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QUESTION & ANSWER: Engine suddenly stops; headlights dim at stops

Jim Kerr QADimming headlights
123RF Stock Photo

By Jim Kerr

Q: I own a 1993 GMC Vandura 3500. It has a 350 CID engine with throttle body injection. I have been having problems with the engine suddenly stopping.

If the van has been sitting for a period of time and I start the vehicle to drive away I will travel for a few feet or a short distance the engine will suddenly quit running.

The check-engine light comes on however when I plug in the code reader the 12 code comes up (system pass) then no other fault codes.

I then pour a little gas into the throttle body and the engine will start up again and the check engine light clears.

I will not have this problem again for the remaining distance I travel.

The van has a full tank of fuel and I changed out the fuel pump and filter.

I don’t want to keep guessing so I am asking if you have any ideas as to what the problem would be.


A: There are a couple likely problems. One is that the base gasket beneath the throttle body would be sucked in by engine vacuum and create a small vacuum leak. If the vacuum leak gets big enough, then the engine can idle rough or stall.

On this year of vehicle, the computer was not smart enough to adapt to vacuum leaks and used a set program based on engine temperature to position the idle air control valve in the throttle body. If there is a vacuum leak, the computer would still position the idle air control valve at a preset position and this can cause the engine to stall.

The other common problem was a sticking idle air control valve. The computer moves this valve in and out in steps to control idle speed. For example, when the air conditioning compressor cuts in, the computer moves the valve out to let more air into the engine.

The increased engine load by the compressor is compensated by the increased air intake. The engine idle stays the same but it also doesn’t get lower due to the increased engine load.

If the valve is sticking, the computer will try to move it to the correct position by sending electrical pulses to the valve motor windings. It doesn’t actually know if the valve moves, so no code is set for a sticking valve.

Replacing the idle air control valve is an easy task. Simply unplug the four terminal electrical connector and unscrew it from the throttle body. Make sure the ignition is off during this time.

Screw the new valve in place and reconnect the electrical connector. To reset the valve position, jumper terminals A and B on the computer system diagnostic connector under the dash and turn the key on.

Wait for a couple minutes and then turn the key off and disconnect the jumper. You are set to go.

What ever you do, don’t try to adjust idle speed with the throttle stop screw. This is a base setting and adjusting it will cause the computer to control the idle air control valve improperly.

Q: I have been driving at night in my 2011 Mazda 6 and notice that when I come to a stop sign, the headlights will dim and almost seem to flicker a bit, They brighten as soon as I pull away from the stop. What could be causing this?


A: The most common cause of dimming or flickering headlights is a poor battery connection.

When you come to a stop, the charging system is putting out less current while the engine idles. During this time, the battery supplies any needed additional current to operate lights and accessories.

If the battery connections are corroded or dirty, then the lights can dim due to the lower output of the charging system at that time.

Clean the battery connections first. Then check that the connections from the battery to the body are clean and tight. Also clean and tighten any connection from the battery to the engine.

This should fix your problem. If it still persists, have your battery tested for capacity. It may be nearing the end of its useful life.

Jim Kerr is a master automobile mechanic and teaches automotive technology. Send your questions for Jim to jkerr@herald.ca or mail them to: Herald Wheels, 2717 Joseph Howe Drive, P.O. Box 610, Halifax, N.S. B3J 2T2

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