1991 3.0L High Idle (3000 RPM)

[mpriley05]

Hello,

My 91 Might Max V6 3.0L has been idling at 3000 RPM at startup for a while now. It won't go down, even after driving. I just replaced the idle air control, but it didn't fix anything. The vacuum line looks OK, and it's not a stuck throttle cable.

Anything else I can check?

Thanks

[mpriley05]

I just tried unplugging the IACV, and it still idles at 3000 when unplugged.

[broken7vessel]

Wow, a 3000 rmp idle is high!

Check your throttle linkage. Make sure it's not binding on anything. You can spray some silicone spray into the cable to keep it moving good. Lightly lubricate all your throttle linkages while you are it.

You can also tell by looking in the throttle body at the butterfly valve. Make sure it is closed with no carbon build up around it. With the engine off, you can have an assistant press the gas pedal while you look at the butterfly valve open. If its dirty, spray a little carb cleaner and clean around the opening with an old toothbrush. Remember, spray only a little at a time. Also check the idle adjustment screw, try backing it off to see if that helps.

One other thing is to check for vacuum leaks.

[mpriley05]

Thanks for the reply.

The butterfly valve is completely closed both at idle and when the truck is off, and the throttle cable doesn't seem to be sticking at all. I used carb cleaner to check for vacuum leaks, but didn't find any.

I drove the truck around a bit last night to see if the ECU just needed to relearn the new IAC, and now the idle is around 2000. That's still way too high though.

It threw a code while I was driving, but the light went off before I got back, so I couldn't pull the code. I'll try again today.

[mpriley05]

I just pulled the intake hose. I can get the truck to stall by covering the throttle body intake with my hand, as well as control the idle by covering the two lower bypass holes with my fingers.

I took out he new IAC to compare it to the old one, and sure enough - it's shorter than the original. I guess it's not long enough to do any work. Anyone have a proper part number for a direct replacement? The one I bought was from 1AAuto.com (part # 1AISC00011)

I also checked the resistance on the old IAC - one of the coils was dead.

[broken7vessel]

Hello again,

That IAC part number shows to be a direct replacement for your engine. Aftermarket replacements can look different than the factory part. Unbolt the IAC but leave the wire connected. Have an assistant cycle the key from off to on (but don't start it). Look to see if that little rod moves in and out with the key cycle. This will confirm that the ECU is sending signal to the IAC, and the IAC is responding. ((I've never tried this, but it's worth a shot: while you have the IAC unbolted, see if you can plug that space on the throttle body somehow and then run the engine with the wire connector plugged in and then unplugged. Observe the rod movement and idle speed.))

Your engine is idling high because of one thing: more air is getting into your engine than it should. Look more into the bypass holes you mentioned. Look for air getting by through another hose hooked into your throttle body / intake manifold. There are multiple emissions purge lines, which are controlled by solenoids, etc. One of these could also be malfunctioning and letting air in past your throttle. Spraying carb cleaner around is good trick for detecting leaks, but only for cracks, unplugged hoses, etc.

It sounds like too much air is bypassing thru a hose somewhere connected to your intake manifold. But first you need to eliminate the IAC has a cause.

Observation, elimination, observation, elimination.... Keep me posted of out how it goes. I have a 91 Mighty Max, but it has a 2.4L. I am not that familiar with the 6G72 engine, but I work on lots of different cars.

[mpriley05]

Thanks for the reply.

I took a video of those two holes in the throttle body on the upstream side of the throttle plate: https://youtu.be/S8SFk97iSXs
One goes down to the IAC, but I have no idea where the other one goes. I can plug one hole, and it slows the idle a bit. Plugging both holes stalls the truck. There is a huge amount of air being sucked in through these holes, so if anyone knows what might be malfunctioning to let so much air through, let me know!

The new IAC is getting a signal (vibrates and moves with key on/off), but I can't tell if it is actually working correctly. Does the ECU control the IAC? If so, is this an ECU problem?

[mpriley05]

I manually closed the FIAV (which is supplied by the second bypass hole in the throttle body). The FIAV spring was stuck open, which is apparently pretty common in these DSM engines. I looped the coolant lines, and the high idle is gone. I backed off the throttle plate screw to lower the idle just a bit more, and now it's golden.

[EvilNavyGinger]

I manually closed the FIAV (which is supplied by the second bypass hole in the throttle body). The FIAV spring was stuck open, which is apparently pretty common in these DSM engines. I looped the coolant lines, and the high idle is gone. I backed off the throttle plate screw to lower the idle just a bit more, and now it's golden.

I'm having a similar issue. 1500-1700 RPM idle. New idle control valve, throttle plate is completely closed. Cheaned out throttle body, BUT I have no coolant in the system yet. Overhaul and testing still, waiting on parts and coolant is soon. No radiator installed.
Will adding coolant make the idle come down?
Does the back of your throttle body have something like this in the picture? This thing was kept running by backyard hillbillies and a bunch of stuff is rigged. I pulled the plug and there is a spring in there. The disc inside looks like it will close off the port if coolant pressure is added behind it, but I have no idea for sure.
This plug should be something else I'm sure.
EDIT-
Removed throttle body and plug. Found out it needed closed off so I shoved the plug in far enough to close the air opening. My adjuster is broke off, found half the PO left in there.
BUT idle is still around 1300.
1-- What's cold start RMP?
2--Should the IAC sound like a stun gun? Not loud, just lots of clicks. Brand new.