BSeriesTurbo

I'm thinkin I've seen some of you post in the eye candy, and that hopefully you might frequent this section...

I work at a German repair shop and am seeing an unusual problem on a 2001 BMW 325i.

We have had the IM off to replace the oil seperator and related hoses, as well as repaired a few other vacuum leaks. The MAF has been replaced due to it being faulty as well.

When driving the car cruising at steady state the car is fine, but try to accelerate at all and it feels as though the E-throttle is opening and closing. It has good power when it is "open", but it will cycle as though you are flooring and letting off flooring and letting off...

No codes in the computer other than p0174 and p0171(lean codes both banks..). I've smoked the entire intake tract and all is well, no leaks present. Good vaccum (~23-25").

Stumped, any input would be greatly appreciated!

NastyNate

Its been a couple years not working on BMW's....but in similar situations ive seen clogged cat-converters on those cars ALOT. They can reproduce the same problem you are describing without any cat-codes(i.e. cat inefficent etc.). Crusing at speed, moderate to fast accel builds back pressure, clogged cat does not allow correct amt. of back pressure out causing engine to choke. Not saying its the problem, but a very common one of those cars. Good luck bro! If you dont figure it out, PM me and I can get ahold of my buddies from the shop I was at...They are all BMW mechs and work on the full range from classic to showroom floor models.

240dreams

Judging by that statement, I would assume that it's probably an electrical problem and not a mechanical issue.

I'm not familiar with BMW's but maybe check: EGR, idle air control valve, throttle position sensor, ...

I'd look into the EGR valve first. Make a block off plate for the port and unplug the connector. Easy. The port itself can be clogged too.

You can possibly try unplugging/eliminating the components to see if the issue goes away.

Does it happen when the engine is cold or hot? All of the time? Does it misfire under acceleration? Or is it just surging?

Archer

I had what sounds like the exact same problem on my e36 and it turned out to be the iacv. good luck man..

Fabrik8

Electronic throttle you said?
How big is the power surge when it feels like the throttle is opening and closing? Also, have you put an OBD2 scanner on it and read the actual throttle position when it is doing this? TPS problems or problems with the position sensor on the throttle pedal can cause all kinds of problems with electronic throttle control. Many of those sensors are redundant and have dual sensors with complimentary-slope outputs so that faults can be detected, etc. though, so that may not be the problem. Usually you would get some fault codes if the electronic throttle was opening and closing with some kind of feedback loop problem. There are a lot of safety measures built in so you aren't piloting a runaway car if you have a TPS failure. I may be making too many assumptions about how BMW has set up that system though, so there may not be redundant sensor outputs and it actually could be a TPS failure. That still doesn't explain a lean code though.

Did anything change once the MAF sensor was replaced? This really sounds like the classic MAP or MAF problems, and on a non-electronic throttle engine I'd say a TPS sensor (but that's hasty on your engine without fault codes).

If you're getting lean codes from both banks I'm assuming that you've got two O2 sensors that are both getting the same lean readings?

I would agree that it could be a clogged cat, although that isn't a very common problem. Generally clogged cats will show up under a bunch of different conditions, like when you let off the throttle, go down a hill and are engine braking, etc. Pressure buildup across a cat usually doesn't just show itself when accelerating, it's more of a flow volume problem rather than a changing flow problem, and you should lose power pretty much everywhere across the board. I'm not sure a lean condition would show up with that either, but I guess it would be possible to get a combination of conditions that could make that happen.

Along the same lines of pressure fluctuations, it could be something as simple as a clogged fuel filter or faulty fuel pressure regulator. The thing that interests me is that it only seems to happen when there is quick demand for fuel, like when accelerating. This could be creating too much pressure drop across the filter, or causing the regulator to act up. The behavior at steady state speed would be different because the pressure would be more or less steady state for a given fuel flow.

I'm sure you've already tried resetting the ECU, but have you looked to see if there is any way to do a min/max throttle position adapt on that electronic throttle? I don't know enough about BMWs..

And I'm not sure about that particular engine, but most electronic throttle equipped engines don't usually have an IACV. The ECU actuates the throttle plate to bypass air the same way that an IACV does, making an IACV completely unnecessary. Some do have both though, I haven't figured out why but I'm sure there is probably a reason. Maybe less wear and tear on the throttle or better control resolution for smaller engines..

I'd really stick an OBD2 scanner on it and monitor it real-time while you try to make it misbehave. Something is bound to show up that doesn't make any sense, and it should be obvious.


Oh, and is it an automatic or manual transmission?

tired hatch

hhmm i know you said you changed the MAF but try unplugging it, the new one could be faulty aswell. this will put it in fail safe that runs on a pre set map. AS for the lean in bank one and two i wouldnt think that the cats would be clogged or it would be rich. however fuel pressure could be a culprit such as fualty filter/ regulator that would lead to starving the injectors. but its really hard to diagnose something without seeing the car. what shop do you work for. my suggestion would be take it to my old shop DP MOTORWERKS Dave is one of the best bmw mechanics i n this area. He actually has the software to read and link to the DME. best of luck

-shane

tired hatch

the Idle control valve could be a problem aswell b/c it will create either a huge vaccum leak under acceleration or wont let it idle at all

User 102020

check your cats.

Fabrik8

Are either of you sure that it has an IACV?

tired hatch

not a 100% on e46. e36 e30 yes but its been so long since ive worked on an e46 that cant remember