Bad MAF?

[agustinz96]

So I replaced the engine on my 01 540i. Ran perfect but every once in a while I'd get a code for bank 1 knock sensor low voltage. So I replaced the knock sensor, put it all back together and when I started the car it was really rough. There was popping noises that sounded like it was coming from the intake manifold. Took the intake off, put some new gaskets on it and it still did the same thing. Car couldn't even idle without me feathering the gas pedal. To me it seemed like an intake problem, maybe a leak or something. I disconnected the MAF sensor and the engine ran better. It was still a little rough and the SES light obviously came on (had no codes before) but way better, and it was able to idle on its own. So my guess is that I must've damaged the MAF when I was removing it. Before I go and order a new one, do you guys think this is the problem?

[MIKYZZ4]

Do not replace the MAF! Disconnecting it only placed the engine system in open loop mode(no input from sensors).
What scan tool, and what were the actual code #s displayed?

[agustinz96]

Do not replace the MAF! Disconnecting it only placed the engine system in open loop mode(no input from sensors). What scan tool, and what were the actual code #s displayed?

Really? I've never heard of that. I thought if you unplugged the MAF and it ran better most of the time it was a faulty MAF (at least that's what I read on other posts) I have an ELM wifi scanner. code was p0102

[MIKYZZ4]

OBD II Fault Code P0102

Fault Code Definition
The Mass Airflow Sensor (MAF) measures the amount of air entering the engine and the Engine Control Module (ECM) uses the information to determine the proper amount of fuel quantity and ignition timing. The Mass Airflow Sensor P0102 trouble code is set when the ECM detects a low voltage output from the Mass Air flow Sensor.


Symptoms
Check Engine Light will illuminate
In many cases, no abnormal symptoms may be noticed
In some cases, the vehicle may get poor fuel mileage


Common Problems That Trigger the P0102 Code
The MAF Sensor is unplugged or the wiring is damaged
Loose or corroded electrical terminals in the MAF Sensor circuit
Faulty MAF Sensor


Common Misdiagnoses
MAF Sensor Replaced when the wiring is bad
Torn Intake Boot or vacuum leak is the real problem

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Did you replace the MAF or Knock Sensor?;with any Cheap Chinese Imitation Part, then there is the answer to your problem.

[agustinz96]

I think you misunderstood what I said. I didn't have any codes until I unplugged the MAF sensor. Then I got the code because it was unplugged. Sorry for not making myself clear

[MIKYZZ4]

Ok, connect MAF, clear codes, start car, let it warm up or go for a short trip.
Report the results or symptoms, lights ...etc.

Sorry about the misunderstanding, I read over your original post, I think I have it now.
Like you, I also suspect an intake leak, or you disturbed something during your time under the hood.
A true intake smoke test, performed with a professional smoke machine, can confirm and pinpoint intake leaks.
You can do a check for the more easily found, or obvious leaks, by removing and inspecting rubber intake boots, and other hoses for cracks&tears.

[Mike WW]

I think the unplug test is if you unplug it and there is no change, it's defective. I'm not sure it can't still be bad with a change, but I think it's slam dunk bad if there is no change.

[JimLev]

If you unplug the MAF and it runs fine the MAF is usually bad.
In his case I think he has another problem he may have created.
Sounds like a vacuum leak.
How's the small rubber hose that connects the CCV tube under the intake manifold to the front of the engine on the drivers side?
Check the jet suction piece that the brake booster line connects to.
If it was just a bad MAK the car should have ran and driven fine with it disconnected, just down a bit on power.

[fullthrottle540]

MAF can indeed make your engine buck, no power, dies at idle. Need to hold throttle just to drive it.... Engine makes popping noises, misfires. It can do this without throwing a MAF code. It happened to me several times, before I finally figured it out.

Not saying you've got a bad MAF, but it is possible.

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If it was just a bad MAF the car should have ran and driven fine with it disconnected, just down a bit on power.

I wish someone would have told me this... lol. Would have saved me a few uber trips.

[geargrinder]

Exactly. The "unplug MAF runs better" data point is not back and white. Because the car goes onto failover/limp tables for MAF when it doesn't sense one (not exactly same as open loop, it could still be trimming based on O2's just not reading actual air mass but instead estimating it from RPM) that will often make a car w dreadful vac leaks seem to be ok again. Pretty sure you've got some crappy vac leak. Check the "vac jet pump" - the little plastic dingus that goes to the brake booster. It's known to break while you work on other things, mighta happened during the knock sensor some job.

- - - Updated - - -

(In addition to checking things Jim mentioned that is)

[mpo]

Bad MAF is a possibility, but I don't see where the low knock voltage comes into play. I think that would be caused by a bad connection to it, or maybe a bad ground. If the MAF and knock sensor share a ground then maybe try to find that. Or at least measure the resistance from the knock sensor ground pin to a known-good ground.

[geargrinder]

Ima gonna go bad knock sensor was legit but during the process a vac leak emerged one way or another... In fact I'd put money on it.

Not uncommon to have manifold gasket prob after some of the KS disassembly steps (like one slipped and got pinched during the bolt up), or for the jet pump to crack but in a sneaky unnoticeable way or any one of several other sneaky vacuum things happen.

- - - Updated - - -

And again the MAF disconnect really leans that way. Basically the motor is volumetric efficiency is virtually same as before, its 'inhaling' more or less exact same amounts of air, just some of it through an unmetered source. Which is why when you take the meter out it runs great, because the default tables are close to exactly what its "inhaling" now anyway.

[MIKYZZ4]

I totally agree with geargrinder, I also think that something was disturbed during his time underhood, now causing an intake leak.

The description of the effects of disconnecting the MAF, and running well on the default parameters, also indicates a fault in fuel mixture
readings/settings. The results of a scan should verify this.

[agustinz96]

I was able to solve this problem. When I did the knock sensor job I removed the ccv from the intake to change the hose and I didn't replace the gasket. When I put it back together there was a big leak from the ccv gasket at the back of the manifold. Put a new gasket one on, and no more issues

[MIKYZZ4]

Thanks for the update. Another unnecessary MAF replacement averted!
Intake leaks cause unnecessary parts replacement soo often!

[geargrinder]

Sweet! Agreed! Congrats!

[Kenny1958]

So I replaced the engine on my 01 540i. Ran perfect but every once in a while I'd get a code for bank 1 knock sensor low voltage. So I replaced the knock sensor, put it all back together and when I started the car it was really rough. There was popping noises that sounded like it was coming from the intake manifold. Took the intake off, put some new gaskets on it and it still did the same thing. Car couldn't even idle without me feathering the gas pedal. To me it seemed like an intake problem, maybe a leak or something. I disconnected the MAF sensor and the engine ran better. It was still a little rough and the SES light obviously came on (had no codes before) but way better, and it was able to idle on its own. So my guess is that I must've damaged the MAF when I was removing it. Before I go and order a new one, do you guys think this is the problem?

Apologies for the thread revival but after replacing the knock sensors did that cure your intermittent low voltage for the bank one knock sensor ??