Light part throttle surging at low RPM, Vacuum leak?

[stangorang]

Ever since getting my car back from the dealer for a lot of work (new turbos, water pump, oil filter housing gasket, injectors) I have noticed a surging sensation at light part throttle. It is noticeable as a driver but I doubt a passenger would notice it unless I asked them to pay attention. You can see a very slight change in RPM on the tach but nothing I think would show up on video.

The sensation is only noticeable at about 10% throttle and while maintaing a fairly constant rpm between 1k-2k.

I searched around and some people are suggesting this is a vacuum leak. Would that explain the surging sensation?

The car is 100% stock right now. I looked at the vacuum lines behind the oil filter housing and they don't look great (frayed weave) but no obvious tears in the tubing. I guess it is conceivable one of them was damaged during the warranty repair (specifically the oil filter housing gasket replacement) or during the process of putting the car back to stock.

Is the next step replacing all those vacuum lines? Would the Cobb AP be able to log this issue?

Thanks for the help!

[Jeller]

I don't want to send you on a wild goose chase but I had a damaged o2 sensor/wiring once that would do something similar to me before, but never throw any codes. But was noticeable when you looked at the o2 sensor voltage.

Take the car back and have them look at it while its still in stock form?

[idnan]

I don't want to send you on a wild goose chase but I had a damaged o2 sensor/wiring once that would do something similar to me before, but never throw any codes. But was noticeable when you looked at the o2 sensor voltage.

Take the car back and have them look at it while its still in stock form?

+1 had exactly the same problem which was sorted by changing both pre cat o2 sensors

[E90Company]

I don't want to send you on a wild goose chase but I had a damaged o2 sensor/wiring once that would do something similar to me before, but never throw any codes. But was noticeable when you looked at the o2 sensor voltage.

Take the car back and have them look at it while its still in stock form?

Makes sense since they might have damaged them in the turbo install.

[stangorang]

Makes sense since they might have damaged them in the turbo install.

But no codes huh? I would have thought something like that would be shooting codes all over the place.

[CannonFodder]

I have the same issue. It is much more prevalent with a tune. I agree with everyone else that it is either the o2 sensor(s) or a leaking vacuum line.

[stangorang]

I was able to capture the surging on my crappy cell. Very subtle as you can see, but enough to the point where you can feel it. That was done in park with a constant throttle input.

[ajm8127]

I would look into getting the vacuum lines replaced. I have noticed something similar while descending hills in low gear at low speed and rpm. I also had an under boost code once I installed the Cobb AP on my car at the Stage2 boost levels, but not with the Stage0 stock-like boost levels. Last weekend I had to remove those two vacuum lines behind the oil filter housing and the ends were dry rotted. I replaced those two piece of vacuum line and the under boost problem disappeared. I have not tested to see if the surging at low speed/rpm/gear went away, but I should be able to on my way to work tomorrow.

[ajm8127]

I didn't notice any surging this morning at low speed in low gear between 1000 and 2000 rpm. I would replace the vacuum lines behind the oil filter housing if you suspect them. Good silicon vacuum line is less that $1 per foot.

See: http://www.bimmerboost.com/showthrea...cilicone-tubes

[Stucks]

I've been having this recently as well....only under load in 6th gear though....and it only happens sometimes, not every time. weird.

[ajm8127]

Here is a log I took this morning of surging, which is still present in my car.



Boost delta is the PID error:

Boost Delta = Requested Boost - Actual Boost

The more negative, the more actual boost is overshooting the setpoint (requested boost). The reason why it is stepped like that instead of smooth is probably due to hardware limitations of the boost sensor or it is a result of the analog to digital conversion in the DME. We are only dealing with a range of 0.03 psi.

You can also see the Actual Load oscillate as the Requested Load stays constant. This makes me thing it is PID related.

[stangorang]

Here is a log I took this morning of surging, which is still present in my car.



Boost delta is the PID error:

Boost Delta = Requested Boost - Actual Boost

The more negative, the more actual boost is overshooting the setpoint (requested boost). The reason why it is stepped like that instead of smooth is probably due to hardware limitations of the boost sensor or it is a result of the analog to digital conversion in the DME. We are only dealing with a range of 0.03 psi.

You can also see the Actual Load oscillate as the Requested Load stays constant. This makes me thing it is PID related.

Nice log, looks like pretty much what I have going on.

Now what's the fix? It is driving me nuts.


I will probably end up taking it back to the dealer but I doubt it will come back to me fixed.

[ajm8127]

Nice log, looks like pretty much what I have going on.

Now what's the fix? It is driving me nuts.


I will probably end up taking it back to the dealer but I doubt it will come back to me fixed.

Is it possible this problem was always present, and that you only noticed it recently?

[stangorang]

Is it possible this problem was always present, and that you only noticed it recently?

No, don't think so.

[stangorang]

Got the car back from the dealer. They double checked their previous warranty work but couldn't find any issues.

Guess I will start with replacing the vac lines this weekend.

[135Hoser]

Got the car back from the dealer. They double checked their previous warranty work but couldn't find any issues.

Guess I will start with replacing the vac lines this weekend.

Drag the SA outside and show them what you showed us. Maintain a RPM and show them the variation in RPM. That's not normal, unless you have Parkinson's.

Get them to actually verify that the O2 sensor voltage is correct and to stop being lazy.

[Sticky]

Drag the SA outside and show them what you showed us. Maintain a RPM and show them the variation in RPM. That's not normal, unless you have Parkinson's.

Get them to actually verify that the O2 sensor voltage is correct and to stop being lazy.

I like the way you think.

[dzenno@PTF]

It might be related to the MAP sensor on the intake manifold although if its bad it'd most likely give you a code but it might be on its way out..has it always been like that?

[ajm8127]

It's also possible that as the car ages, the sensor's response time decreases (this is especially known to happen to O2 sensors, but could happen to any sensor). This decrease in response time would increase the time constant of the control loop, making it slower to respond to error, and more likely to overshoot the set point.

Unless this is causing drivability issues, I doubt the dealer is going to try to troubleshoot it. We are talking about an RPM variance of 200 or so under no load. It's more difficult for a system to regulate itself when the load is low.

Replacing the vac lines is really not a bad idea, however. Even if only for preventative maintenance.

[stangorang]

It might be related to the MAP sensor on the intake manifold although if its bad it'd most likely give you a code but it might be on its way out..has it always been like that?

I like this thought.

I can't push the dealer too much on this one. They basically overhauled a tuned car under warranty. They said without codes they couldn't do much.

[badass335]

So did you ever find out a cause? I'm having the same issue as well. very light throttle and surging. In my case, so much done to my car that i'm losing track!
ECU blew due to mosfets so...

New O2 sensors
New MSD81 ecu
New injectors
New coils
New plugs
New Boost solenoids
New vanos solenoids
New Map sensor on charge pipe
New boost hoses
New forge DV's (replaced older design forge dv's)
New Rbeck pcv
New valve cover gasket
Carbon clean


Now i had the O2 sensors done first. Had to install OEM cats back due to inspection. Figured since i had the downpipes out, i should be proactive and replace O2 sensors. After that driving home, ECU died. Then the new parts kept going in. Maybe the new O2 sensors are defective?

[badass335]

No one has a fix? Suggestions?

[harms]

I'm having this same issue. Did you work your problem out?

[subaru335i]

I have noticed a similar thing ever since I flashed the car with Cobb AP (otherwise completely stock). I usually only notice it within the first 10 mins of driving and only at low loads or if I am cruising at low rpm and higher gear and then start to go up an incline.
It hasn't bothered me enough to actually log it and figure out what is going on but I do notice the actual load oscillates slightly when I feel it so I know its not in my head.
I don't have any codes either and I just assumed it was PID related or maybe related to me never having my intake valves walnut blasted... I will try to see if I can get it to happen this afternoon when it is just in neutral at constant RPM. I have only noticed it in gear.

[ba7raini]

Hey guys, I had a very similar issue which was happening under light acceleration, but it turned out to be from the jb4. I had to change the duty cycle settings and everything went back to normal. But if the car is stock then no idea