My 1992 525i accelerates very slowly, and has almost no power. It will not go above 2k rpm when in gear, though I can rev it fine in neutral or drive. It was thought that it might be the fuel pump, so I swapped that out, but no change in symptoms. I did a stomp test and it gave code 1221 which is Oxygen Sensor 1. The issue started right after I put some HEET in my gas, but that could be a coincidence.

What could cause these symptoms, besides the fuel pump? (Or maybe the used replacement was bad too? Though unlikely).

Hmm, maybe it is the fuel pressure regulator not working right? Just thought of that...

Thats wierd. Do you have a vacuum leak?

I'm gonna have to test the new fuel pump for pressure, and see if it is good. Funny thing is the car was working just fine, then bam stopped working.

That's sounds like something electronics related IMO.

Yeah, check the fuel pump relay.

Ok, I got a pressure tester. I hooked it up at the gas tank... not at the front of the car, but at the pump. PSI reads about 52 when it's idling, and when you put it in Drive and accelerate the pressure goes up to 58 or so. I assume this means the pump is ok.

I noticed something new today, in the daylight. It blows out very thin whitish smoke when accelerating (had an assistant accelerate for me while I watched from behind).

pump is fine then. Squeeze the return line and let me know what happens to the pressure.

Umm, I don't know if this is normal, but while the car is running the needle jumps all over the place between 51 and 54 psi. With it jumping like this it would be hard to notice a small change from squeezing the return line. Is this jumping normal (hooked up at the fuel pump)?

Maybe the throttle position sensor? If it is stumbling, the smoke is probably just from running rich. Test the TPS.

Here is a video of the needle jumping. Does it look like the pressure is correct? This video was shot with the gauge on the fuel rail in the engine compartment, not on the pump. Do the PSI readings look ok? Note I squeezed the regulator vacuum hose twice at approx 15 seconds in the video, hence those two quick jumps. Squeezing the return hose has no effect.

http://www.youtube.com/watch?v=SW4HQyYZI6I

is it automatic if so u might be in limp mode

He'd see that error as well as the 1221. I know a ZF 5speed auto trans on an will cause this kind of symptom (faulty speed sensor or too much slip). But your problem is clearly O2. Just put a new one in if you the fuel is ok. WTF is HEET btw? (Rule of thumb- additives = seldom do what they say and usually do ***something else*** :( )

Is 50+ PSI ok on M50? On M30 and many others it is 43.5 (3 BAR). (Maybe the M50 is 3.5 BAR?)

Why doesn't pinching the return line effect the PSI? Just pinching the vacuum line to the regulator has effect. I've seen reference to pinching the return line several places now.... is this important that it isn't working?

If the transmission is in limp mode would it generate a message on the check control LCD? (I'm not seeing any messages)

pinching the return line with pliers or whatever should cause the psi to spike up, If its not, it hints at your fuel pressure regulator.

Ok, I'll try it with pliers in the morning. Good idea (I still have the gauge hooked up).

Is there a way to know what gear the car is in? Mine just says D on the dash when I put it in drive. I'm thinking maybe the transmission is in limp mode. Would disconnecting the battery and letting it sit for a couple hours get it out of limp mode maybe?

Yes it would, but limp mode doesn't kick in until you get a CEL to flash.

Initially when the problem first started the CEL was on. Can the CEL go out and the car still be in limp mode?

Ok, pinch results. 0 change in PSI.

So next I disconnected the return line from the fuel rail on the engine, and ran a hose from it into a container. Shorted the relay as it tells in the manual (pin 30 and 87) and no fuel goes out the return line.....? Pressure says 51 PSI on the fuel intake side.

I thought pinching the return hinted at fuel pump/filter or FPR.

FPR if the fuel pressure is high and Fuel pump/filter if not.

Should be pumping enough volume to return fuel contantly at idle. IF pinching doesn;t cause spike then you are not pumping enough fuel.

Maybe the pump can't develop enough pressure to trip the regulator to return fuel?

Ok, I did a test at the fuel pump, in the trunk. I hooked up the fuel pressure gauge to the output for fuel on the pump (no T fitting, just hose direct to gauge), disconnected the electrical connector to the pump, shorted the relay in the engine compartment, and then plugged the electrical connector in in the trunk. Bam, the PSI jumped to 70+ for a moment, then quickly went down to about 60psi, and kept dropping (pump going all the time). It eventually dropped to 45psi, and wont rise. I disconnected the power from the pump and plugged it back in, no luck, still not rising above 45psi. Bear in mind there is no fuel flow... the line is blocked by the gauge. Maybe the pump can't generate enough fuel pressure to trip the regulator (as per my previous post)?

What PSI reading should I be getting with this test? I supplied 12.4V to it, freshly charged battery (key off).

Ok, a couple more tests. Volts drop to 11.5v when under load from pump (car off, pulling from battery). Amps going to the pump with this load is 6.4A, with pump hooked straight to pressure gauge. If I reconnect the fuel line to the engine I also read 6.4A load, so it is definitely not flowing gas when engine is off (no gas going through return line, as original test showed).

here is a video. after you short the relay pins 30 and 87, there is a several second delay before the regulator starts overflowing the gas into the fuel container (maybe 3?). during this delay the amp meter reads 6.2A. After a few seconds the fuel pressure leaps up to 51psi, and the fuel goes through the regulator into my container for 30 seconds, generating half a quart of gas (0.4 qts below spec?) and consumes 4.3 amps.

What could all this indicate? Is 4.3 amps abnormal for a fuel pump?

http://www.youtube.com/watch?v=yKoomoS8gDo

It seems the previous backpressure tests got the fuel pump working to flow fuel, or maybe I didn't wait long enough on the first attempt. I'm pretty confused... Before it wouldn't even go to 45 psi. Maybe once the pump has been used for a while it heats up and can not generate enough pressure. Or maybe I had bad contact to when I shorted the fuse, and the pump couldn't get enough current.

Here is another little wierd issue (note needle jump):
http://www.youtube.com/watch?v=qMZDFDyxZyY

Before I couldn't get the pump to even generate 60 psi hardly. Anyway, now it has 70 psi, but it's glunking and the needle jumps.... hear the noise in the video the pump makes, and the needle jumps?

stupid question, but have you disconnected/reconnected the battery during this whole fiasco?

If not, try it... Let it sit for a bit before reconnecting (I waited 10 min).

Limp home mode is a computer mode that limits the rpm's of the car due to an issue the computer finds. A lot of cases involve a knock sensor code (sometimes false) that is not constant. My car sat for 2 years so that was the cause of mine as it just needed to work itself out a bit (and a LOT of cleaning parts, etc.).

Trans Program is where the transmission locks itself into gear (3rd I think) if it finds that the transmission has a problem. And you get that lovely message on the dash.

Ok, squeezed fuel return line while engine was running:

Zap, up to 60 psi. Squeezed it again, tighter. Zap, up to 80 psi (yikes). Ok........... wth.

Car still has no power at all, can't get to 2k rpm hardly. Maybe the transmission is in 4th limp mode.

No I haven't disconnected the battery since the start of this, I am going to go and try that right now.

If the transmission is in limp mode would it generate a message on the check control LCD? (I'm not seeing any messages)

Ok, I got the answer to this question. NO IT DOES NOT SAY ANYTHING GAAAAAHHHHHHH.

Disconnecting the battery fixed it, left it disconnected for 15 minutes. It is fixed now... I thought the limp mode would fix itself, or at least generate a message?! I guess you have to pull the battery to get it back to normal on this car.

It is fixed. Yes, damnit, it is finally fixed. :buttrock

good im glad u fixed it

I blew my whole weekend on this because I thought it would generate a error code if the transmission was in limp mode, and because I wasn't sure if disconnecting the battery would cancel limp mode, even if it was the problem. Lesson definitely learned...

stupid question, but have you disconnected/reconnected the battery during this whole fiasco?

If not, try it... Let it sit for a bit before reconnecting (I waited 10 min).

Limp home mode is a computer mode that limits the rpm's of the car due to an issue the computer finds. A lot of cases involve a knock sensor code (sometimes false) that is not constant. My car sat for 2 years so that was the cause of mine as it just needed to work itself out a bit (and a LOT of cleaning parts, etc.).

Trans Program is where the transmission locks itself into gear (3rd I think) if it finds that the transmission has a problem. And you get that lovely message on the dash.

Thanks for suggesting this, I was thinking to try this just before I read your post, as a last resort. I'm glad and relieved it worked.

Do they have a 'limp mode' if they have a 5 speed transmission? Or is this only on the automatics?

FYI, stomp test is not giving the 1221 error anymore, just 1444 (no faults), now that it is in normal driving mode. I guess limp mode can trigger an o2 sensor warning.