Lean cold start rough idle

[Jrichers1]

I have a 2000 540i sport. I recently started having leaning fuel trims (about +25 stft) on cold start accompanied with very rough idle and even stalling. When driving the fuel trims normalize but get lean again at idle. Once in closed loop it continues until I've been driving for 30-45 minutes then all of a sudden all the fuel trims are very good and the idle is smooth. I have no obd2 dtc's. I've cleaned the maf sensor and have a brand new air filter and spark plugs. The maf waveform doesn't appear funny on the scanner either. I have smoke tested for vacuum leaks (did find a leak by the tb fixed it but has had no effect on the issue and now I'm not finding any smoke from the smoke tester). In vehical diagnostics on my scanner I d get a fault code for 098 dme fuel tank vent valve activation. However my vent valve is being correctly supplied with 12v and I've tested it with a power probe and it functions correctly. Not sure if that would be related. Another side not I have replaced the ccv recently as well. Im struggling to figure out where to look next. I've replaced all the o2 sensors recently but I do know I have a bad cat on bank 2

[seagreen323i]

Any idea on how old the intake manifold gaskets are? I had this issue and that was my issue, it would expand as it warmed up and the problem would go away. Put some washers over the manifold bolts to test.

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[Cobra1956]

Last time the fuel filter was changed?
How old are the coils?

[Jrichers1]

I thought about that too, but the gasket is only a few months old since doing the timing chain guides

[Jrichers1]

Fuel filter was last changed 11k miles ago. I can do a fuel pressure test but I figured that wouldn't explaine why it goes away when warm. Last coil was replaced 9k miles ago but some are older. I can look at rough running cylinders on my scanner when it has the rough idle but it looks like its all 8 about evenly

[Cobra1956]

Well if it's lean on start you are either getting too much air or not enough fuel. As the car warms up you need less fuel. You might even try some serious injector cleaner like Barrymans first.

[Jrichers1]

I can check the fuel pressure tomorrow i can pull the injectors and clean them sometime this week

[Cobra1956]

Just had another thought and don't know if this could be an issue but would a dirty mass air flow sensor cause this issue?

[Jrichers1]

I didnt really think so because it persists for a while after entering closed loop but I cleaned the maf anyways and no difference.

[Cobra1956]

Issue is only when cold? Intake air temp sensor? How old is your coolant? Old coolant can act as a battery and throw voltage that tricks sensors into thinking the car is warm. Just throwing things at the wall here but these are all things I have learned, mostly the hard way.

[Jrichers1]

Coolant is pretty new just put a new resevoir and flushed the system a month ago. I thought of iat sensor too but the scanner is showing about 10° above ambient when the car is warm so it think that's about right. Turns out my fuel pressure tester doesn't have a small enough fitting to work on this fuel rail so I just started to pull the injectors to clean them. Gotta head to work pretty soon so ill have to get back to it another day

[Jrichers1]

So I finished cleaning all the injectors they all had good spray patterns and it didn't make a difference. Im leaning towards thinking maybe its an issue with the dme unless anyone has any other ideas

[Chedley]

seagreen323i's suggestion above -manifold gaskets- seems the most likely to me. But how about warming up the cold engine for a few minutes before driving ??

[Cobra1956]

I would not think DME. Air, fuel, spark. Car runs, you have all three. Car runs poorly there should be something. Lean is either too much air or not enough fuel. I did ask if you only get this when it is cold? I don't remember if we saw a response on that. Tedious but you could check all your lines/hoses for a crack allowing un metered air into the system. One thing you listed was that you replaced one coil a while back. I am not a fan of that rather one goes they all get replaced. Not BMW related but I was chasing an issue like your with my truck. Dealer could not find the cause of the issue and we went through a ton of things. Their scan system assumed that DME was good so I even replaced that and it did not fix it. In the end the cause of everything turned out to be week coils. Very frustrating because I figured their scanner should be able to tell what is wrong....so I would check everything you can. Plug gap, anything you can do yourself.

[seagreen323i]

It has to be a vaccume leak, check your brake booster. If it runs fine once the 02 sensors kick in during closed loop it must be some sort of vaccume leak it can't compensate for during open loop. I had a brake booster leak, brakes flwr fine but I heard a strange sucking noise from it at idle. Changed it and the old one was rusted out.

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[Jrichers1]

So its not a vacuum hose because I did a smoke test and used carb cleaner on the hoses to make sure. It is possible that its the intake manifold and I just couldn't see the smoke coming from down there. Its a new gasket but it could be poorly seated I guess. Its not an open/closed loop issue because the car goes into closed loop within maybe 5 minutes of running and it takes about 45min of driving before the lean idle goes away. I can't just let it idle until it warms up because it runs super rough at first and stalls. I even kept the rims up a little at stop lights to avoid this. The coil replacement was from the previous owner. I've only had the car since last may. He had extensive records so I know what and when its been done but I have done a lot to the car since I've had it. Also spark plugs are new and gapped to .032

- - - Updated - - -

The timing chain guides is the one job I had someone else do which is when the manifold gasket was replaced so I suppose he could've reused the old gasket and I wouldn't know unless I take the intake manifold off so I guess maybe thats the next step

- - - Updated - - -

But another note is the timing chain was done several months ago and this lean issue only just started now

[Jrichers1]

Also the reason I was thinking dme is because I have this phantom code 141 dme electric fan activation that I can't figure out what its talking about.

[Jrichers1]

And btw when I say I smoke tested it I didnt use a cheap smoke tester. It was my buddies $1100 tester (though I used a cigar blown through the brake booster the first time when I found the vacuum leak by the tb)

[seagreen323i]

And btw when I say I smoke tested it I didnt use a cheap smoke tester. It was my buddies $1100 tester (though I used a cigar blown through the brake booster the first time when I found the vacuum leak by the tb)What line did you use to hook the smoke tester up to?

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[Cobra1956]

Coolant sensor could be bad. More of me throwing things at the wall to see if anything sticks.

[Cobra1956]

Also the reason I was thinking dme is because I have this phantom code 141 dme electric fan activation that I can't figure out what its talking about.

Found this:
https://www.bimmerforums.com/forum/showthread.php?1617723-Error-Code-141-Activation-Electric-Fan

[Jrichers1]

I hooked the smoke tester up to the vacuum hardliners that connects to the intake before the throttle body and loops back to the ccv. I didnt think my anxiety fan had issues because ac works fine and as far as the coolant temp sensor, the temp gauge works and the car has no problem going into closed loop. That being said, I will follow your leads and go ahead and test these cts and aux fan. Been busy with some other stuff the last couple days but I will report back after testing those.

[Jrichers1]

@cobra1956 this website and my phone aren't getting along right now so im not sure if you got my response message or not. I tried sending it like 5 times so maybe you got 5 messages lol. Let me know

[Jrichers1]

Well I finally had time to go mess with the car again today and think I may have found the problem. I believe it to be the intake air temp sensor which is built into the maf. I assumed the maf sensor was good by looking at the graphs it spiked correctly without hesitation and flow levels seemed correct however before starting the car I noticed the iat was 15° higher than it was outside. Started the car and it was lean and rough. Turned it off, disconnected the maf, and it ran rich and smoother. Vice versa back to lean. Curious how before it was running lean until long past entering closed loop but today the fuel tri.s evened out pretty quickly. Also checked the coolant temp sensor and it is good. However I think this is enough evidence to warrant ordering a new maf and hopefully it solves the problem. Cant believe I didnt disconnect the maf sooner I guess I just overlooked it after seeing the readings but ill let you know if it solves the issue

[Cobra1956]

Great find! Your thread is a great example of slowly working through the problem. I am not the best using my scanner but I would encourage any who have some type of scan tool to see what it provides so we know what data we have or we need. Most of us understand air, fuel, spark but not what affects those items.The other thing we need to remember is the age of our cars and things do wear out. I thought for sure you were going to report back it was your o2 sensor that was modified. This is just as good. Hope this solves your issues and well done on the hunt!

[Jrichers1]

Well I have bad news to report. I thought it was the maf, but put a new one in and same problem. Mustve been just been a coincidental combination of it being an extra warm day and the car just starting to warm up when I tried that because today was a real cold day again when I put the new maf in. Back to the drawing board. Time to take out the o2 sensor spacer and also look more closely and maybe ill get lucky and find an exhaust leak or something. Sorry the time span of this is so drawn out I just stay super busy so only get a little bit of time here and there to mess with the car.

[Cobra1956]

Is it possible you have a wire harness issue, either the coolant or the maf? If the car is kept outside it might have been visited by critters that like to nibble on stuff.

[Jrichers1]

It has good power and ground. I haven't back probed the signal wires while idling but the waveform looks good and responsive. one of the things I want to try as soon as I get my hands on one is hooking up an oscilloscope

[Cobra1956]

Was just thinking of your situation today while looking up something on my abs swap. Stumbled across a video of a BMW having a lean start issue due to a brake booster leak. Not saying it is your issue but if cold and the booster leaks it will be adding unmetered air. Think a quick check would be to to pull the hose off your booster plug the hose and try a start when cold. Just thinking old brittle rubber when cold might not be giving you the best seal.

[Jrichers1]

Interesting ill look more into that. Also, I was able to get this from identifix. Figured it may be useful to post here in case its ever helpful to someone.

33768Tests/Procedures:1. Check for vacuum leaks on a cold engine. Both at intake and crankcase.

2. Check fuel pressure, it should be 50 PSI at all times.

3. Air mass should read 14 to 17 kg/h, which is the same as about 4.5 g/sec at idle near sea level.

4. Check crankcase vacuum using a slack tube manometer, it should measure 4 to 6 inches water column (0.45 in. Hg or 15 mBar) at idle.

5. Repair vacuum leaks and oil separator as necessary.

6. Reset fuel mixture adaptations in the Engine Control Unit (DME) using a scan tool - then key off 30 seconds, key back on and clear the faults before starting the engine.Tech Tips:These engines have an oil separator that is external to the engine. This separator regulates the crankcase vacuum. Excessive crankcase vacuum can cause lean conditions because air enters the crankcase through the crankshaft seals