ISC is not working correctly.

MAP if equipped would be at thw firewall, with a vacuum hose attached.

KHLow2008
Jul 19, 2012.
Last mechanic just jacked the idle adjustment to hide vibration I suspect he had it at 1k+ Mounts are new. So I reset the idle by ear (500?) The ICS reset and I got high rpms on start then drop (normal/good) and noticeable rpm changes with ac and gear selection. Still vibrating but ever so slightly better. Then I pulled the neg batt to clear the CEL and noticed hunting/wandering rpms that did not self resolve after about 5 mins at idle as per manual. Vibration is almost gone while in gear and no a/c but returns w/ a/c on.

Tiny
Mhooper87
Jul 19, 2012.
If the ISC is non OEM, I would suspect that as the cause. Try removing the cover plate and check the internal gears. If they are dry, get some di-electric greas and remove the gears to apply the grease. The gears could be sticking intermittently and when A/C is turned on, the rpm dropped. Get a tachometer to check the idling speed. Rm should be 700-800.

After disconnecting the battery terminals, the computer needs to relearn so the idling could be erratic.

KHLow2008
Jul 19, 2012.
Followed mfg idle reset - ISC reset and is operating. Still vibrating in gear, can feel to a much lesser degree the same rythmic shake out of gear with ac. Still shakes in gear with ac. Although is a little better now. Rpms are still a little wandering.

Tiny
Mhooper87
Jul 21, 2012.
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