In the early 2000's, I was working as a transmission technician for a Dodge dealership in Omaha. I had been working there around five years. A customer brought in a 1998 Dodge Caravan equipped with a 3.0 liter V6 and a 41TE (604) transmission. The customer complained that the vehicle had no power. He thought it was a transmission issue, so the Caravan was routed to me for diagnosis.
The first test was to scan the system for codes, but there were none to be found. A quick fluid level and condition check verified normal transmission fluid and engine oil conditions. I road tested the vehicle to verify the customer's complaint, and the transmission shifted well despite the lack of power that was verified during the test drive. It seemed to have symptoms typical to a plugged catalytic converter. I then performed the plugged exhaust vacuum test, which proved to be inconclusive and vague. To be certain, I unhooked the exhaust from the manifold and test drove the vehicle again. There was still no power. This ruled out the exhaust system as a possible cause.
Next I started looking at all of the engine sensor parameters (i.e. throttle position, engine coolant temp, manifold absolute pressure, intake air temp, crankshaft and camshaft position sensors, etc.) and all readings and oscilloscope wave forms read normal with no irregularities or glitches. The next step I took was to then evaluate fuel pressure. Fuel pressure was approximately 50 PSI, which is within specifications, along with normal Long Term Fuel Trim values. I also checked for solid secondary ignition and had good blue spark at the spark plugs wires.
The van was making no abnormal noises and sounded normal during both start-up, and while it was running. As I was going through more possible scenarios in my head, I started to consider spark and camshaft timing. The camshaft and crankshaft position sensors were obviously working since the engine started and ran, so I could rule those out. I knew that the crankshaft position sensor was mounted in the transmission bellhousing and read off of the flexplate/flywheel on this application, but since there were no abnormal noises during cranking and while running the engine, I really did not think that the Flexplate was something that I should be suspecting at this point.
A quick check of the distributor spark timing indicated no problems there. My next step was to evaluate the condition of the timing belt, and the possibility of it jumping a few cogs and throwing the camshaft timing off. Following my suspicion, I removed the timing belt covers for careful inspection. All timing marks lined up perfectly and the belt was in good shape. Yet another theory spoiled again, and I was starting to think that this was going to be quite a challenge and a great opportunity to learn something new.
I then proceeded to do a compression test which would also allow me to inspect the spark plugs at the same time. The spark plugs appeared normal, and the compression test was within specifications and consistent across all six cylinders. As I was still searching for ideas, I next considered if a worn or broken camshaft could be the cause. Since the cam sensor reads from the front of the engine (via distributor), I felt I should not rule out a broken camshaft as a possible cause. Since the compression test doesn't necessarily guarantee a healthy camshaft, I also wanted to make sure there were no flat lobes. I then removed both valve covers and verified that there were no broken or worn camshafts.
Naturally, I am starting to second guess myself at this point...what am I possibly forgetting to check. Keep in mind that there were no abnormal noises and all PID data readings are normal. It seems that I would need to do a visual inspection of the flexplate to verify its condition. Is it possible that the flexplate has cracked and turned on itself. Could it do this in a manner that could cause it to throw off the timing and still not make any abnormal noises while starting and/or running. At this point in my diagnosis I could not rule this out as a possible cause, and it was a longshot but I was desperate and running out of things to check. So I turned the engine to top dead center and removed the crankshaft position sensor to find that none of the notches in the flexplate were lined up with the crank sensor provision... ah ha. Of course I had to ask myself, "Why didn't I check that first?. I needed to remove the transmission to inspect the flexplate, so I discussed this with the Service Manager. When we received the customer's approval, I removed the transmission.
Once the transmission was removed, everything looked okay with the reinforcing plate in place on the flexplate. My last hope was that when I removed the reinforcing plate, I would see the problem. I removed the eight bolts holding on the reinforcing plate and the flexplate to the crankshaft, and sure enough... the reinforcing plate was hiding the fact that the Flexplate was cracked and had wedged itself in such a way that it stayed tight and noise free during cranking and running conditions. This was definitely not what I expected to find as the cause, but I guarantee you I will never forget it. Since that time, there have been a couple times in my career where this knowledge has really paid off.
The bottom line is that you should always consider the flexplate as a possible cause for lack of power on these Chrysler V6 engines with bellhousing mounted crank sensors, even with no abnormal noises. This does not occur very often, but having this knowledge may prevent a few headaches as you diagnose similar issues. Learn from my mistake and don't rule out the flexplate too soon during your diagnosis, as it just might save you some time.