I HAVE REBUILT 5.7 TPI WITH THIS UNUSUAL PROBLEM. WHEN CAR IDLES DOWN AND DURING NORMAL DRIVING CONDITIONS AFTER THE WARM-UP, IT MAKES THIS SHUDDERING BUMPING NOISE CAUSING LOSS OF POWER AND SUCH. IT ISN'T AN IGNITION PROBLEM BECAUSE I'VE CHECKED THOROUGHLY. PLEASE TALK TO ME AND TELL ME WHAT YOU THINK.

 

Low power and shudder at low RPM? If the ignition is intact, consider checking compression, fuel delivery, and the valve train.

Lacking any further information, pinpointing the problem can be difficult.

 

If there is vibration and studder at idle, there is likely at least one dead or weak cylinder. Try to isolate which cylinder(s) is weak using a power balance test, then concentrate on that cylinder.

After the engine is running at normal operating temperature, carefully remove the spark plug wires one at a time. Plug boot pliers are very helpful for this, since the spark will try to find any path to ground (you). You may find it easier to remove the wires from the distributor, but there is a chance of crossfire as a result of this practice. Removing them from the spark plugs is a little safer.

As a spark plug wire is removed, the engine idle should change. There should be more shaking and a drop in RPM before the ISC can compensate. If you experience a change, install the wire back onto the plug and move on to the next one. If you discover one that makes little or no change in teh idle, you have found a dead or weak cylinder.