Folks,

My 1986 745T is on the fritz yet again....I had had a small exhaust leak when one of the gaskets between the head and the exhaust manifold decided to die on me, which slowly progressed into a big exhaust leak. That leak is now fixed. Needless to say, this was causing slight (but not as huge as one might imagine) performance issues. With the leak fixed and the turbocharger spinning up, there was more pressure on the system than ever. When my mechanic had pulled my turbo, he'd noticed that the fresh air hose that ran between the airbox and the input of the turbocharger was, how shall we say, a bit decrepit. I've since replaced this $175 hose, as it definately had a hole in it. I also found a similar hole worn over time through the rubber tube which runs from what appears to be (what I think is) the vacuum-controlled wastegate actuator and the large 2ish inch hose that is the output of the turbocharger.

I assumed I had the problem licked, drove the car for an hour or so, and as I pulled into my driveway, the confounded thing started doing the following things again:
1. Running rich
2. Backfiring if I put my foot into it at all
3. Can't exceed about 45/50 miles per hour
4. boost will only build EXTREMELY slowly...if I put my foot into it at all, it's a lost cause, but if I nurse it along, I can get up to 45/50.
5. All this is accompanied by what sounds like a backfire sound (if I put my foot into it at all)
6. This is a completely intermittant problem.
7. Rough, seeking idle, but only sometimes

From a cold start, the problem does not manifest itself.. In fact, in the several times I've gotten the problem TO manifest itself, it's been AFTER the car has had a long, hard slog, and has performed marvelously... I'm quite frustrated and have checked ALL rubber for signs of extreme wear/cracking/etc for obvious reasons, and have found no other problems, including on output side of the intercooler, where I did find yet another problem with the rubber and replaced the rubber accordingly. I also thought it was possible that the little wastegate-controller thing that is bolted to the front of my head and interconnects the output and input to the turbocharger was malfunctioning, so I disconnected the vacuum to it, plugged the line, and, after a drive such as the one defined above, this problem would still occur. Consequently, I'm relatively sure that's not the problem, but you never know.

HEEEEEEEEEEEELLLP!!! I'm pulling my hair out over here....

Alex Perez


Does *ANYONE* have any suggestions?

Sounds like an AMM (air mass meter) to me, but I'm not sure when it would be most likely to present itself. The AMM is that black, plastic, roundish thing about 6" long located at the front left of your engine compartment on top of the air filter.

The part manages air flow to the engine (it's located just ahead of the turbo), so when it fails the mixture goes off (wrong data sent to ECU, mixture miscalculated) and you would probably run really rich (backfiring, etc.). Also when it fails the car reverts to limp home mode and would be similar to what you describe.

When the problem is present you might try turning the car off, disconnecting the harness to the AMM, and restarting. If it runs just as crappy its the AMM.

The good news is you can replace it yourself in about five minutes, the bad new is that the part is expensive... from a discount online supplier a new Bosch AMM will run you $200-$250. FCPGroton has good prices on AMMs and offers rebuilts as well, but I don't know if I would want to screw around with a rebuild AMM for $50 less. BTW, make sure you replace the thermostat in the airbox when you do the AMM... if the thermo fails it will fry the AMM with hot air from the engine. Better safe the sorry.

-rt

RT,

You know, I'd not even considered the AMM, although I know exactly where it is. I should probably clean my air filter first, because it's been an AWFULLY long time since I've done so. Are there any procedures for cleaning AMM's? I know the actual metal sensor inside is not to be tampered with, but can the screens on either side of the AMM be cleaned? I noticed quite a bit of blackish residue on the screen itself. Fortunately, I've got an AMM that I can swap out from a buddy's parts car in order to see if this is actually the problem. At least this will save me $200 to $300 if this is not the problem.

Another thought I'd had was that the problem could *POSSIBLY* lie with the vacuum-controlled regulator that sits on the front end of the fuel rail and controls the amount of flowback into the tank for the fuel recirculation. If this were intermittant, I was thinking that this might cause the problem I'm experiencing, because all the fuel that wasn't getting to flow back into the tank would try to go into my cylinders instead, causing my car to run extremely rich and unreliably....it's just a theory, and was the only one I had before your reply..any other suggestions? I'll try this AMM scenario out tomorrow in the morning when I've got some time at home.