Strange idle problem

Radical_Ralliart

Radical_Ralliart

Member
So i’ve had this issue since i bought my 2.4L lancer since november of 2022 so i thought i’d ask about it now. If anyone else has experienced this please let me know what you did to fix it

So the issue is that when the engine is at operating temp, and at idle it randomly will fluctuate, drops to 200-300 rpm and jumps back up to 700-800rpm. It does this over and over for usually anywhere between 2-5 minutes at a time.

It will sometimes stall the engine out, but i can start the engine again just fine.

I went to a mechanic to have the ecu scanned with the engine running and driving, but every sensor showed normal signs, except idle speed that was fluctuating while at idle.

Bear in mind i’ve replaced alot of parts around the engine. Heres a list of what i’ve replaced

-alternator
-all 4 coilpacks
-all 4 sparkplugs
-sparkplug seals/valve cover gasket
-battery
-oil pressure sensor
-maf sensor
-thermostat
-map sensor
-front oxygen sensor
 
Was the MAF replaced with a genuine unit or aftermarket?

Give the throttle body a clean! It does a lot of work controlling idle speed.

Have a friend turn on the ignition and hold the throttle down. While doing this, unplug the throttle and switch ign off. The throttle should now be held open for easy cleaning. Best not to manually move an elec throttle body.
 
SDate42 said:
Was the MAF replaced with a genuine unit or aftermarket?

Give the throttle body a clean! It does a lot of work controlling idle speed.

Have a friend turn on the ignition and hold the throttle down. While doing this, unplug the throttle and switch ign off. The throttle should now be held open for easy cleaning. Best not to manually move an elec throttle body.
Click to expand...
I swapped it with a mitsubishi maf from another lancer that had way lower km’s done on it, problem remained the same afterwards.

I did try cleaning the throttlebody a while ago but i’ll try it again, will the throttle plate stay open after i unplug it?
 
After reading what you have done so far to it it doesn’t leave much to do except fuel pump/filter and injector clean, but I’m thinking throttle body as well, like SDate42 suggested above, but I would go find another throttle body and swap it over with yours and see how that goes before moving on anymore.
 
Is there an Idle Air Control Valve for the 2.4L? For the CE generation, when warm, the ECU dictates the IACV should open/close more than when cold. When it starts to fail, the idle will hunt in particular when warm, as the ECU constantly tried to read the steps within the IACV motor and cant do so
 
donki said:
Is there an Idle Air Control Valve for the 2.4L? For the CE generation, when warm, the ECU dictates the IACV should open/close more than when cold. When it starts to fail, the idle will hunt in particular when warm, as the ECU constantly tried to read the steps within the IACV motor and cant do so
Click to expand...
From what i can tell i think the throttle position sensor and idle air control are all built into the electric throttle body. Theres only one plug that goes into it so i may have to just get another one
 
There is no separate idle air control valve in the 4G69 - the throttle plate does it all. The ecu will open or close the throttle an extra amount in response to the A/C compressor clicking in and out and also for electrical load e.g if the fans kick in.
One often overlooked thing is that the throttle body has a stop screw to prevent the throttle plate jamming closed when it gunks up with egr filth.
1693440525064.jpeg
To adjust this (with engine off), press the throttle plate to fully close against the stop screw with your finger. The unpowered state is significantly open. Adjust the screw so it allows the throttle to fully close and then wind it back in until you just feel the throttle plate start to open - lock it there. The ecu tests and learns the open / closed position of the throttle every time the key is turned off (not on). It has an el cheapo dc electric motor instead of an accurate stepper motor so it will continuously make mistakes when trying to set the throttle for a stable idle... tight arse Mitsi :angry:
The throttle can be forced fully opened and closed by hand without risk of damage.
For abnormal idle rpm dips, try to identify if they coincide with A/C or fans presenting an extra load on the engine. If this is the case, there could be a fault in the A/C compressor or fan controller (drawing huge currents). It could also be the egr valve not fully closing so the ecu is having a hard time compensating for this.
The primary response of the ecu is to manipulate the ign timing to maintain idle, the secondary response is to open or close the throttle.
Any cheap elm327 type of OBD2 plug will log and display rpm, throttle position, ign timing and load which should point out what the problem is.
This is what it should look like with no A/C, no electrical load, fully warmed. Note the ign timing is doing all the work to hold idle, the throttle is sitting firm on 0.5% in this example and the load is constant indicating airflow volume is also constant. With A/C on, you will see a step up and down in the load and TPS as the compressor kicks in and out. Same deal with the radiator fan. There is always a small rpm dip as the ecu can't react to fast load changes before the fact.
1693440229698.png
Hope this helps clear up some misconceptions about the drive-by-wire throttle.
 
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