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Doug
Doug, ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 8606
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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Mitsubishi 3000GT SL: 1995 3000 GT SL. Engine overhauled.

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1995 3000 GT SL. Engine overhauled. Now car has a cold idle stall problem. What to do?
Hi,

What is your cold and hot idle speeds right now?

The idle can be adjusted manually, however it is not meant to be.... unless you think it was misadjusted during overhaul?
Customer: replied 4 years ago.

I haven't check it with a tach, but it sounds and feels slow when cold. A slight hold on the gas peddle keeps it running cold. Hot is OK.

Thanks.

If that is the case, it is possible that it simply got adjusted while apart and needs to be dialed in again.

But first can you verify if the idle speed changes when you turn the AC on or turn the steering wheel a half turn, and if so how does it change?
Customer: replied 4 years ago.

k, the hot idle is 900 rpm, the a/c drops it to about 700 rpm on the cluster tach. It gets close to stalling. The garage floor is smooth and turning the steering wheel doesn't drop the rpm.

Thanks.

If the idle is dropping when the AC is turned on and the power steering is not causing the idle to go up when turned, it is most probable that you have an issue with your idle speed valve. This component adjusts the idle as needed to keep the engine running whether hot or cold (an issue you are seeing, since it only idles well hot), and it also adjust the idle up to compensate for load when the AC is turned on or the power steering pressure switch is tripped (neither of which you are seeing).

This definitely was not a problem you experienced prior to the rebuild, is that correct? Or are we not 100% certain of that (recently purchased car etc)?
Customer: replied 4 years ago.

Yes, it happened after the rebuild. We've owned it 15 years, It's at 131,000 miles. It got engine work because it wasn't passing California smog and was sending oil onto the catalytic converter. The problem happened after that. Cold idle stall is the problem that really needs to be fixed. The a/c doesn't seem to kill it, but it drops the rmp to 700 rpm when the engine is hot without compensation to increase rpm back to 900. I haven't checked the a/c effect when cold. It's a garaged car.

Thanks.

Your symptoms all point directly at an idle speed control valve issue, however it is a bit surprising to have this just happen to occur during the over haul.
Have we carefully checked around the throttle body (specifically around the bottom of it) to make sure nothing is unplugged?
Customer: replied 4 years ago.

No hissing and nothing looks unplugged.

Thanks.

As strange as it is, it may just be that the idle valve decided to fail while the engine was being overhauled. I don't usually subscribe to coincidences myself, but given the information you have provided, it is really all that fits.

-idle is low/poor when cold, but fine when warm
-turning on AC bogs down the idle
-loading the power steering pressure does not increase idle

Those three things altogether can only mean an issue with the idle valve. If you only had the cold idle problem we could say perhaps the valve is dirty or the throttle is too dirty and needs a cleaning... both very common issues. However with no increase in idle speed via AC or power steering operation, that is only going to be an idle valve malfunctioning.

If the valve is plugged in and the wiring isn't damaged etc (again it is on the bottom of the throttle body), that really only leaves the valve itself for failure. While it is possible for the computer end of the system to fail, it is not terribly likely without numerous other problems.
Customer: replied 4 years ago.

Is this an obvious fix a local mechanic can do?

Most experienced mechanics can handle this. If you tell them just what you told me in your answers:
Stalls when cold, idle seems low
Idles fine hot
AC causes idle to drop
Turning the steering wheel does not increase idle

They will quickly arrive at the same conclusion. Replacement is straight forward.... unbolt the throttle body from the intake (disconnect plugs and hoses) and then unbolt idle valve (sometimes called fast idle valve, idle speed control valve, idle air control valve, etc) from the bottom of the throttle.
The harder part is getting your hands on one... due to the age and rarity of this model, it is going to be a special order anywhere you go. You can get used ones from salvage, just be cautious there as these do fail a lot so there are strong odds you would get a bad one if you went used.
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