Every now and then an interesting diagnostic problem comes into the shop. Many of these problem vehicles come from other shops and many of the vehicles have been backed up to the parts store and loaded up with everything in sight. This 2001 VW Cabrio was one of those vehicles.
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You might think a car this old would be a car that nobody would invest any money in, but please keep in mind, beauty is in the eye of the beholder and when a person loves their car, takes great care of it and keeps up on the maintenance, the vehicle will last a long time. While I had this car apart, the lady owner walked into the shop and loudly exclaimed, “MY BABY.” I think she had a love affair with this car.
"Baby" has a problem
The story of the problem started out about 6 months earlier. The lady owner was on her way home from work on Friday evening. Her commute was 134 miles. She was 50 miles from home when the engine lost power and died. The car was loaded on a flatbed and hauled to a shop that specialized in VW, Mercedes Benz and Volvo vehicles. The shop found the problem was caused by a bad ignition coil, so a new coil was installed. The engine started and ran OK and was driven about two weeks before the engine stalled again.
The vehicle was hauled back to the European shop, only to find the shop had closed its doors so the vehicle was hauled to another shop for the needed repairs. At shop #2, they replaced the ignition coil, put in a used ECM, installed a new distributor and put on a new MAF. The vehicle was driven for two weeks and stalled again. The car was taken back to the shop where they found the ignition coil had failed yet again. A new OE coil from the dealer was installed. This repair also lasted about 2 weeks, then the stalling problem reoccurred. At this point, the car was brought to my shop to be fixed.
Each time the engine stalled, the problem has been the ignition coil quit making spark, and installing a new ignition coil got the engine running again. Any time I hear something like this, my mind always wonders if the problem is really an ignition coil or is the defective ignition coil a product of something else that is causing the ignition coil to fail.
Let's take a look
With the Cabrio at my shop, I found the engine could be started and it would run about 15 minutes and then stall. The engine could be restarted and it would run, and then stall again. Each time the running time would get shorter until it would not run at all.
On any problem like this, my first step is to always take a look at anything stored in the computer’s memory. A scan tool was hooked up and all modules scanned for any clues that might have been stored in memory. There are a few codes stored in the ABS module and two codes stored in the PCM. Code P0341 (Camshaft Position Sensor circuit range/performance) and P0102 (MAF flow too low). The P0341 code is set by the CMP and the CKP signal correlation being wrong. My gut feeling is the P0102 code is set by the engine not breathing properly. Clearing the codes, starting the engine and running a few seconds until it will stall will cause both codes to come back. At times, the codes must be cleared before the engine will restart. This engine will run with the CMP sensor unplugged so the P0341 code is not an issue with the stalling problem.
My first question was, “What is going away, spark or fuel? To answer this question, I used two current probes and my lab scope. One current probe went to the ignition coil power feed while the second went to the #4 fuel injector power feed. I also hooked a voltage probe to the CKP and CMP sensors. The engine was started and allowed to run until it stalled. At this point, I have my first test results to find a direction.
What has changed?
In Figure 1, the waveform shows a problem with the ignition coil current. Something has caused the coil on time to suddenly increase to the point that would overheat an ignition coil. My next question is, “What is causing this problem? Is it the ignition coil itself, is it an input to the PCM or is it the PCM that is causing the destruction of the ignition coils?"
|Figure 1 - Scope capture of the CMP voltage, ignition coil current & fuel injector current. I found the reason for the engine stall when the ignition coil control was lost.|
The ignition system on this engine is a distributor ignition system with the CMP (Camshaft Position Sensor) housed inside of the distributor. The ignition is triggered from a CKP (Crankshaft Position Sensor) that reads from the reluctor on the crankshaft. The CMP has no effect on the ignition system and is used only to identify cylinder 1 position. Let’s take a look at a wiring diagram to see how this circuit is designed.