IAC valve likely cause of vehicle’s stalling problems

Hey there, time traveller!
This article was published 20/09/2002 (8437 days ago), so information in it may no longer be current.

QUESTION — I have stalling problems with my 1988 Cutlass Cierra Brougham. It has a 2.5 litre 4-cylinder with 128,000 km, mostly accumulated on long trips. The engine runs good at high speeds. The problem is when I stop, it stalls. Most of the time it stalls when the air conditioner is on and I am stopped at a stop sign with small incline.

It seems like I need a little more fuel or rpm, but it’s not there. If it doesn’t stall, it hesitates. If the car is cold and engine rpm higher, it’s OK. Here is what has been changed so far: gas filter, engine computer, computer chip (revised prom), basic injection tune up, injection flush, and sparkplugs. The fuel pressure, engine computer scan, and all sensors are normal. Can you help?

ANSWER — It sounds like you are having a problem with either the idle air control valve or the minimum idle air setting. The idle air control (IAC) valve is a stepper motor valve fastened onto the throttle body and controlled by the engine computer. The computer sends electrical pulses to two separate windings inside the valve to move it either in or out. This controls the flow of air through a bypass passage in the throttle body, which in turn determines engine idle speed.

I have changed many IAC valves because they were sticking and causing stalling problems. Dirt or varnish inside the valve causes them to stick, but cleaning them internally with a chemical cleaner will damage the electric stepper motor, so replacement is the best solution. However, before buying a new valve, the minimum air rate should be checked.

Newer cars have an adaptive learning ability for the IAC position to adjust minimum air rate, but your car has only set positions programmed into the engine computer. The minimum air rate is adjusted by grounding terminal B of the diagnostic connector beneath the dash while the ignition is on. Disconnect the wires to the IAC valve, with the ignition still on, and start the engine. A warm engine should idle at 550 to 600 rpm with all accessories off. If the idle speed is incorrect, clean any varnish around the throttle plates, and then adjust the idle stop screw to the correct idle speed. Reconnect the wires to the IAC valve and drive the car over 40 kph to allow the computer to reset the IAC position. This should cure your stalling problem.

QUESTION — My 1988 Pontiac 6000 is experiencing some clunking noises that I hope you can help me with. About two years ago, I replaced the struts, springs and ball joints. Within about six months, I started to hear some clunking noises in the front end whenever I encountered potholes or other smaller discontinuities in the road. My main concern, however, is that with hard turns either left or right, the car experiences a loud clunk in the driver’s side front suspension. Successive turns in the same direction yield no noise, but as soon as a turn is made in the opposite direction, the clunk is heard again.

I took the car to a garage but they could not specifically isolate the problem, suggested it may be the strut and advised that I should wait until it got worse. I am thinking it may have something to do with the strut mounts. Do you have any suggestions on what it could be and how to confirm the problematic part(s)?

ANSWER — Because the noise occurs when the car is turned hard either way, the struts would be the logical place to start looking for problems. I doubt if the top strut mounts are causing the problem, unless they are loose. A bad top mount usually binds, so that the steering is stiff or doesn’t return to centre properly.

Instead of an internal problem with the strut, I would guess the noise is caused by the coil spring snapping into a different position each time the wheel is turned. Perhaps a plastic insulator at the end of the spring has broken, allowing the spring to bind, or the spring may be broken itself. Inspect the coil spring carefully for breaks or rusty wear marks and I think you will have found your problem.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca