Rough idle in gear 1990 Mirage throttle position sensor or idle speed

Tiny
MHOOPER87
  • MEMBER
  • 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.
Monday, April 16th, 2012 AT 2:47 AM

48 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
I would look for a vacuum leak. Have the intake system smoke tested to find it. Intake gaskets are common.

Roy
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Monday, April 16th, 2012 AT 2:49 AM
Tiny
KHLOW2008
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Some trouble codes do not trigger the CEL for older models so it is good to check for trouble codes.

https://www.2carpros.com/articles/mitsubishi-trouble-code-definitions-and-retrieval-procedure-for-odb1-system

Try above link if you want to check for trouble codes.
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Monday, April 16th, 2012 AT 11:50 AM
Tiny
MHOOPER87
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If I have no CEL do I have codes?
Do notice what sounds like an exhaust leak that I only hear in gear stopped at traffic and after isc lowers rpms revs back up then drops to rough idle.

Thanks this is driving me nuts. 1 month and no one has a solution. Will get smoked and advise.
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Monday, April 16th, 2012 AT 1:54 PM
Tiny
MHOOPER87
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I am correcting original post. Transmission jumps in / out of gear most notably when I'm off the gas. Also seems to shift prematurely. Thanks again.
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Monday, April 16th, 2012 AT 1:57 PM
Tiny
KHLOW2008
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Not all codes trigger the CEL and for transmission, most codes do not show unless you use the diagnostic method to check.

If you are not going to follow advise, I will stop here.

Have a nice day.
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Tuesday, April 17th, 2012 AT 1:40 PM
Tiny
MHOOPER87
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Smoke test was negative. Maybe I led on to know more than I do. How do I perform code check via diagnostic method? Thanks!
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Tuesday, April 17th, 2012 AT 5:54 PM
Tiny
KHLOW2008
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For engine trouble code, click the link provided in previous post.

Transmission Self-Diagnostic Test Procedure

Connect analog voltmeter or LED lamp to diagnostic connector terminal No. 10. And ground. See Fig. 7. Turn the ignition switch to the ON position, and TCU memory contents will start immediately.

The sweeping needle of the analog voltmeter or the blinking LED lamp would indicate the fault code.
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Wednesday, April 18th, 2012 AT 1:53 PM
Tiny
MHOOPER87
  • MEMBER
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Mechanic says he doesn't have software to run an engine diagnostic code check? Is there special software required? He did find fixed idle and set idle screws "way out of specs". No more revving / wandering RPMs but still rough idle in gear at full stop. I hear a slight exhaust (?) Leak only when: lights on, turn steering wheel, press brake pedal when in gear does seem to coincide with rougher idle. Mechanic says its negligible and thinks its as good as its gonna get. I can't believe it. He says 10 cycles of 35 mph to 55 mph required to recalibrate ECU/ECM. And to run a tank of Hi Octane to smooth it out.

The car wants to run smooth in gear and does momentarily on slowing down but once at a full stop starts running rough & shaking. It smooths out as soon as I let off brake and start to roll just a little. Will try a trans specialist (AAMCO?) For Trans codes. Thanks again.
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Thursday, April 19th, 2012 AT 12:22 AM
Tiny
KHLOW2008
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Did you read my replies? I outlined the procedures for retrieving both engine and transmission trouble codes.

Your idling speed is too low and not stable. If you did not clean the throttle body, do so. Have the valve clearances checked as well.
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Thursday, April 19th, 2012 AT 1:14 PM
Tiny
MHOOPER87
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No codes on engine or trans. Cleaned throttle body no difference. Latest mechanic says torque converter is bad. Going for rebuilt trans - any suggestions or good prices - or places to steer clear of? I see rebuilds for $500 sound about right to you? Thanks again.
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Sunday, April 22nd, 2012 AT 8:04 PM
Tiny
MHOOPER87
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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.
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Wednesday, May 16th, 2012 AT 12:32 AM
Tiny
KHLOW2008
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Check the throttle position sensor adjustment. Have a cpompression test done.
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Wednesday, May 16th, 2012 AT 4:16 PM
Tiny
MHOOPER87
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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.
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Friday, June 22nd, 2012 AT 9:36 AM
Tiny
MHOOPER87
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Does/how the new ecu sync with new ISC? Is it necessary?
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Friday, June 22nd, 2012 AT 9:49 AM
Tiny
KHLOW2008
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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.
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Friday, June 22nd, 2012 AT 1:08 PM
Tiny
MHOOPER87
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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.
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Wednesday, July 18th, 2012 AT 8:39 PM
Tiny
KHLOW2008
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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.
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Thursday, July 19th, 2012 AT 6:46 PM
Tiny
MHOOPER87
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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.
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Thursday, July 19th, 2012 AT 7:01 PM
Tiny
KHLOW2008
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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.
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Thursday, July 19th, 2012 AT 8:12 PM
Tiny
MHOOPER87
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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.
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Saturday, July 21st, 2012 AT 12:25 PM

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