Iac question

[Mike1919]

Does that look normal?

 

[RonD]

My '94 4.0l Manual trans idles at 650 warmed up.

Turning on AC(and defrost) sends 12volts to the computer so it will use IAC to idle a bit higher, about 750.

So no, it is not normal IMO.

It reads like AC compressor is pulling down the idle which IS normal if it was high to begin with.

 

[Mike1919]

Yeah thats what I'm thinking. That top picture is about 650 right? It looks like the computer always thinks the compressor is engaged causing the high idle.

 

[Mike1919]

So where does the ecu get the signal to turn up the idle to compensate for the load of the compressor?

 

[ranger1997]

When the compressor is powered on the ECU also receives a signal from the circuit to increase idle.

Also the power steering has a pressure switch that sends a signal to the ECU to increase the idle for parking etc.


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[Mike1919]

Although we can rule out any electrical issues because it runs fine without the iac connected huh?

 

[Earl43P]

When the compressor is powered on the ECU also receives a signal from the circuit to increase idle.

Also the power steering has a pressure switch that sends a signal to the ECU to increase the idle for parking etc.


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Only the 4 cylinder has the PS pressure switch.

 

[RonD]

Although we can rule out any electrical issues because it runs fine without the iac connected huh?

It should idle at about 500rpm or stall with IAC unhooked, it shouldn't "run fine".
If idle is higher then there is either an air leak or throttle plate mis-adjustment.
Idle must be down that far so IAC can set a minimum idle, 650rpm.

 

[Mike1919]

Well then it appears i would have a vacuum leak. Is there any way to check if its the lower intake gasket before tearing into it?

 

[RonD]

I like to use starting fluid(ether) in a spray can.

Engine warmed up, IAC unhooked:
First eliminate smaller hose leaks, unplug each hose from vacuum manifold, one at a time, and plug the port with your finger, if idle drops then that hose has a leak.
PCV valve hose is different since it is suppose to leak a bit at idle, test that hose with engine off make sure it has no cracks, replace PCV valve with correct stock valve, they are NOT all the same, the amount of air they "leak" is designed for the engine system.

After confirming none of the ports on the vacuum manifold has a leaking hose, check caps as well, then time to use the starting fluid.

Just so you know what to listen for, remove a smaller vacuum hose from vacuum manifold and spray some ether at the open port, RPM should increase and you may hear a pinging noise, ether has a lower ignition point so can pre-ignite.

I use a short hose for lower intake, place hose near lower gasket and spray into hose, pretty simple, move hose around until you have tested the areas.

Ether does ignite, thats the point of using it, so watch it around hot exhaust or spark plugs and coil, but it does evaporate very fast as well, which is why I like it better than brake/carb cleaner.

 

[Mike1919]

Thanks for the replies. I have ruled out all if the hoses by unplugging each one and plugging it with a cap and nothing changed. I've also been spraying ether all over where the plenum meets the intake. I'll test the lower intake tonight. The head gaskets were done 50,000 miles ago so I wouldnt think the intake would have a leak unless the bolts already backed out.

 

[grumpyguy]

Thanks for the replies. I have ruled out all if the hoses by unplugging each one and plugging it with a cap and nothing changed. I've also been spraying ether all over where the plenum meets the intake. I'll test the lower intake tonight. The head gaskets were done 50,000 miles ago so I wouldnt think the intake would have a leak unless the bolts already backed out.

Mine did in 20,000.

 

[Mike1919]

Well i read the writeup on replacing the intake gasket and it seems iI have the easier version being I dont have an egr valve.