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Condition, cause and correction—connecting the dots
Friday, November 6, 2015 - 08:00
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Some readers will likely remember an RX7 I mentioned in a sideline a few years back that kept eating rear axle bearings. It started when the car was barely six weeks old. The car came to the dealership for noise in the rear end, and the rear bearings were a mess, so they were replaced and the RX7 left quietly.  A few weeks later, the car was back with the same noise, and the bearings were destroyed again. After the third time, the field service engineer authorized the replacement of the entire rear axle. A month later, the car was back with the same problem, and so Mazda bought it back and sent it to one of their training centers, where the instructor found the problem purely by accident during an electrical class.

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The battery-to-engine ground cable had never been tightened when the car was built, and so every time that rotary engine was cranked, the starter would find ground through the body by way of the park brake cables into the rear axle, where it arced on the bearings, then went at the speed of light up the driveshaft into the engine block to the starter housing, and the starter would spin. Enough arcs on those polished surfaces and they would give up metal and lose their luster in a big way. And after miles of driving, and nobody anywhere could look at the ruined bearings and figure out the origin of the ruination.

Problems like this one are the spice of a troubleshooter’s life, and usually after the repair is complete and the head scratching is done, we can draw ourselves a map from condition to cause to correction. Of course, we have to sort out the causes and the effects along the way, and that can require some mental gymnastics.

More recently, one of my students complained that after every time he gassed up his 2008 Kia Rondo he’d have to wait about ten minutes before the vehicle would start. That was a head scratcher too, and after a bit of thought and some research we found out that the canister purge valve was always open, and that all the vapor being shoved out of the empty space in the gas tank during refueling was being forced through the open purge valve into the intake. When he attempted to start the engine immediately afterward, that heavy hydrocarbon vapor, along with the double pulse from the injectors during engine crank, would dampen the spark plugs enough to cause the no-start condition until the plugs dried off enough on their own to begin firing again.  Careful with his money, this guy wanted to know how to test the part before ordering a replacement, and I showed him how to verify that air (and hydrocarbon vapor) could pass freely through the old purge valve no matter whether it was energized or not.

Gathering data and sorting it out requires a sharp-minded clear thinker, and everybody who’s in the know will agree that a really good troubleshooter in our field needs to be just a bit smarter than the average bear on a number of levels. So let’s launch into a couple more stories as we travel the winding road to the B2300 brainteaser.

Here's the mazda B2300 in all its glory. The exhaust bolt that had melted away let the header pipe drop enough to reduce backpressure, which caused the P0401 and let air in to confuse the O2 and ruin fuel trim. This patch job is still in place but will eventually be removed.  
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