BMW running problem


I have a 1984 BMW 318i. When it gets to 3.5-4K rpm, it starts to miss and the tachometer starts to jump. What’s wrong with it?

How old are the spark plugs, wires, dist cap, rotor, etc.?

For just a moment I could not satisfy myself that BMW did not use CIS injection in this car, so I looked. Your car uses the much more prefered (at least by me) Bosch L-Jetronic. Check all the things mentioned by Goldwing but if you care to divert from the normal diagnostic process just a bit remove the air flow meter (it is connected to the intake by a big rubber boot) and look inside,find the little flapper door and see if you can move it through its normal range of movement (perhaps 90 degrees if not less)A backfire out the intake can jam this flapper.

I put a clutch in one of these cars and with the trans out the engine hangs down a bit. This hanging down effect pushed the distributor cap so a that it was not seated correctly. I did not notice this cap was off to the side and gave the engine a crank, the spinning rotor busted the cap. This leads me to tell you about something else you should check, the rotor has a rev limit function. Give it a good visual inspection

The engine has been rebuilt with 25K miles on it and all parts are new except maybe the ignition coil. Also, the vacuum diaphram doesn’t work.

You really put miles on fast. On the 3rd was the first time we hear from you and now the engine is rebuilt with 25k on it. I have no idea what vacuum diaphram you are talking about.

LOL! I bought the car in mid-Feb with the rebuilt engine and it had that many miles on the engine when I got it. The odometer doesn’t work; it’s stuck on 225,132; the fuel gauge only works when it’s hot outside, however the empty light does work. It misses at 3-4K rpm and the tach jumps around and when it idles, it’s never steady; it goes up and down, up and down. Almost every part and sensor on it are new and I don’t know what’s wrong with it. Also, what is the black plastic piece that mounts under the bumper behind the fog lights called because mine is missing and I need a new one?

Was the idle air control valve replaced? I’d start there.

Yes it was, maybe a month ago.

I don’t know if it matters, but every time I start the car, I have make sure that the alternator is charging by switching on a bunch of stuff. Most of the time it works, but every once in a while it won’t. I have to plug this rigged thing into the engine just enough to get it charging and go. If I leave it plugged in or plug it in too much, it kills the engine.

The trouble may be coming from the alternator. It may be generating excessive ripple voltage which could cause the tach to jump. Have it checked out.

Sounds like a number of problems. Do you have the Bentley manual on it? It seems you need it, well worth the money:

I do have the manual.

You do not mention if you checked the first thing I told you to check, the flapper inside the air flow meter.Was this engine rebuilt to solve this drivability problem? There was no other BMW that generated as much buyers remorse as the 1984 318i, I am sure you have the only one in existance with a rebuilt engine as most just wanted to be done with the car.

I know for a fact that none of my problems stem from the alternator and that none of them are fuel-related.

This car was presumed to have a blown head gasket or cracked head about a month or less ago as it was blowing antifreeze out the tailpipe. The cooling system was completely flushed out and had a $60/oz stop-leak stuff put in there (I forget the name of it). It seems to have fixed a LOT of things, but the odometer still doesn’t work, the fuel gauge still doesn’t work (fuel light does), and it still misses, but not as bad - usually. The air box is in discussion. Also, I’m looking for part number 51 13 1 884 452. I’ve had it ordered, but after it came in from Germany bent twice, the company said no more. Any help on any of this?

Look at the hose that goes from the air vane meter to the throttle body. If there’s a crack in this hose it will cause the engine to miss under various driving conditions. If the hose has no cracks then the vane meter may be the problem.


Blown head gasket? Fix-it stop-leak? Don’t count on it fixing a lot of things. Have you done a compression test?

Forget the head gasket. I need help on all the other stuff.