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Can luck be considered diagnostics?

Friday, November 1, 2019 - 06:00
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Are you superstitious? I must admit that I am, just a little bit. What harm does it do to toss a pinch of salt over your shoulder after you knock the salt shaker over on the table? However, if you “knock on wood” to ward off evil spirits and something never happens, does superstition have anything to do with it? Some folks call that good luck and therefore, it has nothing to do with superstition.

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The days of the Chevy Vega

When I was very young and impetuous, I successfully drove across the country. Some of you may have also done so when you were 21 as well, and most of you will think, so what? Well, I did it in a 1972 Chevy Vega. Do any of you older folks now affirm that it was indeed, quite a feat? Those younger might not know how problem-prone those cars were, but I can assure you they most certainly were.

It was on the second day of our trip when the car started to surge at a steady throttle. Having not even made it halfway across country (West Texas, to be precise) before trouble occurred, was not a good sign of how the rest of the trip might end up! The surge went away if I accelerated or decelerated. As it got later into the day, the surge continued to get worse. My wife and I were getting worried, so I pulled off to the side of the road to see if I could determine what was causing the problem.

Knowing if you’re working with a current DTC is important when making an accurate and efficient diagnosis.

A visual inspection revealed nothing was obviously wrong. The surge was dramatic at idle, causing the RPMs to raise and lower but not rhythmically and the engine was running very rough.

Tired, hot and exasperated, this inexperienced mechanic leaned against the air filter housing — and to my surprise the engine smoothed out. Completely surprised by what had happened, I lifted my hand off of the air filter and the engine started stumbling again! I repeated my actions several times to make sure it was not just coincidental.

I shut off the engine and took the air cleaner assembly off of the carburetor. I noticed the top two-thirds of the carburetor was loose because screws that passed through the throttle body into the underside of the carburetor bowl had vibrated loose and backed out. Air was bypassing the carburetor’s venturi between the two loose parts causing the engine to surge. On acceleration, the carburetor was enriching the mixture enough to prevent a surge. On deceleration, a high-intake vacuum pulled the loose parts together. While cruising, there was too much air entering the intake for the amount of fuel delivered. After performing an emergency repair, I felt fortunate and continued on my way. 

Found by accident?

Have you ever accidentally found the solution to a diagnostic problem? Have you ever gotten that lucky? I think all of us experience that every now and then. As a matter of fact, very recently it happened to me again. In my defense, I tried to fix this 2011 Chevrolet Malibu LT with a 2.4L, DOHC, 16v engine and 4-speed automatic transmission the right way, based on the information the vehicle provided, but that information led me nowhere near what was causing the problem.

2011 Chevy Malibu

The customer complaint was “check engine light on” without experiencing any problems driving it. The vehicle arrived in the shop with five ECM codes, three in the Power Steering Control Module, one in the Vehicle Theft Deterrent Module and two in the Generic OBDII side. It was quite obvious that some electrical faults were present recently. The technician cleared all codes out of all modules and found the ECM would immediately set two diagnostic trouble codes (DTCs) as soon as it was started.

Naturally, the shop eliminated all of the basic causes for so many problems such as a bad battery or faulty alternator before calling me. I did some research for them and found a technical service bulletin TSB referencing chafed wires underneath the rear seat, which may cause multiple faults, DTCs and/or customer complaints. The shop technician checked the harness in question and found absolutely nothing wrong with it. 

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