Car Doctor Q&A: Auto Diagnostic Tools From Under The Sink

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Well, Car Doctor John Paul is back at it. Over the last few weeks, he’s diagnosed problems with a leaf blower, an old garden hose and even foot powder. This time he’s going under the kitchen sink. We’re betting next week he comes back with a plunger.

Q. Is there any way to tell what cylinder of the engine in my old BMW is causing a skip without a scan tool?

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Years back you could pull a spark plug wire and see if the engine reacted. Today if there are spark plug wires you can’t easily get to them. I tried pulling fuel injector wires but that didn’t seem to make much of a difference. Do I need to buy more tools or bring my truck to a shop?

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A. There was a time when my favorite tool was an engine oscilloscope that would perform a cylinder balance test. Then as cars got more sophisticated a scan tool did a pretty good job of identifying which cylinder misfired. But what do you do when you have hand tools and no fancy electronics?

A simple and pretty foolproof method is to let the engine run until it is warm/hot, then take a spray bottle with water and spray the exhaust manifold at each cylinder.

Properly firing cylinders will get so hot the water will instantly turn into steam on the hot manifold. The manifold at the misfiring cylinder will not vaporize the water.

Additionally if the skip is caused by a faulty ignition wire spraying the wires with water will quickly pinpoint the faulty wire.

Spray the wires from one end to the other and look for electrical arcing, when you see the arcing you have found the faulty wire.