Testing IAC

[jotapulga]

Hi all,

Anyone ever tried applying 12V DC to the IAC and seeing how much pintel/seal travel there was? I tried the above and didn't see any noticeable movement, even reversing the polarity. All I heard was a solenoid or vacuum type click.

 

[Maniak]

I can usually tell it is going by how it starts.. If the idle doesn't jump to 1800 or so on initial start (and come right back down) I start to worry..

When I'm actually checking it.. I turn on the a/c and if the idle doesn't go up slightly (slightly higher than it was without the a/c) I know my aic isn't working right..

at this point I clean it.. if it works better great.. if not.. I buy a new one.

~Mark

 

[jotapulga]

Maniak,

I tried the A/C test, here's how it went.

Engine warm, in drive, stopped at a light, turned on A/C, no change in RPM, rough idle (as usual).

Shut vehicle off for approx. 2 hours.

A/C still on, started vehicle, drove for approx. 15 miles, stop & go/fwy, idle higher than normal 800-900 RPM and smooth, stopped at a stop sign, turned off A/C, idle dropped and rough again.

Sound like a bad IAC?

 

[Maniak]

The first test you did does sound like the AIC..

The 2nd test doesn't.. the 2nd test shows that the aic was running (at least something was lifting the idle.. and the aic is the one that normally does it)..

The AIC should have lifted or at least kept your idle the same (it might fluctuate a little but it will at least stay the same) when the a/c turns on.

you might have a more serous problem. Before we replaced the heads on our '92 our idle was always rough.. no one could tell me what it was.. so I did a compression test.. and found 1 cylinder only had 40psi (rest had 160).. since they are all supposed to be over 110psi and within 10% range (from lowest to highest) I knew something internal was bad..

It might be time to do a compression test.. (This is assuming you spark plugs and cables are good).

~Mark

 

[TPLYNCH]

Jot, do you have the SOHC engine?? rough idle is a symptom of the bad manifold gaskets (O rings?). Search 00M12 for more info. Its part of the extended 72K warranty for the tensioners and these gaskets on the SOHC 4.0

 

[jotapulga]

Thanks for the info on 00M12. I had that done last year as PM. The rough idle started after I got it back from the dealer after a CEL (P0303). They did a carbon blast, fuel injector cleaning, replaced a coil pack, plugs & wires. I personally have cleared the DTC 4 times but the P0303 keeps coming back. I've checked the #3 plug and wire. Resistance is OK and it's getting spark, though the plug looks brand new like it's never ignited fuel. I used a stethoscope on the #3 fuel injector and it's clicking. It's been back to the dealer 3 times and they can't locate the problem. It seems that every time I take it in something goes wrong so I've decided to start doing the non-warranty work myself. I've learned alot monitoring this discussion group.

Here's a list of what has been done to it since the P0303 and the rough idle.

Cylinder carbon blast
Cleaned IAC/Throttle Body
Cleaned EGR - installed gasket, there was no gasket before
Replaced MAF (got a DTC)
Replaced DPFE (got a DTC)
Replaced fuel filter
Replaced Coil Pack (extended Warranty covered it)
Peplaced Plugs/Wires (though the #3 wire has a gray boot and the rest have black boots)
Replaced both aft CAT O2 sensors (got a DTC)
Checked EVAP solenoids/hoses (front & rear)
Checked EVR
Checked PCV

Do do next:
Check Crankshaft Position Sensor
Yank Upper Intake Manifold & Clean
Replace #3 Fuel Injector

Any other ideas?

 

[TPLYNCH]

Are you/they sure they replaced the O-ring gaskets correctly. Someone else on the board had it done,and they screwed up and didn't replace them properly and had the same problem....

 

[jotapulga]

TPLYNCH,

I'm not sure they replaced the O-rings correctly. After the 3rd time taking it back to the dealer I asked them to take it apart and put it back together again, thinking that one of the intake manifolds wasn't installed or aligned properly. They told that a 25-year ASE Certified Master Mechanic had checked it out and he could find nothing wrong. (Take that for what it's worth.) I'm hesitant to pull either of the manifolds myself. I think all I'll need to put it back together again are an upper intake manifold gasket and lower intake manifold gasket and O-rings. Sound correct? I'm fed up with any dealer's work at this point. It was running great before the dealer touched it, even with the P0303 CEL.

 

[pwood75]

i just replaced my iac on my '95 the other day, and i was having the exact problem that you described. when in park, mine would idle up and down from about 500rpm to 1800 rpm and sometimes it would just die. I could sometimes get it to stop surging when i shut off the a/c. when i would put it in gear while sitting still, it would not surge. mine would also drop in rpm when coming to a stop, almost to the point of dying. i picked up the part from o'reillys for about $78. don't go to autozone, they don't even stock the correct one. if you are not sure that it is the iac, then you could also have a vacuum leak, or an intake manifold leak. if you think your intake manifold gasket is sucking air, try spraying carb cleaner around the edge of the manifold, and if it idles up, you could have a leak.

 

[jotapulga]

I'll try the carb cleaner test. My idle jumps between 625-650 about every second or two. I know that isn't much but it really makes the truck shake. When I come to a stop the idle dives like it's going to die. I'll probably end up buying an IAC. Though after dumping so much cash into it lately (and getting nowhere) I'd like to be sure of the problem.

Thanks.

 

[Spas]

Did you ever find out what was wrong with it?

 

[Opera House]

Since no one mentioned it

The IAC has an internal diode and if you reverse the polarity as one poster said he did, you will either short out the diode or burn it open. Either of these are bad for the computer. The voltage is pulsed 12V and the diode absorbs the kickback voltage. Shorted or open it will destroy the driver transistor. If you test it on a bench, always use a brake lamp in series with the power to limit current. One test uou can do on the vehicle is take the white wire and stick a pin in it to make contact. You can then bring this wire to ground through a lamp and the engine will speed up.