Tech: Solving the 3.0l V6 Idle Problem Short Cuts

Author/ Photographer: Phil Hansford, June 30, 2000 (Drawings: 89 Factory Service Manual)



tn_tach.jpg (3299 bytes)
Tired of seeing that tach vibrating at idle?

If you have a Mitsubishi 3.0l V6 with a few miles on it, chances are you have at some point experienced this: In neutral, or with the clutch in, the idle surges up and down by itself, between ~600 and 1500 rpms. If you took it to a garage, they probably told you that the whatchamacallit needed to be cleaned out, and a hundred dollars later, you walked away poorer, but satisfied that at least your tach was steady again. This article will show you how to clean out those nasty carbon deposits, so you can spend your hard earned money on more essential things, like gasoline (Sorry, no tips to get around that one!). You can use any of the commercially available throttle body/carburetor spray cleaners, or a gasoline-soaked rag as your cleaning solution.

Step One: Identifying the problem

tn_throttle_body_apart.jpg (4450 bytes)
Exploded view of the throttle body/air intake

This problem is almost always caused by carbon deposits which form on the air intake, throttle valve and components of the throttle body, most importantly the Idle Servo Control Motor (sometimes referred to as the ISC, IAC, or "stepper motor"). These components are quite accessible under the hood, and can be easily cleaned out, in just a few minutes.

Start by identifying the parts you will be working on: (Refer to the "exploded view")


Step Two: Open it up!

Now that you have identified everything, its time to dive in! Start by loosening the hose clamp that keeps the air intake onto the throttle body. Its just one bolt, which you can loosen with a Philips screwdriver, or a small socket. When its sufficiently loosened, firmly pull the hose off the throttle body, and pull it to the side, out of the way. Note the small rubber hose which plugs into the underside of the air intake hose: don't forget to put this back on after you're finished cleaning, or you will create other problems. Now that the air intake hose is off, you can look right into the mouth of the beast: you'll see a black, soot-like deposit over everything.

tn_snorkel1.jpg (6647 bytes)
Left hand (driver's side) look at the throttle body/air intake
tn_snorkel2.jpg (6133 bytes)
The hose clamp you need to remove to pull off the air hose
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Don't forget to reaattach this small rubber line on the underside at the end.
tn_throttle_blade1.jpg (7001 bytes)
The mouth of the beast! Notice the brass colored throttle blade inside.

Step Three: Removing the ISC motor

tn_stepper1.jpg (11713 bytes)
The ISC motor as it looks after the air hose is out of the way.
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The ISC fasteners are highlighted, but unplug that harness first!
tn_ISC.jpg (6112 bytes)
tn_isc2.jpg (4624 bytes)
A closeup of the ISC fasteners, and a disassembled ISC.

With the throttle body opened up, go ahead and remove the ISC, or stepper motor. Start by disconnecting the harness, then remove the Philips head bolts which keep it on. Be careful of any gaskets that may be in place. These bolts may be sealed with silicone, so be patient when removing them. Once the ISC motor and the air intake hose are off, its time to do some cleaning.

*Be aware that these carbon deposits are very dirty, and usually require liberal amounts of solvent to remove from your hands!

If you really want to get down to the nitty gritty, you can also remove the throttle body from the plenum: just remove the Throttle Position Sensor, and be prepared to silicone the gasket in between the throttle body and the plenum when you button it back up.This is not essential for success, but it may allow you to remove more of the soot.

Step Four: Clean it out!
tn_air_hose.jpg (5384 bytes)
Plug this hole before spraying solvent into the throttle body!
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Don't use solvent on the rubber seals and gaskets: be sure to clean the tip.

Start by stuffing a rag down the bypass inlet (small hole just before the throttle blade), to keep the solvent out of the passage. Spray solvent into the throttle body, and let it soak, before removing the deposits with a rag. Alternatively, use the gas-soaked rag to rub off the deposits. Be sure that the throttle body walls and blade are clean.

When cleaning the ISC motor, be careful to avoid the rubber insulators and gaskets with the solvent, as it will dry out and crack rubber.  Instead use a clean rag on the rubber parts, and solvent on the metal parts.

Allow all these parts to dry, then reassemble the throttle body, and your idle should be as smooth as peanut butter! I'd also throw a bottle of "fuel-injection cleaner" into the gas tank, as well (An additive with "Techron" in it seemed to work good for me). If you're looking at "chunky peanut butter", and you still get stumble during idle and/or acceleration, you'll probably want to look into a professional fuel injector cleaning.

*Special thanks to Lloyd Schwartz for diagnosing this problem for us from the beginning!

Contacts: Related Links:
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