'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??