Diagnosing rough idle

[Spetz]

Hi everyone,

My idle isn't as smooth as it should be.
I have replaced the usual culprits and have done all the usual tests with no result.

Would the best option be to take the car to a mechanic who has an oscilloscope to diagnose this?

Car is a 2003 Mitsubishi Diamante.
Items replaced:
Fuel filter, injectors serviced, ignition module, spark plugs, spark leads, distributor cap and button

Tests conducted:
Compression test, vacuum test, vacuum leak test, ECU and all sensor test (all sensors reading within normal parameters)

 

[BrownBox88]

My Galant has the 6G72 which isn't much different other than being a 3.0 motor. Did you check the idle air control motor and clean the throttle body itself? EGR and the EGR control solenoid can cause rough idle too.

 

[Imp4]

I recently had this on a 2001 Galant 2.4 and the condition was worse with a cold engine.
I was able to diagnose a faulty IAC.
New through Rockauto was $175. I was able to hit up the local Pull-a-part and pickup two for $10 each. This was good thing because only one worked. But it did solve the problem for $20 invested.

IAC for your Diamante is about $120 on Rockauto, but seemingly much cheaper at $36 from 1aauto.com.
I would try to diagnose this condition on your car and pull one from the bone yard near you if it turns out to be the issue.

 

[mk378]

Is the rpm correct? Use a service tach; the dash tachs are not very accurate especially at idle.

Also do a cylinder drop test to see if the misfiring is based in one cylinder or generalized.

 

[Imp4]

You could also check for a vacuum leak, the MAF sensor, or a coil that is going....

 

[Spetz]

Hi everyone,

I have cleaned the throttle body and MAF sensor too.

I connected a laptop with the MUT software which gave me all the readings and the rpm was as it should be an stable.
I reflashed the ECU to raise the rpm in D by about 80rpm or so which made the idle smoother but I didn't want to go up any more than that (I believe it is now at 700 from the original 620rpm).

I have also checked the EGR by blocking it off completely with a blanking plate and there was no difference to idle

 

[spackard]

Motor mount?

 

[Spetz]

I can feel the engine jerk while I have my hand on it.
1 mount is new OEM (the roll stopping one that takes most of the force) and this has of course made the rough idle less noticeable but it is only masking the issue.

My original question is whether an oscilloscope will diagnose a rough idle?
I've booked my car in on Wednesday next week to get checked. Hoping whatever the issue is it is found

 

[Lubener]

Originally Posted By: spackard
Motor mount?

That's what I am thinking.I usually them them all together.

 

[G.Frost]

Valve lash adjustment? Sometimes running PEA, combustion deposit detergent, like Red Line SI-1 will clean up a lumpy idle.

 

[SatinSilver]

I remember someone on here recently trying all sorts of things that didn't work. But then they added a quart of Lubegard Red to the trans after doing a drain and fill. And voila much smoother idle.