One dim headlight and stalling truck

Question: My right headlight has become very dim. With the help of a neighbor I replaced the bulb but it didn't help. Do you have any ideas of how I could fix this myself?

Answer: Please check the brightness of the right front parking light and turn signal, comparing them to the left side. If they are similarly dim, the problem is a loose/dirty ground connection. This is typically a black wire that is attached to the vehicle body with a screw - probably within a foot or so of the headlight.

If it's just the headlight that's dim, try wiggling the headlight connector while observing brightness. If changes in brightness occur, the terminals within the headlight connector may be loose or corroded.

If this doesn't point to the problem, check to see if your vehicle employs separate fuses for each headlight, and remove/reseat each fuse as a test. If none of this helps, professional diagnosis will be needed.

Q: My 1992 Ford pickup truck has been acting up in the last year.

I have taken the truck to several different good mechanics, but nobody seems to figure out what is going on with it. My truck runs really well, but when I put it in park and/or neutral, the motor dies.

I am wondering what you think of it and if you have encountered that kind of problem before. You seem to know a lot about motors and I am excited to hear what you have to say about it.

A: Your confidence is appreciated but may be a bit optimistic. Many times stalling problems on vehicles of this vintage can be rectified by cleaning the throttle body and giving careful attention to the idle air control device.

Your throttle can build up carbon deposits on the back side of the throttle blade and throttle bore, affecting closed-throttle airflow.

The IAC (idle air control) device allows a metered quantity of air to bypass the throttle, adjusting idle speed in compensation for loads such as the air conditioning, power steering, and drive or reverse selection. Should the IAC device also become dirty/sticky it may not be able to wear both hats effectively when sudden load changes occur, with a gunked up throttle.

Cleaning both the throttle and IAC is a first step, followed by a check of IAC responsiveness.

Other possible causes of at-idle stalling could be a flaky throttle position or MAP (manifold absolute pressure) sensor, a connector or wiring harness fault that momentarily breaks a circuit as the engine shifts in its mounts, or possibly a loose battery or ground cable, or computer ground.

If the truck stalls frequently, it's certainly worth checking the scan tool's data list for oddities leading up to the stall out.

A '92 vehicle employs the earlier OBD-I diagnostic system that provides limited but useful system information. I'm not sure why your local techs aren't getting this resolved if the problem can be readily duplicated. If it's intermittent, that makes things a lot more difficult.