Engine light likely traced to faulty EGR

Hey there, time traveller!
This article was published 13/08/2004 (7833 days ago), so information in it may no longer be current.

QUESTION — The check-engine light for the fuel injection on my 1991 Ford half-ton truck keeps coming on every time I drive. When I start the engine, the light stays out until the engine is warm and the rpms go over about 2,000 rpm. Then it stays on until I turn the key off. I replaced a cracked driver’s-side exhaust manifold because a friend said it could be causing the problem, but the light still comes on. What should I do now?

ANSWER — Your friend has some good advice. The cracked manifold could have caused the light to come on because it allowed extra air to get to the oxygen sensor. However, on this truck the problem is related to engine speed. The computer control system is complex and many things could cause the check engine light to turn on, but I suspect the Exhaust Gas Recirculation (EGR) system.

At about 2,000 rpm, the fuel injection computer opens the EGR valve. The actual movement of the valve is measured by the pintle position sensor mounted on top of the EGR valve and the information is sent to the computer. If the amount of EGR commanded by the computer and the feedback information from the pintle position sensor do not match, the computer turns on the check engine light and stores a fault code.

Checking the fault code can be done with a test light and jumper wire at the underhood diagnostic connector, but I would recommend having a repair shop do this for you. Jumping the wrong terminals could damage the truck’s computer! The code helps a mechanic narrow the search for the fault.

If the code is for EGR as I suspect, then the problem may be caused by a sticking EGR valve, inoperative vacuum or vent solenoid, or a leaking vacuum line. The vacuum and vent solenoids are located at the left rear side of the engine and the computer uses them to operate the EGR valve. They should click faintly when a warm engine is run above 2000 rpm. The vacuum lines run from the solenoids, past the exhaust manifold, and to the EGR valve. That cracked exhaust manifold may be the culprit after all. Leaking exhaust gases may have burned through one of the vacuum lines. Carefully check all the vacuum lines for any leaks or burned areas and repair any that would stop the EGR valve from working.

QUESTION — The fog lamp bulbs on my 1990 Pontiac Grand AM burn out after only a few hours of use. I purchased this car used two years ago and already I have put in seven new bulbs. I doubt that all these bulbs had manufacturing defects, so what is causing them to burn out?

ANSWER — Two reasons for the fog light bulbs burning out prematurely come to mind. The first is oil contamination on the bulb. The bulbs are a halogen type that operate at much higher temperatures than regular bulbs. If there is any oil on the glass, then that area of the bulb operates at a different temperature and material vaporized from the bulb filament collects there. Soon, enough material has been lost from the filament that the filament “opens” and the bulb stops working.

Holding the bulb with bare hands can place oil from your skin onto the bulb glass. The remedy is to clean all replacement bulbs with rubbing alcohol or methyl hydrate before installing them. Then be careful not to touch the glass surface!

Excessive vibration may be the second reason these bulbs are burning out. There is supposed to be a wire retainer clip that holds the bulbs firmly in place and reduces vibration. I have seen cars with these clips missing. Check to see if your car has them.

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