Who has fought & solved unstable idle issues? (M54 3.0)

[Bdc101]

I have been suffering some symptoms over the last year or so in my 2003 X5 with M54B30 (5-speed manual):

1. Fuel trims exceeded codes in DME but not on dash (frequent)
2. Fuel trim codes on dash occasionally (only on long drives and only a couple of times)
2.a. I haven't kept perfect records but I believe I am getting only lean codes, not rich.
2.b. I do get these codes on both banks 1 and 2 at the same time.

3. Unstable idle: on/after long drives (only when completely warm) car will stumble when putting it in neutral and sometimes die.

Using both this forum and Bimmerfest I've learned that these sorts of problems can be caused by a variety of things including MAF, vacuum leaks, ICV, DISA, and others. With this in mind I have checked/fixed the following:
1. Replaced intake elbow (3 yrs ago, but have double-checked the integrity recently).
2. Removed and cleaned idle control valve with brakleen. Verified that it is freely oscillating when the key is in the "on" position. The valve was dirty but had no real buildup that was visible.
3. Inspected airflow meter and verified integrity of all downstream clamps, vacuum lines, and hoses. (Note that I have not replaced the MAF and may do so if I have no better ideas.)
4. Replaced DISA with the GAS upgraded unit and O-rings.

I see a plethora of threads describing the problem when searching, but few actual solutions. For this reason I am starting a new thread and would be grateful for anyone else to share their experience with problems of this sort and how they fixed them. Thanks in advance guys!

[nick325xit 5spd]

I would start by checking fuel pressure because that's pretty easy and inexpensive to do. I'd also want to see what the MAF readings are and see if how far off of target they are. If you can swap with a known good one, it's probably worth it. Next, it's time to do a smoke test.

Basically, the three likely possible causes are fuel pressure, MAF bad, or vacuum leak. DISA / ICV could leak and cause a vacuum leak, but they're only flowing metered air, so they shouldn't cause lean codes.

[80stech]

There is a ton of stuff that can cause this but often overlooked is a missfire due to a weak coil or poor spark plug. A compression test would probably be a good idea. Worn valve guides could be another issue.

[nick325xit 5spd]

Originally Posted by 80stech

There is a ton of stuff that can cause this but often overlooked is a missfire due to a weak coil or poor spark plug. A compression test would probably be a good idea. Worn valve guides could be another issue.

This is a good point. Misfires can be read as too rich, causing the car to lean out until it causes more misfires in other cylinders.

Injectors are a potential cause there too. A LEAN misfire looks like a RICH condition to the O2 sensor.

[Bdc101]

The smoke test is a good suggestion and I will look into getting that done next.



I am seeing realistic airflow values from the MAF (using my Foxwell real-time data). That doesn't mean that the MAF is truly reporting correct values, but the signal looks like the right shape and I believe the idle reading was about 3.5 g/s which is reasonable. I have read about others replacing the MAF when it gets this old, but it doesn't seem to fix everybody's problems.



I see the fuel pressure suggestion a lot, but in my searches I have never read about any BMW owner who actually found a fuel pressure problem causing a problem like this, or fixed this problem by replacing the pump. It seems like it would be a very odd way for the fuel pump or pressure control valve to fail. However, I fully expect and intend to eat my words if and when I am proven wrong.

[nick325xit 5spd]

Originally Posted by Bdc101

I see the fuel pressure suggestion a lot, but in my searches I have never read about any BMW owner who actually found a fuel pressure problem causing a problem like this, or fixed this problem by replacing the pump. It seems like it would be a very odd way for the fuel pump or pressure control valve to fail. However, I fully expect and intend to eat my words if and when I am proven wrong.

I wasted a lot of time diagnosing a 330i many years ago when I should have thrown a fuel pressure gauge on it early on. It's not the most likely issue based on the symptoms, but a failing pump can cause these symptoms. And since the M54 fuel rail has a schrader valve on it, it's SO EASY to check this. You don't even have to disconnect a line. Note that it's important to check fuel pressure both at idle and under load. Preferably with the car hot as pump performance can decline as it heats up.

Edit: It's also worth noting that a number of people have reported issues with the Hengst fuel filter/regulator assemblies.

[Bdc101]

It's easy but I have to buy tools to check it -- you can't just use any old air pressure gauge with a schrader valve on it, right? Or can you?

[nick325xit 5spd]

Originally Posted by Bdc101

It's easy but I have to buy tools to check it -- you can't just use any old air pressure gauge with a schrader valve on it, right? Or can you?

Most of the auto parts stores will loan out a fuel pressure tester. Don't forget that a lot of these generic tools can be borrowed for free!

Or, you can get them pretty cheap on Amazon.

(And no, don't use an air pressure gauge. It might work, but it's probably not fuel rated. And you won't want it around afterwards.)

[Bdc101]

That's a great suggestion, I think I will try that this weekend.

[X5chemist]

What is normal fuel pressure idle and under load? I've been leaning to check mine. About to hit 15700 miles, so it's up there. A new fuel filter was installed during an oil pan gasket replacement.