by Jim Kerr

Walk into any repair shop and ask the technicians what kind of automobile problem is the hardest to diagnose. They may answer “squeaks and rattles”, or “vibrations”, but most would tell you that “driveability problems” are the hardest to repair. Driveability encompasses a very wide range of vehicle problems, from transmission shifting to hard starting. Driveability problems can cost a lot of money and time.

A while back, I had the task of locating a driveability problem on a co-worker’s Buick Park Avenue. The make and model are not important because this story is typical of many cars and trucks on the market. This owner, let’s call him Bob, had a hesitation or stumble in his car whenever he was cruising at 80 kph. The hotter it was outside, the more the problem appeared, but when he stepped on the gas, the car accelerated fine.

Bob does a little of his own work, but hadn’t had a tune-up in a long time, so he took the car in and asked them to do one. The sparkplugs were replaced, timing was checked, and all the other usual inspections were done. Everything looked good. The next warm day, Bob was driving the car and the stumble was still there. Disgruntled, Bob took the car to a different repair shop and told them what had been done.

This shop rechecked everything the first shop had done, took the car for an extensive test drive over a period of days, and got the car to act up. They determined the problem only occurred just as the clutch locked up (engaged) in the transmission’s torque converter at 80 kph, and the problem was with the transmission. After charging for the diagnosis, they sent Bob to a transmission repair shop.

The technician at the transmission shop drove the car and thought the problem was not in the transmission but rather it was a problem with the engine. Now Bob was totally disgusted! He had spent several hundred dollars and still didn’t have the car fixed. Off to a third repair shop.

This shop went through all the diagnostics, confirmed it was a transmission problem, charged him of course for the time spent, and sent him on his way. Back to the transmission shop! Bob told them it had been checked out by two repair shops, and both had told him it was the transmission. Then Bob told them to overhaul the transmission! $2500 later, the problem was still there. Then Bob came to see me.

I just happened to have a group of students working on driveability problems at the college where I teach automotive mechanics, so I told him we would take it on as a project.

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 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 OK. 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 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 $4000 in repair bills! So 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 it just takes time to find a problem. A technician has to make the vehicle act up before they can diagnose the problem. 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. Give the technician ample time to locate the fault. In this case, time spent on diagnosis would have saved a costly transmission overhaul.

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