2001 325 Unstable idle, stalling, jerky acceleration

[SilvrEclipse]

Hello all,

I have a 01 325 5 spd with about 150k on it. This car has been having these issues for the last few years, I have worked on them on and off but haven't been able to figure out whats going on. The car does not have a CEL on, I have the PA soft software that doesn't show anything that would relate to these issues. I do have a traction fault from a bad yaw sensor and a seat belt tensioner fault (which is why the lights are on the dash).

These are the issues and the frequencies I see them.

1) Pull up to the light and the car has an unstable idle, see first video link below. This happens almost every day. I normally stab the throttle a few times and it settles down. If I let it hunt it tends to stall 50% of the time. This condition is worse when the AC is on or a high electrical load (lights on etc).
2) When the car is really heatsoaked from sitting in traffic, the throttle response is very jerky, on and off the throttle. Example, cruising in any gear at a constant speed, if you accelerate the car will jerk/miss/stumble heavily before accelerating. See the 2nd video below, its hard to see but the rpms drop maybe 200rpm when I hit the throttle before the car accelerates. The car will also jerk sometimes when coming off the throttle. This maybe happens a few times a year because I don't get stuck in traffic like this often. Its always related to long periods of idling in traffic.

Like I said, the car does not have a CEL on and runs pretty good for the most part. I have done a smoke test on the intake system and didn't find anything. There are so many lines buried in the engine bay of this car I could have missed something. The idle issue sounds like a vacuum leak to me, but the jerky acceleration sounds like a throttle body. I have cleaned the throttle body and check the intake boots for tears.

If anyone has any ideas or other things I can check, I will gladly take the advice. I am about ready to just take the car to BMW and let them figure it out.

Thanks

Video #1 - https://youtu.be/vGprXq3x1qc
Video #2 - https://youtu.be/zi_obyOpNjU

[yjsaabman]

Check the output of the alternator. Especially seeing the idle change when load is applied. If the voltage regulator is failing it can cause issues like this with no faults. It simply can't keep up with the changing loads and demands and all of the electrical systems will suffer.

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[Vorsprung46]

Theres more potential vacuum leaks then the boots, and based on the symptoms i bet you missed one


CCV (Valve and Hoses)
Hose to Fuel Filter
Upper and Lower tubes to the MAF
All SAP hoses
Valve cover gasket
DISA seal
ICV seal
theres also small caps on the back of your manifold that can leak

[TonyRome]

Have you ever rebuilt the Vanos?

[SilvrEclipse]

Finally getting back to working on this issue with the car. Over the last year, the car has been running about the same, no better no worse. In the last few weeks I've really been trying to diagnose this issue. I've also thrown a lot of parts at it to catch up on some maintenance. The interesting discovery I've made is that most of the time, the fuel trims are reasonable. But when the car gets into the unstable idle, both short term fuel trims go way negative, -20+ on both banks and the car tries to stall. My theory is that the car is starving itself for fuel causing the unstable idle. If the car does happen to settle down, the fuel trims go back to normal.

In the last few weeks I've replaced the battery, alternator, rebuilt vanos, new valve cover gasket, new plugs, new intake boots, new air filter, new front 02s, fixed the cracked caps on the back of the intake manifold.

I've downloaded obd fusion and tried to get some meaningful data, but the refresh rate is absolutely garbage. The best I can do is about 2sec data points if I only log the bare minimum.

After all the new parts, car does the exact same thing. First test drive out and the fuel trims went to -25 before the car stalled. Short term Fuel trims are reactive to the 02 sensors, which is why I changed them in the first place. But not seeing any improvements, I'm not sure where to go from here. Any advise?

[SilvrEclipse]

I was able to get the car to stall today. You can see the STFT tank down to -28%, and it appears that the 02s are hanging up in the rich zone. It makes sense why the ECU is pulling fuel but why are the 02s saying the car is rich?

The car starts to run worse as more fuel is being pulled out. So I'm under the assumption that the 02s are incorrect and the car is not running as rich as it thinks. If the motor was so rich that even pulling out that much fuel was still not enough. The car would be bogging down like it's flooding before the fuel trims even react.

Screenshot_20230329-171808.png

[E39 Newbie]

Have you smoke tested to make absolutely sure you have no vacuum leaks?

Have you tried disconnecting the MAF when it acts up to see if the problem improves?

Have you checked your fuel pressure?

[SilvrEclipse]

I have tried to smoke test the car in the past, I'm not sure that my home built tester did a great job, but nothing jumped out at me. I've checked as many vacuum lines as I can find, replaced anything that looked suspect. Also check the ports on the rear of the manifold.

I have driven around with the maf disconnected. It seems to do better, but it's hard to say if that's just masking another issue.

Haven't checked fuel pressure. I should be able to do that easily.

Car has been acting worse lately with the warmer weather. The higher load at idle with the ac on makes the idle worse. And it's always the same story, fuel trims go way negative every time.

[E39 Newbie]

If disconnecting the MAF doesn't improve things then it's probably not a bad MAF. Negative fuel trim indicates too rich. If you've replaced the upstream O2 sensors then the only place left to look is the fuel system. First is to see how much fuel pressure you have. It's supposed to be 50psi, and it should maintain a steady 50 regardless of RPM. Too low and you'll get lean conditions, too high and you may get rich conditions.

Then you have to check injectors. Pull the plugs. Are any wet with gas after this happens? Injector testers aren't expensive and one may pay for itself after the first use.

BTW, your smoke tester has to put out sufficient pressure to force the smoke past marginal seals and o-rings.