1998 BMW 328 - Code

P0121 code bit replacing throttle sensor didn’t help.
Idles smoothly but stutters at 1100 to 1500 rpm. Ok at highway speed. Not vanos related. MAF replaced already too. Manifold air temperature part of maf.

Sounds like an air leak, is it an M52 engine?

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Thank you for responding.
Yes. M52 engine. 160K miles. 2nd owner.
The mechanic tested for air leaks. None last week.
However, today I think I’m hearing a lot more air whistling under the intake manifold. Now suspecting the oil separator/ccv. Bad diaphram?
I wish it were easy to unplug the oil separator from the manifold, and plug the manifold hole to confirm that’s my problem. I just wanna fix the real problem.
What threw me is the bucking at city streets speed. 1200 RPM and light load.

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No problems, they did suffer from the pcv valve And pipes under the manifold, there not to bad a job if you’re replacing the pipes( there are 3) You can do it without taking the manifold off. The other things to check When you’re in there are the intake pipe after the air mass meter where it splits to the idle valve , and the DISA flap above the throttle body- they can have the flap fall off. I’m a BMW tech but we don’t see many in the network any more. Just shout if you need anything else :+1:

Thank you for the comments.
A noisy signal in the TPS had been suggested. So I took unmounted it off the throttle body, and twisted the throttle up to 1200 RPM by hand: The huffing symptom persisted. QED. (I assume a fall-back algorithms in the computer compensated for the incorrect TPS signal)
Removed ccv assembly and plugged the hole in the manifold: Huffing symptom persisted. QED.
I noticed the hose from the exhaust air pump solenoid control valve (the solenoid valve right next to the vanos solenoid valve) to the check-valve was cracked and crumbly. The check-valve was dangling next to the block.
I’ll replace the hose(s) (the bad OEM hose was black with white stripe), and cross fingers. I discovered the routing at
Common problem or rare problem?
It’s a vacuum circuit I wasn’t expecting. A smoke test probably wouldn’t have found it because there’s a check-valve in the line.
I wonder if there are any other vacuum circuits nobody knows about.
Why do you always find what you’re looking for in the last place you look?

Yes it will run on a substitute value, we’ll done finding it👍Luckily we don’t see most of the emissions equipment seen on US cars

It took forever to discover that one line from the “T” goes to the vacuum reservoir on the Exhaust system page:

Vacuum tested at the reservoir. Thankfully no leaks in that branch.
Should I use high-temperature silicone rubber hose?


You did well tracing it, you might have a future a a Bmw parts advisor. There certainly not the easiest. Have a good weekend :ok_hand:

The vacuum control for the air injection is restored with better than average quality tubing.
Long term fuel trim went from -8.5% to zero.
I drove it a few miles. Still have pending P0121 TPS code, P1188 and P1189 mixture codes.
Still have some stutter at light to no pedal at 1200 RPM.
Still banging my head against a wall.
I discovered one connector not plugged into anything. It’s under the intake manifold.
The radio needs the key to turn it back on.
I think I’m not done.

Still sounds like an air leak to me, does your scanner read live data? Can y see what the air flow meter is reading?

The app can hook readings to a value display, or to a dial or graph.
The #2 oxygen sensors are now far less erratic. Under some driving conditions and at idle they’ll settle down at around 0.5V±0.1. They slammed from rail to rail before.
Charcoal canister input? I can’t figure out where that is.
The vacuum line from the vacuum reservoir goes to a box under the fuse box against the firewall. Is that routing of the vacuum line to the trunk?

At warm idle you should see 14kg/h from the airflow meter if not unplug it and try it, they have a habit of not storing a code , when unplugged they should run slightly richer. The charcoal canister is the right behind the rear wheel arch liner from memory. If you sespect a vacuum line issue get a short length if and cut bits stick a bolt in the end remove the one from the car and blank them off. The ones I’ve worked on have no vacuum pipes going to the bulkhead ( maybe emission related?).

matches my symptoms of ASC light on, stuttering, and p0121 code.
The thread said the gas-tank pressure sensor was loading the 5V reference for the TPS. The sensor is located, unfortunately, in the RH wheel well behind a mud flap, and on the back side of the overflow tank where you can’t reach it.
Hoping for a clean job. Noooo… no today.
The input to the TPS was about 5.02V with or without the tank pressure sensor in circuit. With it disconnected, the surging symptom is still there.
[i.e. Smooth idle, but advance the throttle to raise the RPM to about 1200, and the motor starts surging or “huffing.” The air sucking noise sounds like it’s coming from under the intake manifold near cyl #4. No significant fluctuation of the mpg dial under the speedometer either]
Another dead end. I had such high hopes.
12.3 KG/hour MAF value (34.16g/s) at idle. That’s 12% low. Still an air leak?
The long term fuel trim is now zero% (was -8.59% prior to replacing those vac tubes)
The ACS warning light in the bottom middle of the instrument cluster always comes on a few minutes after start-up. I have a feeling it’s a real part of the problem.
I’m pretty convinced the TPS sensor produces a constant, noise-free signal when the engine is making the huff, huff, huff surging noise.
It idles smoothly, so I haven’t been motivated to clean the idle control valve.
I don’t have an extra MAF to try.
Should the car start up with the MAF disconnected? This one doesn’t. A youtube video showed an E36 m3 starting with the MAF disconnected.

One more thing. I found a plug dangling from the bundle of wires under the intake manifold next to the block. It looked like it hadn’t been plugged into anything for a long time. Are all the plugs supposed to be plugged into some solenoid or sensor?
I don’t think it’s related to the surging problem because that was sudden onset only a couple of tanks of gas ago.

If it’s a two pin rectangular one it’s probably the air temperature sensor, they were only on the old m50’s but looms still had the wiring

The manifold air temperature sensor is along the bottom of the manifold next to the vacuum line to the reservoir with the branch to the exhaust air injection control valve. With the throttle body off, you can see it sticking up into the air path. I took it out when trying to push the tube onto the nipple on the manifold; it was in the way. It’s connected, and I can read it from the OBD-ii scanner.
page 58 implies that the engine should continue to run if the MAF is unplugged,
but it will quit if the IAC valve ‘has a problem’.
So I guess I’ll try to clean it.

Is the IAC valve cleanable? or is it prudent to replace it?

How about the MAF reading at idle being 12% low?

Sorry that’s taken a while I’m out of town, IAC valve usually just needs a clean, either brake or carb cleaner. This only affects the idle so unless it’s stuck open it shouldn’t mess with the throttle response.
You should read around 14 kgs unless the car is almost stalling. Like you said unplug the air mass meter and it should run a lot better if that’s the issue.

To eliminate any question, I replaced the IAC. (more $ than I like, but the bees paid for it out of honey sales)
The moving part in the middle of the IAC didn’t appear to be sticky or gummed up.

The MAF reading increased to about 14.4KG/h.
I assume reducing the air leaks cause the MAF reading to increase.

When I unplug the MAF, and I start the car, it runs for a few seconds and dies; does that mean I still have a sizable air leak?
When the engine is idling, when the MAF is unplugged, it dies.

When I drive it, the bucking/surging is reduced by 80 to 90%.

The ACS warning light keeps coming on. Is that related or a separate problem?
I’m gonna search for ACS & P0121 reading this evening. Possibly related.

the dangling plug went to a one-pin connector near the generator.
Can’t figure out what it is. I remember now disconnecting it so I could get my hand in the space around the oil separator.

thank you for your patience and suggestions.