After making up a diagnostic plan, we ran the car on a chassis dyno at highway speeds, but we could not get the car to hesitate. We had to drive the car on the highway to duplicate the conditions and make the car act up. The problem did occur when the transmission locked up the torque converter clutch, but this also places a higher load on the engine, so we thought it was an engine problem.
Rechecking all the work done previously, including fuel pressure (which causes many driveability problems), we narrowed it down to one cylinder misfiring intermittently. Switching spark plugs and fuel injectors between cylinders did not correct the problem. The spark plug wires were new, but they checked out fine as well: time to look deeper.
Camshaft lift and timing were checked, the intake manifold was removed to check for a hidden vacuum leak, and the cylinder compression was checked. Everything appeared okay. Working in a systematic way with students, the diagnostics had taken three weeks, and we still didn’t have it fixed.
Finally, we replaced the complete distributor with a known good one off another vehicle. The car ran fine! A close inspection of the distributor showed a small carbon track inside the distributor cap where the spark had jumped. This is what had caused the misfire! Now, lest you think I am a little slow and should have found this right away, in my defence, the distributor cap was new. Bob had replaced it himself before the tune-up, to save a little money. Also, the carbon tracking could only be seen with a magnifying glass.
A small $20 part had cost Bob over $4,000 in repair bills! What can we learn from Bob’s experience? First, don’t jump from shop to shop. Each repair shop rechecked the work done by the previous shop and charged for it again. If the first attempt didn’t repair the car, take it back. They know what they did and won’t spend a lot of time rechecking it.
Secondly, tell the repair shop what the problem is, not what you think causes it. If Bob had said the problem was a hesitation, the repair shop would have still done a tune-up because many driveability problems are fixed with a tune-up, but they would have looked specifically for the hesitation. Bob told them to do a tune-up, and that is what they did.
Finally, sometimes finding a problem just takes time. A technician has to make the vehicle act up before they can diagnose the problem, and if the problem doesn’t appear, a technician can only guess at what is wrong. Many costly parts are changed for no reason because of guesses, so give the technician ample time to locate the fault. In this case, time spent on diagnosis would have saved a costly transmission overhaul.