IAC Test?

[chevimp327]

First Post. Bought a 94 TS 3.8 a few months ago and am real sorry I did. But trying to make the best of it. The previous owner said it had an O2 code but I could not verify it as it has the infamous OBD 1.5. I discovered it had a "hunting idle" problem and from this forum I decided it must be the TPS. So replaced it and the O2 sensor as it has 279K miles. It ran great for about 3 days, no hunting idle or CEL. Then the present problem popped up. It has intermittent Hi-idle and no speedometer or odometer function. The tach still works. This problem will go about 15 engine start/engine stop cycles (4-5 days) then for 1 cycle will work fine with normal idle (750RPM and speedometer works). Then back to problem for 15 more cycles. I paid to get the code scanned and it was a 1640 which from the forums I believe was associated with the TCC and evap purge sol. I disconnected pins A and D from Transmission plug with no change in symptom. Checked for vacuum leaks and changed out a couple of suspect hoses, also with no change. Disconnected purge solenoid and plugged hoses with no change. Took out IAC ,cleaned, ohmed contacts A/B @50 ohms, C/D @50 ohms, and adjusted pintle extension back to about 1" as per manual (it was about 1 3/8"). Put it back in and problem went away for 2 WHOLE CYCLES!! Thought it was fixed for good but on 3rd cycle its back. So my question is how to test this IAC out of the vehicle, pwred up to check for pintle movement binding or sticking. I don't want to change it until I know its bad/suspect for sure as I'm cheap and also unemployed at the moment. Also what is the correlation between Hi Idle and no speedometer function? Or is this just one of those PCM sending out crap signals causing the "tail chasing syndrome"?

[Blue Bowtie]

The VSS and IAC are separate systems. The only "connection" is the software in the ECM. The IAC target position calculations will respond to VSS inputs to provide decel fuel cutoff and to add steps once the VSS drops to about 12 MPH to prevent engine stalling.

Short of commanding the IAC with a bi-directional scanner about all you can do is remove it, place it in a plastic bag with the electrical connector in place, and turn on the ignition (without starting the engine). The pintle should retract. There is a chance that the pintle will extend then retract, so placing it in a bag is important to catch the pintle and spring if it extends too far - It's better than trying to find the parts on the garage floor.

If you can connect a scanner you should be able to control the IAC to some extent by commanding a position.

It's good that you removed and cleaned the IAC but applying a couple drops of clean engine oil to the pintle rack gears when the pintle is removed is also a good idea since the cleaners used usually wash out most of the lubrication (lithium grease).

[chevimp327]

Oh happy day. I finally scrounged up the bucks and got a new (refurb) PCM and its been 3 days of normal idle, no CEL, and the pleasure of knowing how fast I'm going. Cruise even works good. Now all I have left to fix is the leaky water pump, inoperative pwr mirrors, driver's side door latch and lock switch, #6 cyl exhaust manifold crack, pwr sliding door motor and latch, broke off rear hatch knob, inoperative steering wheel controls and horn, intermittently staticky radio, and AC. Gonna start with the driver's door latch, if the water pump will hold out, as I've probably walked an extra mile or 2 to the driver's door and back to the passenger side after forgetting its broke. But lots of future Forum search material. Thanks for the help Blue Bowtie.

[Meridivs]

I recommend you consider changing the water pump immediately since a serious leak can develop a lot faster at this stage leaving you stranded on some remote road.

We had the same symptom a few months ago and it took 4-6 weeks to go from a small leak to a major problem which would render the car unusable.

A water-pump with gasket will go for as low as 20$ on rockauto.com, way better than towing your car or replacing and engine after a cooling-system failure.