Cold weather stalling--tracing the issue

[ryanrrivas]

Hi everyone,
After reading many of the posts on this forum as I've done work on my 2002 325i, I'm finally signing up and reaching out because I've come to a standstill in my repairs.

The major problem I have is this: When the temperatures gets below about 35-40 degrees (Fahrenheit), my car has trouble starting, often ending in stalling, which gets worse as the temperatures drop lower. On the first startup, the engine will turn on, reaching about 1000rpm, then drops and stalls. The second start up, the engine will rev normally, reaching above 2000rpm, stabilize a bit under 1000rpm but eventually stalls out. On the third startup, depending on the weather, it will be a repeat of the second startup--or it will finally turn on. If I get to a fourth time, things will typically run normally.

Also, usually throughout this startup process, curiously, the check oil light comes for a second and then goes out.

After this initial startup problem--once the engine warms up a bit--everything else runs normally. No evident problems. For a while I was getting lean codes in both banks, but that goes on and off. No other codes, or issues, associated with this problem, which leaves me at a bit of a loss.

I've done lots of work in the last few months. When I first started working on the vehicle, the issue was a leaking valve cover. No startup problems (there was also no cold weather). But as I've done more and more work, the startup issues, when they started showing with the weather, have continued to elude me.

Here's the list of jobs I've done to date:
Valve cover gasket repair
New ignition coils
New MAF sensor
Cleaned idle air control valve
Cleaned throttle body
Replaced DISA valve o-ring/cleaned off DISA valve a bit
Replaced CCV system, along with a few vacuum lines and intake gaskets
Just replaced VANOS seals yesterday (when I did this, I found oil in the camshaft position sensor--cleaned it out with some MAF sensor cleaner, which is all I had at hand; also with the VANOS seals, they clearly needed replacement, but didn't notice any major difference after replacing which struck me as odd, given all I read about the benefits of replacing the seals)

I'm starting to think that there might be a faulty sensor somewhere that could be causing the startup problems, but again, I'm at a loss at this point. I thought for sure it was the VANOS seals after doing all the other jobs, but started her up this morning and had the same issues... Again, there's no codes on the car, only the occasional lean codes, which come and go...

Please help!

[Muzz258]

Do you have a vacuum Leak?

The proportion of unmetered air from a vacuum leak to metered air is higher at idle and lower at revs. Do this test.

With a hot engine at idle, check the 4 fuel trims, 2 short-term and 2 long-term. If any are > 8% then you likely have a vacuum leak. Rev the engine to 3,000 rpm for 30 seconds and watch the short-term fuel trims. If they reduce significantly, might even go negative, then it is confirmed that you have a vacuum leak. Smoke test the engine to find the leaks.

If the short-term fuel trims do not reduce significantly, then it’s something else.

[MarcoZandrini]

It sounds like the engine isn’t going into the “open loop” cold start mode. Is the secondary air pump (SAP; located on the inner fender on the exhaust side of the engine compartment turning on? When the engine computer (DME in BMWspeak) is in this mode it uses a rich air/fuel mixture to get the catalytic converters up to temperature. It also turns on the SAP to make sure all of the gas is burned before the exhaust gets to the converters.
The first step in diagnosing a problem is to scan the DME for codes. I recommend you get the OBDFusion app for your smartphone or tablet. Then check the E46Fanatics website for threads on OBDFusion in which jfoj gas commented. His post will tell you which communications adapter to buy and how to set up the app.
Is the fuel pump original? When was the fuel filter last changed?

[ryanrrivas]

Thanks to both of you for the replies. I'm new to DIY repairs, and I've been learning as I go, so many things are still beyond my knowledge. In response to Marco, the SAP is functional and turns on (it makes the vacuum-like noise as the car starts), but I have not changed the fuel filter, nor to my knowledge has it ever been changed... neither has the fuel pump. I had some suspicions that something might be going on with the fueling system, so I think changing the filter, and then perhaps the pump, will be my next line of attack. Thanks for your suggestions and insight.

To Muzz, I've had thoughts about there being a vacuum leak, but to my knowledge, I replaced or at least inspected the vacuum lines as I was doing the CCV job and everything seemed to check out. I've been wanting to get a smoke test just to be extra sure, but after reading more about the fuel filter, I'm starting to think that that might be the cause of the lean codes and hard starts. If a new filter doesn't solve the problem, then I'll for sure get the smoke test. Again, thanks for your suggestions.

[MarcoZandrini]

There’s a fuel pressure test valve on the fuel rail. Item #2 in the link is the valve’s cap. Fuel pressure has to be at least 50psi at all engine speeds and loads.

https://www.realoem.com/bmw/enUS/sho...diagId=13_0900

[ryanrrivas]

Here's an update about this situation. After installing a new fuel filter, I continue to have the same problems at start up. Again, once it warms up, it runs beautifully. After all this work, I'm kind of at my wit's end, and am now wondering how problematic this situation really is. Could there be any damage being done with these repetitive start and stalls? Or can I just deal with the issue.

Another question: What other components are involved in the cold start cycle?

[Kohler00]

Here's an update about this situation. After installing a new fuel filter, I continue to have the same problems at start up. Again, once it warms up, it runs beautifully. After all this work, I'm kind of at my wit's end, and am now wondering how problematic this situation really is. Could there be any damage being done with these repetitive start and stalls? Or can I just deal with the issue.

Another question: What other components are involved in the cold start cycle?

I'm in the same boat as you, mate. Cold starts nearly always stall on the first try and then the second is buttery smooth.

In my case I'm certain it's a vacuum leak as my LTFTs are sitting at ~11%.

If you're uncertain with how fuel trims work I'm sure you can find a quick YouTube video to explain them, they're pretty simple.

Definitely worth checking out before you start tossing money at parts. A simple OBD app and Bluetooth adapter for your car are a must have if you're going to DIY.

To answer your question about cold start components, there's actually not many. As previously stated the SAP is one of them. I could be wrong here, but I believe the DME has a certain map to make the engine run rich on a cold start. The SAP simply attempts to warm the cats up quicker so the car can go into closed loop. I believe the SAP is the only "cold start only" component.

Personally I would attack the vacuum leak angle, these cars, in their old age, are notorious for having them.

Sent from my SM-G950U using Tapatalk

[MarcoZandrini]

Kohler, you’re close! The DME does use an overly rich fuel/air mixture during cold starts. The DME checks the coolant temperature at the sensor in the cylinder head between cylinders 5 and 6 to determine when to use this map. The SAP blows air into the exhaust manifolds to make sure all of the gas is burned as unburned gas can damage the catalytic converters. Once the temperature gets to a certain valve the DME reverts to closed loop operation.

OP, buy/borrow a fuel pressure gauge that has a hose and install it in the fuel rail’s shrader valve. Put the gauge on the windshield so you can see what happens. Let the car cool overnight. Check the reading before starting the engine. There should be residual pressure when you check it in the morning. Then turn the key to the run position without starting the engine. This primes the fuel pump. Check to see if the gauge shows increased fuel pressure. Then start the engine and watch the fuel pressure gauge. When was the fuel filter last changed? The filter gas the fuel pressure regulator which may be failing. The only brands of fuel filter to buy are Mahle and Keyser. Both supply filters to BMW.

[RetroBeamerBoy]

Im having the same issue+ misfires on cyl1 after revving.