intermittent no start problem, strong crank, engine flooding

[Flying squirrel]

Well I thought I'd seek the advice of the forum again. My '95 525i with 262,000 miles decided not to start the other day.
It cranks strong. It sometimes starts and sometimes doesn't. It has been running great and when it does start it runs strong.
It happened suddenly the other day. At first it tried to start and the sputtered. I had a mechanic look at it and couldn't diagnose it because it started running when he looked at it. The next morning it wouldn't start. I tried again later, still no start. Today it started as if nothing was wrong.
The mechanic seemed to think its a fuel delivery issue. I replaced the fuel pump a few years ago. I'm thinking it may be the fuel pump relay. I'm going to throw a new one in tomorrow. I figure for $13 it's worth a try.
Any thoughts would be appreciated. I'll update if the relay seems to do anything.

[Layne]

Could be anything really. The crankshaft position sensor is a likely culprit. Could be a fuel delivery problem, but it would be very rare to happen only at start up and never happen while running.

[ross1]

Agreed with above. If you intend to throw parts at it until fixed I'd also suggest the crank position sensor

[Jackson42]

Had similar problems with my E30 and the 535i. Turned out to be a defective fuel pressure regulator in the 535i and a defective check valve in the E30's fuel pump. Would only happen once in a while. Might be worth your time to do fuel pressure tests to see if one of these is affecting your car.

[squirrelhill]

I have an e34 touring with 261k miles. Last summer, I was experiencing periodic engine flooding during cold starts (i.e., car sits for more than 8 hours). In response, I installed a new coolant temp sensor, FPR and reman green top fuel injectors. This resolved the cold start flooding for many months, however, with the cold weather in D.C., the flooding issue has reappeared. Specifically, I am experiencing hard cold starts and can smell loads of unburnt fuel when starting the car. On occasion, I need to first clear the chamber by pulling the fuel pump fuse before I can get the car to start.

My question is whether there are any other likely culprits/checkpoints for the leak down besides the ...coolant temp sensor, FPR and/or leaky injectors. Could the leak down be caused by a faulty check valve in the fuel pump? What about a failing o2 sensor? Just trying to get a feel for any other potential leak down points to check on before going back to the injectors, FPR, etc.

[ross1]

Low fuel pressure means lousy fuel atomization, which is especially important in cold temps. Worth checking into. M50 s/b 3bar or ~51 psi regulated.
It's not O2 sensor as the car starts in open loop, which means it's not listening to the O2 sensor anyway.

[5mall5nail5]

Coolant temperature sensor also (and might as well check intake temperature sensor)

[shogun]

My question is whether there are any other likely culprits/checkpoints for the leak down besides the ...coolant temp sensor, FPR and/or leaky injectors. Could the leak down be caused by a faulty check valve in the fuel pump? What about a failing o2 sensor? Just trying to get a feel for any other potential leak down points to check on before going back to the injectors, FPR, etc.

If the check valve in the fuel pump is faulty, the fuel would run back into the tank till the pressure in the fuel line between pump and engine bay is down. But it would not cause flooding of the cylinders.
You have already checked/changed the fuel pressure regulator, so that should be o.k. Leaving the fuel injectors.
Before you pull the fuel injectors, you can test the fuel pressure for leaks, see workshop manual for the procedure https://www.bmwtechinfo.com/repair/main/

[ross1]

If flooding in warm weather I'd be more inclined to think a coolant temp sensor but in very cold it's going to want full rich anyway. Plus, you've recently replaced this. Pretty sure intake AIR temp is ignored until it goes closed loop, so probably not a factor here.
I don't think any ignition issue as it manages to clear itself once the over fueling is relieved.
Checked fuel pressure yet?

[mossyoaks]

Time for an OEM Bosch CPS. Don't cheap out. This is one part that when it fails can leave you stranded.

https://www.fcpeuro.com/products/bmw...m3-12141726065

[moroza]

... very rare to happen only at start up and never happen while running.

Not if it's a slow leakdown of the fuel check valve.

[ross1]

Not if it's a slow leakdown of the fuel check valve.

Our OP also has another thread regarding flooding on cold start w/cold ambient temp. which led me to also think of low fuel pressure. Along with his mechanic that makes three votes for this.

@OP, How about one thread so we can keep track? Have you checked fuel pressure yet?

[shogun]

good idea, I merged the 2 threads now

[squirrelhill]

I am not the OP (Flying squirrel), but understand why our threads were merged re: similar topics and user names. Thanks to all who responded to my questions.

On my end, I ordered a fuel pressure gauge and will take the appropriate readings once the weather breaks a bit. Also, based on Ross' comments, I may be wrongly diagnosing the issue as a flooded condition, when perhaps it is a fuel starvation issue caused by a fuel pump on its way out. The good news is that I have a new Bosch fuel pump in my emergency bin and may throw it in since I do not know the history of the current fuel pump and would appreciate the peace of mind (touring has 261k miles).

I will update this thread if I find out anything definitive/helpful. Otherwise, I am going to step aside so that the OP has the full stage.

[Layne]

I don't think any ignition issue as it manages to clear itself once the over fueling is relieved.

I don't agree. It's easy to run with a weak spark when the plugs are dry and hot. It's very hard for them to work wet. If the spark is weak at startup, when the mixture is rich, and the plugs become wet, they can then fail to work and become even wetter, losing all hope of firing up until the excess fuel is cleared. I would at least replace the plugs, they probably need it anyway. Luckily for the M50 and M60, there's nothing else to the ignition system except the coils which are robust and certainly all would not fail at once, and the coil boots, which can be easily checked for the correct resistance.

[5mall5nail5]

FWIW low fuel pressure can cause a flood - if the pressure isn't enough to properly inject the right amount of fuel at cold start it'll just puddle and flood.

[ross1]

I don't agree. It's easy to run with a weak spark when the plugs are dry and hot. It's very hard for them to work wet. If the spark is weak at startup, when the mixture is rich, and the plugs become wet, they can then fail to work and become even wetter, losing all hope of firing up until the excess fuel is cleared. I would at least replace the plugs, they probably need it anyway. Luckily for the M50 and M60, there's nothing else to the ignition system except the coils which are robust and certainly all would not fail at once, and the coil boots, which can be easily checked for the correct resistance.

Yeah, you are right about this. While I still think a fueling problem, (This one does manage to eventually fire wet plugs once the fuel is cut off)I shouldn't offhandedly rule out ignition.