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Doug
Doug, ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 8562
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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Mitsubishi Montero Sport LS 4x4: when starting out from a stop,

Customer Question

when starting out from a stop, the car hesitates momentarily (no power) then surges forward with full power expected. Injectors , coils, plugs, electronics, timing belt and tensioners, etc are new, began suddenly. Enguine light comes on within a mile of driving after extinguishing.
Submitted: 1 year ago.
Category: Mitsubishi
Expert:  Doug replied 1 year ago.
Hi,
When was the last time the throttle unit was serviced?
Customer: replied 1 year ago.
don't know, the idle is lower than normal. It has been in shop 6 wks. vaccuum system has been checked
Expert:  Doug replied 1 year ago.
Thanks, ***** ***** raises a flag there.
Is the vehicle present and able to be looked at, or is it still in the shop as we speak?
Customer: replied 1 year ago.
at shop
Customer: replied 1 year ago.
but I can see it
Customer: replied 1 year ago.
if phone call costs just type plz
Customer: replied 1 year ago.
?
Expert:  Doug replied 1 year ago.
I'm sorry we have a glitch when the phone call request goes through... I will try posting again.
I have two different tests/inspections I would like to make.
The first one is as follows:
Start the engine and let it idle.
Watch the tachometer and press the AC button. Does the engine speed go up when the button is pressed, or does it remain the same and/or drop when the compressor engages?
Steer the wheel hard to one side to load up the steering pump; when the pump loads up does the engine speed go up or down?
Let me know on those, and then we will perform the other inspection.
Customer: replied 1 year ago.
I'll reply tomorrow
Expert:  Doug replied 1 year ago.
No problem.
In case you get here before I do, if you are seeing the engine speed increase correctly then I would like you to next pull the intake hose off the throttle body and inspect the inside for any signs of debris/gunk on the inner walls. If there is anything other than silver in there (open the throttle for a full view) we will want to clean it thoroughly.
Customer: replied 1 year ago.
When first starting (today) the idle was 1000 rpm, turning on A/C the rpm dropped to about 400 and the engine faltered. when warm turning wheel to right rpm does not change, A/C rpm drops
Expert:  Doug replied 1 year ago.
Thanks, ***** ***** pretty incriminating information right there.
The purpose of that test was to determine what the IAC (idle air control) valve is doing... and the answer is a pretty solid "nothing".
Under normal operation the IAC will open and shut to adjust the secondary air flow through the throttle body. This is done to help the engine idle properly similar to a choke on older cars.
When the engine is started it will normally adjust to get your idle to a reasonable rate since cold operation and hot operation require very different amounts of secondary air flow. This is a big portion of your difficulty with the idle in general as well as take off.
The AC check was to force the IAC to move. When the AC is turned on the computer sends a signal to the IAC to open allow more air to enter the engine, compensating for the additional load of the AC compressor. When working, pushing the AC button will cause the engine speed to increase by 100-200 RPM, then when the AC compressor engages it will settle back to the original engine speed (sometimes plus 100 RPM, depending on the load).
With the AC causing the engine speed to drop from compressor load and -not- raise before engagement, that IAC is not working. You can check the connections for damage/corrosion as well as the wiring for any signs of rodent damage etc, but otherwise it is likely just going to be a faulty IAC... they are pretty common to fail. I usually recommend factory ones on these, as we did used to see some aftermarket ones short out prematurely causing additional issues, but here of late we haven't seen a lot of that anymore so aftermarket is probably passable at this point (and a ton cheaper).
It would still be a good idea to clean out the throttle as I mentioned before, but knowing that the IAC is not idling-up properly which will cause your issues anyway, that needs to be a priority.

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