Am I right or wrong??

[Edgefevah]

So I start my truck and it idles at 2000rpm. Unplug the IAC and it goes down to 900-1000. Is this a tell that the IAC is bad?

I have searched numerous times here so I am well aware that there are other things that can cause this.

I am planning on getting a multimeter here soon to test the wires and what not.

But for now, what's the BEST way to test whether the IAC is working?


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[martin]

My 91 idles at 800, if I unplug the iac it drops to 400. I would guess yours is working.

 

[RonD]

I would say you have a Vacuum leak first, or throttle plate is not closing all the way.
If you did the throttle cable "mod" you may have tightened it too much.

When you unplug the IAC(idle air control) valve and idle goes down that means the valve inside is moving, so it is not stuck.
The IAC is not a sensor it is a controlled "vacuum leak" operated by the computer, IAC valves are used on all EFI(fuel injected) engines.

When engine is first started computer will open IAC valve all the way, RPMs should go up to 1,500+, so high idle on startup is normal, then the computer checks ECT sensor to get the engine(coolant) temperature, cold temp should cause idle to drop to 900-1,000rpm, warm engine temp 700-800rpm, this is assuming gas pedal(TPS) is not touched during startup.
Cold idle will slowly go down as engine warms up, 5 to 8 minutes of running should get idle down to about 750rpms, time it takes depends on outside temp.

I would warm up engine then unplug IAC, if idle is above 600rpm check that throttle plate is closed all the way, then unplug 1 vacuum line at a time on the intake and plug that port with your finger, see if idle drops.
PVC valve is suppose to be a vacuum "leak" at idle, so skip that hose, unless it is cracked.
If all ports seem fine then you will need to use starting fluid or carb cleaner around the intake seals to see if there is a gasket leak, 4.0l often gets intake/head leaks, re-torquing intake can usually fix it.
You would get a surge in RPM if carb cleaner is sucked into engine via vacuum leak.

 

[Edgefevah]

So I went out and did a video of startup and warmup of my truck. currently loading onto photo bucket.

at start up it idles @ 2000-2100rpm then slowly over a 5minute time period gets down to 1250-1350rpm. at which point it does start to surge a little.

After I let it run at 1250-1350rpm for awhile I unplugged the IAC and it idles down to 1000rpm which is a lot better. I then plugged the IAC back in and it goes up to 1250rpm.

Check every vacuum port attached to the upper intake and did not notice any change in idle. I haven't done the Lower Intake Gasket check yet as I couldn't get to the store to buy some carb cleaner.

The other 2 things I want to look into are the MAF and the temp sender(the one on the drivers side of the front of the lower intake.)

Also found out that someone had put the "zip-tie mod" got rid of it and no change.

 

[Edgefevah]

bump

 

[RonD]

Can you close the throttle plate more manually?
On some throttles there is a set screw under the throttle cable linkage on the intake, make sure someone hasn't set it too high, engine should idle at 500-600rpm with IAC unplugged.

Unplug IAC, block PCV valve hose, remove air plenum from intake manifold, have a look at the throttle plate, make sure it is closed all the way, open and close it manually a few times, there should be only a slight gap at top and bottom.
Start engine, put your hand over intake opening slowly, starving engine of air, as hand completely covers intake engine should stall, if it continues to idle, stumbling from lack of air, you should hear the vacuum leak.
If it stalls then vacuum leak is "probably" not the problem.

Make sure TPS(Throttle Position Sensor) is not holding throttle plate open, you can remove it and see if throttle plate closes all the way.

Also if you have cruise control check to make sure that cable isn't holding open throttle plate a bit.

 

[Edgefevah]

Well RonD I looked at the throttle plate and I can't close it anymore then it is. so I presumed to put my hand over the intake to see about vacuum leaks, the engine died right away which is a good thing So i checked that set acre that everyone tells you too NEVER touch and sure enough it had been touched by someone so i backed it off till it was just touching and sure enough it idled down to 900-1000rpms make me happy happy happy!!!!!!

 

[RonD]

Good job