Dear Tom and Ray:

I have a 1987 Pontiac Fiero GT with a V-6 engine and a five speed manual transmission. While the engine is running, the tachometer needle wiggles up and down plus or minus 500 rpm, seemingly randomly. Strangely enough, this problem goes away when the car is accelerating, but returns when decelerating or idling. Over time, this problem has extended to a pulsation of power from the engine that I can feel, but can't hear. What do you think the problem is?
Carlton

RAY: Well, of course you can't hear anything, Carlton. You're driving a Fiero.

TOM: I think the problem is a worn out timing chain. The timing chain drives the distributor shaft, which send the signal to the tachometer. And if the timing chain is stretched, the distributor shaft will wobble, and the engine firing will be erratic.

RAY: When you step on the accelerator, the increased torque (rapid twisting) of the crank shaft takes up the slack in the timing chain, so the problem disappears during acceleration. But as soon as the tension is released (when you take your foot off the gas), the timing chain loosens up and the tach starts fluctuating again.

TOM: By the time you read this, your timing chain will probably have broken, in which case you'll know that we're right. If not, have the chain checked out right away. If you act quickly, you may save yourself some towing charges. Good luck.