Hi Everyone.

This week we had to troubleshoot an older Buick Park Avenue for a misfiring complaint. The owner had already been trying to fix this by changing some parts, primarily the plugs, wires, and he had swapped a couple of used ignition modules and coils in and out with no luck.

When I road tested the car, the misfire was quite apparent. The most important aspect of it was that I could hear some back-firing with the miss fire, and that's really important to notice. If the problem was one where a loss of fuel or compression was occurring, you don't get the kind of backfires that I could sense. Right from the front seat of the car, I knew this was a spark issue. The problem is this old Buick (1992 Park Avenue) doesn't have the sophisticated electronics that today's cars do so it was time to really dust off some of the skills.



 

Since I suspected that problem was ignition, and especially spark timing related, it made sense to see if the module was able to command the coils to fire. The first capture you will see shows that there is indeed a problem with the module turning the coils on and off consistently.

The red trace is the current flowing through the coils as controlled by the ignition module. The blue trace is the camshaft sensor signal, and the green trace is the crankshaft position sensor. Of particular importance is the voltage drop that is occurring on the ground for the cam and crank sensors which is provided by the ignition module. The tan trace at the bottom is the electronic spark timing output from the PCM. Notice how the faults in the coil current match up with the signal error from the ECM to the ignition coil. I remember when these cars were common and this misfire condition would occur. We didn't have four channel scopes, and we didn't have the low amps current probe like we do today. There was definitely a point back then when we could be forced to try a known good part, and this cars condition would have potentially been just one of those. At the same time I had to stay aware of the fact that the customer had already been throwing some used parts at this, and was I looking at the original failure, or one that he inadvertently added to the car.

The next capture helps narrow down the possibilities. I had to be sure if the ECM was making this mistake all on its own, or if it was still an input issue.

The voltage drop on the ground circuit is quite obvious and you can see it in both the crank sensor in  green and the cam sensor in blue.

We need to see the distributor reference command, the white wire from the module to the PCM during the misfire.

With all of the data collected, it was time to talk to the customer. At first he had us put together an estimate, but once we did that he sprung the old "I can buy that part cheaper" routine on us. The PCM was going to cost us almost $120 from the dealer, and he was claiming he could buy an aftermarket reman under $90. One of the problems is that there is still a chance that the ground voltage rise could be the cause for the PCM miscalculating the spark timing. It's also possible that the distributor reference signal could be corrupted at the PCM even tough it was OK at the module. There was going to be more work required to drop the PCM to make those measurements, and time had to be spent removing, cleaning and properly tightening the connections at the engine block and the coil mounting bracket. But the customer wouldn't approve any more time, all he wants to do right now is replace the PCM. I'd say he has a little better than a 60% chance that the car will be repaired with the computer replacement but I also know that without getting to make these last checks, there is still room for another discovery.

The really tough part about this is without the meat and potatoes part of this repair, the sale and replacement of the computer once it is confirmed to be defective we don't charge enough to make ends meets with diagnostics at this level. The pricing pressure we have always endured would have the cars not showing up at all if we truly priced that correctly, so the repairs have always been relied on to subsidize the diagnostic portion. It was neat to see the data from one of these and be able to save it for future reference and training. But if we can't earn a living doing it, we have to wonder why we even learned how to and equipped ourselves to do it.