Awful loud noises on startup - '86 535i
'86 535i sat 2 years before I bought it then sat 3 years in the garage. Had lots o stuff to replace - head gasket (it overheated on the 2 mile drive to my home), clutch plate/disc/throwout bearing, pivot pin, trans seals, guibo, etc. Lots o stuff.
Took my time before starting first time. Turned engine over by hand, plugs out, forcing oil into the pressure sensor hole to lube bearings, etc. Ran fuel pump to empty old gas (didn't even smell like gas).
Since it had to be run to 80C for final head torque, I slapped on a spare AFM instead of the whole original assembly. After cranking a couple seconds, it started, a bit rough for a second or two, then smoothed out. Idled fine, revv'd fine, let warm up (for final head bolt torque), a-okay. ICV really catches engine speed nicely when spinning down after a throttle blip. Ran cool, no overheat, smooth and very quiet. Maybe one valve clicking. Great!!
Re-torqued the head, adjusted valves, and put the original AFM/filter assembly on.
The next time I started it, it made a TERRIBLE racket, some of it sounding like backfiring in the intake, but hugely loud. Almost like something mechanically wrong.
Still, it started in a couple seconds, then smoothed right out and ran like a charm. It restarts perfectly once it's been run, but ever since that first noisy occasion, it does the same thing the first time it's started on any given day. Still, it starts, smooths out and then runs perfectly.
I suspected cold start issue, but remembered it started okay the very first time it ran, so I put the spare AFM back in before trying to start, and... it started fine. No huge noises, etc. It was cool in the garage, 60s maybe, but it had been like that before.
What's going on?? Lean start??
Re: Awful loud noises on startup - '86 535i
I assume the original AFM is not binding??? If not, it appears it is kaput. Put it back on again and see if you get the same symptom. If so, pop the cover and check for wear on the resistance track at the wiper arm idle position. Reposition arm and recheck AFM installed.
compared electrically to the other AFM
and there was essentially no difference. They had a little difference in resistance from wiper to common when the flap was fully closed, but the resistance rose quickly to equal the other. The NTC temp sensors read same value; they were in the same room so same temp.
I fixed the exhaust leak, and started it once since. Not quite as loud, but sounded like a clear snap! -snap! -snap! almost like an amplified electrical spark, in time with the engine firing.
The air bypass setting was not fully closed, so about 1-1/2 turns closed it. Still, it's easy to see the O2 sensor quickly overcoming any attempted mixture richening by pushing on the AFM vane, and listen to the engine RPM drop back as it compensates. O2 behaves normally, jumping between low volt and 0.85 V.
the spare AFM works fine...
...which really is puzzling, since the original one gives the same resistance readings, temperature reading, and the track isn't intermittent.
Oh well, at least the spare works.
Hate to be a broken record here but if I were you
I'd change out that old distributer cap and rotor. my mint 88 sat covered in a garage for 8+ mo. and had run perfectly. The first start was a backfire from the AFM area, racket, them wouldn't start/run. Went down the entire checklist but avoided the dizzy cap and rotor(kind of a hassle) New AFM, new ecu, new relays fuses (ALL), new air boot, new plugs, wires,.......finally opened the cap and found a nice coat of moisture inside the cap and the contacts...2 years old....rotting and useless. Replaced with Bosch...started right up...oh...and with a perfect idle, which most E28s don't seem to have. Very happy.