Sorry to cross post... I reallized after posting in the Mk IV section I should have posted here.
Ok so check this out. 2001 Jetta 2.0 GL manual trans, manual windows, AZG engine code. production date 10/00, late 2001 production model. I am by no means a newbie mechanic, nor am I new to electrical troubleshooting but this one’s insane. I’ll give you the full backstory first in order to not waste anyone’s time. I know that some of these details may seem irrelevant, but you never know so here goes…
A few weeks ago my little sister found herself in need of a car. After some looking, she and I found a super clean 2001 Jetta 2.0 stick shift with only 82,000 miles. I had a look over it and found no problems whatsoever aside from a battery in need of replacement. So she bought the car. The next day, we bought a brand new O’reilly battery and installed it in the car. The morning after that, the car decided not to start. It would crank over just fine, but it didn’t even think about trying to start. My sister called me. I went over to look at it. Sure enough, the car was cranking over but wouldn’t start. Everything else still worked fine, the lights, the stereo, the door locks, it just wouldn't start. I immediately noticed that the CEL and EPC lights were not illuminating with the key on. As I’m sure you all know, with the key on, but engine off, CEL and EPC lights should illuminate. I found this strange so I plugged my laptop into it and tried to scan it with Vag-Com. (I’m an Audi owner) No communication. Vag-Com was unaware that the car was even plugged in. I immediately figured the ECM must not be powering up.
So I run down a checklist in my head, ignition switch, ECM fuses, ECM relay, ECM wiring, or the ECM itself. These are really the only things it could be in my mind. ECM fuses are good. Wiring all appears to be good. I was doubtful that it was the ECM. So I figured probably ignition switch. Unfortunately, at this point the car decided it was done acting up and suddenly the ECM power returned and the car started and ran fine and the ECM had not stored any fault codes.
The next morning I get the same call from my sister. I go to her house and this time I just cycle the key a couple of times and the ECM power returns and the car decides to run. Not being supremely confident in my abilities, I took the car to a VW specialist shop in town to check it out. He looked at it and said that it is almost certainly the ignition switch. So we had him replace the switch. No change. The car stalled out almost immediately after the switch was installed. Still no trouble codes. After seeing that the ignition switch was not at fault, the mechanic offered to put the old switch back in a refund our money, but we knew the switch is a common problem in these cars, so we decided to leave the new switch in. The mechanic advised us that it was probably an ECM issue and recommended we take the car to the dealership. So we did. This is where things get more fun.
The dealership advises us that they’ve seen this car before. In February of this year the car had been brought in for a similar issue by its previous owner. The issue was diagnosed as a bad ECM. So the ECM was replaced. The car didn’t even make it a month from that time before the issue came back. The dealer then diagnosed it as a bad replacement ECM and replaced it again. They also advised me that if it turns out the ECM is bad, they will replace it for free under warranty. By this time the issue had become sporadically intermittent, so the dealer wasn’t able to consistently replicate the issue in order to troubleshoot. The dealer kept the car for several days, after which they tried to tell me it was a bad fuel pump. I called bullsh!t on that and asked them how a bad fuel pump could cause the CEL to not illuminate with the key on, engine off. I told them it sounds like a bad ECM and that they should replace the ECM. So they call me back later and tell me it’s a fuel pump relay. I told them I was doubtful, but to go ahead and replace it.
I went to pick up the car, paid them their $180 and tried to drive away. I made it less than 2 blocks before the issue came back and the car stalled out again. I went back to the dealer and made them take the car back and refund my money. The next day I went in and talked to the dealer’s general manager and service manager. At this point they decided to go ahead and replace the ECM under warranty. So I got the car back last week with a bill of $0 thanks to the warranty and it seemed to be much better. For about 2 days.
For 2 days, it ran perfectly. Then the problem started up again. At this point though, the car won’t just stall out and refuse to start. The problem is so intermittent that the only thing it does is briefly lose electrical power to the engine while driving, at which point, the engine dies, but then power comes back and the engine re-fires and keeps right on going. All of this in the span of less than a second. It’s just enough to make the car buck momentarily, and then it’s fine.
So now I’m figuring it must be an issue with ECM power or ground coming into the ECM. The ECM ground wire only travels about a foot from the ECM and attaches to the bulkhead near the wipers and it’s in fine shape, with no measurable resistance back to the battery. I also measured voltage at the back of the ECM harness while the car was running on both the ECM continuous power pin and the key-switched power pin. Good power at both, even when the car stumbles.
To add to the confusion, the car did something strange just a few minutes ago. The car has been sitting in my garage since last night. Just now I went out to take another look at it. When I opened the driver’s door, the fuel pump cycled as if I had just turned the ignition on. The key wasn‘t even in the ignition. The pump just cycled on its own when I opened the door. I tried it again and now it isn’t doing it. Maybe this has nothing to do with my issues but I thought it was worth mentioning.
So what am I left with? ANOTHER bad ECM? That seems highly unlikely and I doubt the dealer would replace the ECM again under warranty given the circumstances. I’m stumped. If anybody has some real in depth experience with the electrical systems in this car and knows what might be causing this, please chime in. I don’t want to be a jerk, but if you don’t really absolutely know what you're talking about on this, please don’t. It’s not the O2 sensor, no matter what your mailman’s brother-in-law says.