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The perils of automotive diagnostics and repair

Monday, January 1, 2018 - 08:00
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Years ago we troubleshot a Grand Prix that had run just fine until the owner’s cousin had changed the intake manifold gasket and afterwards it was skipping dead on cylinder 2, so she asked if we could have a look at it. This was an engine skip – how hard could it be? First, we checked the obvious stuff (spark plug, compression, etc.) and came up short. But what we did find was that the number 2 injector didn’t sound right with the stethoscope, so we replaced that injector with a known good one, but to no avail. We then checked the entire injector circuit for shorts of any kind and excessive resistance, pin fit at the ECM and the injector, current flow through that circuit with the injector artificially energized (0.8 amps) and ran a temporary circuit overlay. Nothing changed.

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When I finally scoped the injector pulse and compared it to the others, the pulse was strangely narrow, so I called a local salvage yard and obtained a replacement ECM. No cigar. Not even close. I replaced that ECM with a second one, because the salvage yard had a bunch of them on hand and they were only $20 each. I double checked everything. This made no sense at all. Finally, I Scotch-locked that injector’s trigger wire to the adjacent injector’s trigger and the car ran great from then on with no more problems. Remember, this was an OBDI system.

This is a comparison of the actual scope trace of the narrow pulse (left) and the normal pulse (right). These patterns were captured with the old Snap-On DDC

I didn’t like that temporary fix, but one of the GM engineers who was as stumped as I was told me those early GM ECM injector drivers can each handle 4 amps, and it’d be just fine carrying two 0.8 amp nozzles. Even if it had burned out a driver and I needed to keep digging, I still had two other good ECMs on hand. One way or another, that Grand Prix holds the distinction of being a grueling fueling enigma that still has me wondering to this day.

Burning in bad info

In the world of politics, news media and other sensitive areas, some have discovered that you can repeat some supposed fact enough that most of the hearers begin to believe it, regardless of its veracity. Our customers – some of them anyway – can also convince themselves that they know what’s wrong when they have little or no useful data except the symptom. Then there are those who have a vehicle concern and somebody they know who seems to have a bit of automotive knowledge makes a superficial jackrabbit diagnosis, hopping quickly across the high points without doing much else. And don’t you love those customers who bring you some parts they want installed based on an offhand diagnosis made by somebody who either doesn’t know how to do the work or “doesn’t have time?”

Even when we begin to gather data scientifically, we can still misfire on our diagnosis, and anybody who claims they haven’t been there isn’t being truthful. For just one example among many, I would have sworn in a court of law that the left rear axle bearing was ruined on my aunt’s ‘92 Crown Vic – after all, that’s where the noise seemed to be coming from, and it changed for the worse with a swerve to the right – as it turned out, she had a noisy left front tire and for some reason the noise was telegraphing to the left rear.

Back in the early ’80s a guy wanted me to replace his carburetor because two different shops using offhand diagnostics told him they didn’t do carburetor work, but that it needed replacing. One of them even claimed to have used an ignition scope and was a tune-up shop. It was a small carburetor on an inline six, so first, I bought a $6 Delco carb kit before I did anything else. Afterwards, I did a mild throttle snap and found it dropping a cylinder under load. I identified the cylinder, replaced a bad brand-new spark plug, and fixed that one.

And then there are quite a lot of people who will play the blown head gasket card without having seen anything other than an overheating issue. I don’t know how many times I’ve heard that, and I experienced it once myself on a 1993 Camry I checked for a friend beside the road. That one had split its radiator, overheated and was puking hot, sweet-smelling geysers out of the filler neck when we refilled it and fired it up. After it came to the shop on the hook, I wanted to show the class how that kind of head gasket failure looks and smells, but all those symptoms were gone – all it needed was a radiator. Go figure.

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