Let's play: fuel or spark?

[BennyM]

Based on the following symptoms, what would you suspect?
For the last few days, I've had to crank the starter longer than usual before the engine roars to life. In a couple of instances, I've cranked and stopped, waited a few seconds, then turned the key once more with success. It seems to require more effort if it has been run recently--so fuel pump check valve maybe? I figured it was the fuel pump because that thing is noisy as hell, but then my other e34 ran for years with a noisy fuel pump. The FPR is only a year old. So, maybe it's a crankshaft position sensor?....why do we have sensors for both crank and cam, btw? Don't they rotate in unison?

One other bit of info that might help, a couple months ago, just during one trip, the engine decided to bog down if I gave it a lot of throttle. It idled fine, but if I tried to get the revs up, it would act like I was driving up a steep hill with a trailer even though I wasn't. I pulled over. Turned it off, waited, then started again and it ran just fine ever since. Figured it was a fluke, but maybe these two intermittent symptoms are related.

I haven't done any real diagnostics yet: A: because it's freakin' cold outside and B: because the symptoms are so intermittent. Thoughts?

[whiskychaser]

I haven't done any real diagnostics yet: A: because it's freakin' cold outside and B: because the symptoms are so intermittent. Thoughts?
I've never heard my fuel pump run. Maybe thats a clue? No point in asking what your fuel pressure is or what happens when you pinch the FPR hose as you cant do it. Provided it still starts and doesnt let you down, I'd make sure the battery is well charged and check it out when it gets warmer. Not be facetious. Just practical

[Tiger]

You say the fuel pump is only a year old? What brand? Check fuel pressure then change the CPS.

[BennyM]

The fuel pressure regulator is less than a year old. The fuel pump is original as far as I know.

[Tiger]

Check the fuel pressure then. It is probably the fuel pump that wore out. At least the checkvalve at the fuel pump is already bad.

[BennyM]

New evidence of my suspicion: In the last couple days, I ran my tank rather low, and as the needle got closer and closer to R, the car was harder and harder to start. It didn't matter if the thing sat for a 5 hours or just for 5 minutes. This morning, with maybe a couple of gallons left in the tank, the engine just barely started. Whenever I turned a corner or did anything to upset the balance of the car (ie: fuel sloshing around) it would bog down. Filled up at the gas station a few blocks away and, with a full tank, the engine started with, say, 5 cranks instead of 10, and the car drove normally the rest of the day. The pump also got quieter. Very interesting.

[632 Regal]

Definitely sounds fuel related in all your posts, change your fuel filters as a first.

[Grantus]

What is the fuel tank venting arrangement on these cars? It might pay to check that out, or try starting with the fuel cap off.

Pull the plugs and have a look at them. Check the fuel filter.

I always look at the simple stuff first.

[BennyM]

I will change the filter and check the pressure in a few days and report back. Drove it around today with no running issues, though. It just cranks a few more times than it should. I would expect it to run consistently poorly if the filter were clogged, but it only runs poorly with less than 1/4 tank of fuel. Usually, the M50s crank over 3 or 4 times at the beginning of the day, and maybe once or twice after a short rest. I really feel like the check valve in the tank isn't keeping the line pressurized in between runs. That's what my gut says, don't know what the fuel gauge will say.

Does anyone know where this check valve is? I can't find it on RealOEM, so I assume it's built into the pump.

[Bill R.]

On yours its built into the pump, on the 535i and the early m20 525 it was on top of the pump before the pressure regulator, it was/is available for the
535 and m20 525 as a separate part, on yours not so. You can just put a check valve in the line on the outside of the fuel tank right where the hoses attach to the tank.

Check this link to an earlier post for the part number of the check valve you can install inline externally
http://www.bimmernut.com/forum/5-series-bmw/40658-well-poor-old-535-her.html

[BennyM]

Bill,
This is awesomely fantastic. I will order it from Patrick tomorrow. Thank you thank you.

[632 Regal]

Bill is here! Great to see a post from you with merit... miss this stuff.


On yours its built into the pump, on the 535i and the early m20 525 it was on top of the pump before the pressure regulator, it was/is available for the
535 and m20 525 as a separate part, on yours not so. You can just put a check valve in the line on the outside of the fuel tank right where the hoses attach to the tank.

Check this link to an earlier post for the part number of the check valve you can install inline externally
http://www.bimmernut.com/forum/5-series-bmw/40658-well-poor-old-535-her.html

[BennyM]

Checking in. Replaced the fuel filter, which looked like it had already been replaced. Checked fuel pressure with engine running. Shut it off. Pressure dropped 30 psi almost instantly. Pressure was nil within a minute. Unfortunately, the replacement check valve hadn't arrived yet, otherwise I would have replaced it right then and there. Thanks everyone for helping me narrow it down.

[632 Regal]

Check valve seems to be the obvious culprit, keep us updated.

[BennyM]

Replaced the check valve per Bill R's procedure and all is well now. Thanks everyone!

[Tiger]

Glad you got it... Like I said... check fuel pressure first... which you finally did and discovered it is the checkvalve after all.