I have an '86 M635 (9/85 build) that is having a strange problem that nobody seems to have nailed yet. I've got some experiments in the works (e.g. loaner stuff on the way), but thought I'd post here in case somebody had seen this before.
Apologies for the long story...
History:
For a while, the car would start and idle fine, but wouldn't accept throttle - would stumble/stall/die. I needed to figure it out before SharkFest 2016. After replacing the fuel pressure regulator just in case, I focused on the AFM, and borrowed one to test. I was swapping it in when I saw the rectangle adapter on the airbox side, I thought I had the wrong AFM (Bosch part numbers don't tell you much, and my normal contacts weren't handy - it didn't occur to me that my K&N was round and factory/stock might be a rectangle...shows what an amateur I am!), so I put all the old stuff back. On a whim, I cranked up the car and to my surprise it was great! I took it for a run and it was great! The next day, it started and had the same problem. Arg. The morning after, I took the AFM back out to inspect the hose between the AFM and the intake plenum, buttoned it all up and the damn thing worked! Ran great! Until the next morning. Same thing. I took it all apart again and made sure everything was really tight because SharkFest was days away and I was traveling. I returned home with a known-good M635 AFM and a new AFM-->Intake hose. But the car started, ran great and took me all the way to Chattanooga and through the weekend.
Until the ride home.
On the highway, it began to stumble briefly and just cut off. I pulled to the side, swapped in the "new" AFM and the hose, and...same issue.
. My buddy and I tapped and poked at stuff under the hood, unplugged and replugged stuff and something worked. Awesome again. For 35 miles or so... This time we were a little more structured, and noticed that what worked was unplugging and replugging the AFM.The car made it back home with only a couple more instances, both of which were solved by the unplug/replug trick, as were the later occurrences of the problem. When it wasn't having the issue, the car started, idled and ran great. Until it didn't.
Shortly before SharkFest this year the problem got worse. The trick had stopped working. It would idle fine, but accept no throttle.
I took it to my mechanic, and warned him it had been pretty intermittent for me. He eventually replaced the Throttle Position Sensor and it fixed it for him.
I picked up the car. It was running a little rich, and popping when I let off the throttle or under deceleration. Before my buddy Greg and I left for SharkFest, we moved the AFM spring a couple clicks to lean it out and it seemed much better.
Until it wasn't. About 20 miles in it did the stall/die thing at a steady speed (like it did on the way home last year). I pulled over, did the "trick" and it worked. For about 50 miles. After the second occurance I did the trick and drove it to a (sorta) nearby BMW dealership. They got the above story and were puzzled. The one tech who had seen something like this said he'd like to see what the fuel pressure looked like when this was happening, but had no idea about cause. They offered to let me keep it there for the weekend if I didn't want them to take a look. Given the highly intermittent nature, the cost of BMW dealership labor and the fact that nobody in that shop had seen an M6 in a decade much less worked on one lead me to park it and rode to SharkFest sharkless (but it was great fun anyway :))
I spoke to several people over the weekend, and there were a number of theories. Everybody seemed pretty mystified by "the trick" since it ran counter to a lot of the theories (why would it solve a bad relay or wonky fuel pump, etc.)
I came to get it yesterday, and it was flawless the whole 90-mile drive home. I drove it to work today (less than 20-mile round-trip) and it was still fine. It is still a little rich. It is a federalized car, and I do have an O2 sensor, but I'm not 100% sure it actually has a Johnson box or whatever. I need to find/follow the wires and see. (I'm probably going to rip all that crap out now since they aren't inspecting for it and the car wasn't designed to run like that. I also have Fahey's swappable cat/straight-pipe setup that will always be in straight-pipe mode other than inspection-time soon. Anyway...)
I'm not going to count on the problem solving itself. The main suspects are:
- Connector to the AFM
- ECU
- Fuel Pump
- Relay
- Something else electrical
We did notice in the first pull-over of this year's trip that the ground strap that goes to the valve cover was missing, so we replaced it, but the second bork happened 50 miles later.
I'm thinking having the ECU re-flowed wouldn't be a bad idea even if it doesn't turn out to be that, but I'd love to hear any other theories. I've got a loaner M635 ECU coming at some point soon.
Again - apologies for the very long story, and thanks in advance for any help!
Pic of my car today just for fun!
