My 1988 Mustang has an excellent idle. However, when I stop at a traffic light/stop sign occasionally the engine will shudder, shaking the car a little and then die.

It does not matter if it is winter or summer or if the car has been driven just a few miles or many miles around town completely warmed up. I can go days or many miles without the engine dying. Then out of the clear blue sky the engine will shudder and die and it may or may not happen again during that drive.

A friend told me it might be caused by a faulty fuel pressure pulsation dampener. Because of the age of the car, this part has been difficult to purchase. Do you agree? Or, what else might be causing this problem?

Jim Thomas, Huntsville, Ala.

If there’s one thing I’ve learned in this business, it’s that anything is possible. I believe the occasional roughness and stall-out you’ve encountered might be caused by a dirty throttle body, a sticking idle air bypass valve or a dirty exhaust gas recirculation (EGR) control solenoid vent-filter. Let’s check these areas before considering replacement of the pulsation dampener.

Modern vehicles use an idle air control (IAC) valve to regulate idle speed. Most IACs are an electric solenoid/pintle valve, which allows a metered quantity of air to sneak around the closed throttle, providing cold-start fast idle and compensation for accessory loads. The powertrain control module (PCM) pulses the IAC as needed to obtain desired idle speed. A sticky IAC or dirty throttle can cause roughness/stall-out during abrupt maneuvers such as a rapid stop or sudden U-turn. Throttle body dirtiness occurs because of carbon build-up and can be easily cleaned, using an approved solvent and a rag.

EGR valves deliver a small quantity of exhaust gas back through the engine a second time, reducing oxides of nitrogen, a toxic exhaust emission. EGR operation does affect engine performance and must be accurately controlled to provide acceptable driveability. A solenoid valve, somewhat similar to the IAC, delivers vacuum to the EGR valve’s diaphragm at part-throttle and above (never while idling). A vent-filter prevents vacuum from being accidentally trapped as the engine returns to idle. Should the EGR valve linger on during a rapid stop because of a dirty vent-filter, extreme roughness and stall-out are likely. The filter resides beneath a black plastic snap-off cap atop the EGR control solenoid.

Readers to the rescue: Remember Windel Newby’s occasionally clattering Tempo? Jerry Whittle and Ken Ziolo reminded me of another possible cause, a faulty harmonic balancer, which is a common fault on that car model. This part bolts to the front of the crankshaft, and contains or is attached to the bottom belt pulley. The balancer consists of a metal hub and outer ring, separated by a rubber band. Should the balancer begin to wiggle apart, clattering or rubbing noises are very possible. Good job readers!

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.

Write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190; or e-mail to under-the-hoodjuno.com. He cannot make personal replies.)

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AP-NY-11-25-05 0614EST