I might have a big problem

[DDK's 525]

Ok, i have a 90 525 M20. She has lost some power off the line but at 3500-4000 rpm's the power just is flat meaning it wont go and the engine starts to ping at 4500. The exhaust has an airy sound to it as well. Any clue as to what might be going on? I know its vague but any ideas would be helpful.

[Tiger]

Probably AFM. Check for engine trouble code.

[DDK's 525]

When I check the codes, all i get is one short flash then a long flash. Would low compression cause this? Just thinking worse case. Is there a way to test the AFM?

[whiskychaser]

Maybe have a look at this and see if it helps with the codes:
BMW E34 Website
AFIK they dont make a car that indicates compressions via ECU codes. You need a compression tester for that.

[Tiger]

Very unlikely to have a low compression great running car unless you suffered catastrophic damages like lost of oil.... lost of coolant and overheated to death.

Dead AFM... with engine running... unlplug the harness and no change... would be dead.

[Bill R.]

If it has a bad catalytic converter, ie plugged or melted internally it won't have any power at all in the upper rpm's. And it will make a weird hissing sound.

[ahlem]

I'd look and listen carefully for vacuum leaks. How many miles? What's the cap and rotor look like? You can purchase a low cost spark checker from Harbor Freight and see what the spark looks like in a general sort of way at each plug. If your injectors are clogged that could cause those symptoms as well as low fuel pressure. Start with the cheap stuff. Look at your plugs and see if they are a nice grey brown color and in good condition and gapped properly. Any recent work done on the car worth re-checking?

[DDK's 525]

If it has a bad catalytic converter, ie plugged or melted internally it won't have any power at all in the upper rpm's. And it will make a weird hissing sound.

Ding Ding Ding... We have a winner! So I was driving back from the airport and it seemed to have a little better throttle response so I floored it to see what would happen and then BANG! Something in the exhaust blew a hole (got much louder LOL) and I had all the power back. I haven't had time today to get underneath to see what let go but my guess is the cat. Thanks for all the help guys.

[Bill R.]

Now you need to determine what killed the cat before you buy a new one.... Raw fuel from a bad spark plug, plug wire or coil etc will ignite in the cat causing extremely high temps which melt the core of the cat.... If you had a problem with the engine missing or running rough, something is causing too much fuel to be dumped which can and will damage the cat.

[DDK's 525]

After looking under the car, it was actually the muffler that let go. I don't have it out yet to see what might have caused the issue. The next question is will the muffler and resonator from a 94 525 fit a 90? I can get the assembly for $75 and bolt it in if it will work. Anybody know?