Rough idle when the engine is cold only

[Jonathan]

hi
I am a car newbie here, my ride is a 1991 Mitsubishi Galant GSX, non turbo, DOHC. Recently I am having some rough idling issues upon starting when the engine cold. The RPM drops to about 500 and if i try to drive it, I have to press on the gas pedal to keep the idle up to 1000 and keep from stalling, it drives pretty unsteady. But once the engine warms up in about 20 to 30 minutes. This does not happen when the engine is already warm, for example I already drove somewhere in it, parked and started it up again. It drives smoothly as if nothing is wrong. But the fumes out of my tail pipe smells like gasoline. and i have horrible gas mileage ( about 3 km per liter )

I went to a repair shop, they cleaned the Idle Air Controller and throttle, and they suggest that my MAF needs replacement. So i got myself a surplus MAF (we dont have many OEM parts here specially for older models), which the mechanic assured me is working fine. After the installation, i still get the rough idling on cold starts, but the mechanic suggested a workaround – pumping the gas pedal 4 to 5 times before cranking the key ( after the check engine light went off). When i do this, the engine starts fine, reving at 1000 but it reverts back to rough idling when i dont do this. As before the car is fine when the engine is warm.

my question is
1. by pumping my gas pedal – what does it do to “cure” the rough idling? what condition does it create, so that the rough idling on cold starts will not happen

2. Did it cure my horrible gas mileage, since it has just been 3 days,i had no chance to confirm if my gas mileage has improved. i filled my car to full tank and will measure my gas consumption

thanks in advance

Topic

[Lorrin Barth]

Since there is no pump as on a carburetor I can’t see how pumping the throttle would do anything. Maybe the throttle is sticking and not closing properly. You might try wiping out the bore at the butterfly with solvent.

The most common cause of your complaint is an intake vacuum leak somewhere after the MAF.

[Jonathan]

Thank you BarneyB. i will give that a try and hunt for the vacuum leak. thanks again

[James Cheyne]

A few other things to look into are the following: TPS (Throttle Position) Sensor, EGR Valve malfunctioning, a bad PVC valve, M.A.P. Sensor, Idle Air Control Valve, and the Idle Speed Actuator. I have a 1995 Plymouth Voyager with a 3.3L V-6 made by Mitsubishi and the culprit was the Idle Air Control valve. Hope this helps.

[Lorrin Barth]

Yeah, on the older Mitsubishi vehicles you can expect to have to change the IAC about every 80,000 miles. However, this problem will produce the wrong idle not necessarily a rough idle. However, it may want to stall.

I have some experience with cleaning Mitsubishi IAC valves and found it to be futile effort. The valve does more than control idle, it is active throughout the driving range.

[college man]

What sticks out for me is that the car takes 20-30 minutes to warm up
which is a long time. If the car stays that cold the coolant temp sensor
will keep the mixture rich. You may have a t-stat problem.

[Jonathan]

@ collage man – thanks for the input, hmmm my temp sensor is new, just changed it last week, but i will take a look at it again thanks.

@ barneyb – i will take it to a new mechanic and have him evaluate the IAC. thanks

@ Jc822 – i will tell my mechanic to evaluate these thanks

thanks guys, as i am a newbie, i dont trust myself to fix the car, but i will take it to a new mechanic and will bring this as a checklist.

i will update you guys on what happened.
again thanks
Jonathan

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