M50 cranks but won't fire

[bissellh]

My 1992 BMW E34 525i with M50 non vanos engine will not start. The symptoms: The check engine light does not illuminate at all preventing me from doing a stomp test. The engine does turn over, but does NOT fire or start.

The last time I drove it, some lights on the speedo were flickering. The ABS and check engine light flickered. The engine actually cut out momentarily
on the highway, but started up again without having to pull over.

Just to make things more complicated, here are some things I did to the car recently:
1. replaced fuel pump and fuel level sensor. This seemed to impove engine performance. The engine had been stuttering while pulling away from stop lights. The fuel pressure must have been low. The gas gauge had been flaky. The new fuel level sender seems to have improved this behavior, but I have not been able to run the gas low in the tank in order to test this since the car will not start anymore.
2. I wired an electic inverter directly to the battery under the back seat so I can charge my electric golf push cart battery on the way home from the golf course. It has automatic shut off in case the battery gets low. Plus, I have one of those voltage regulators bolted on there. I hope I did not damage any of my electronics with this thing.
3. I switched out the master and slave brake cylinders recently and got some air in the ABS unit along the way. I have not had time to take it in to get this resolved yet. I hope the ABS is not sending some sort of kill signal to the ECU.

These 3 things probably have nothing to do with anything. I suspect that my ECU/DME/Motronic is bad, but I am not sure. My car is so flaky, I usually think the engine harness needs replacing when something quirky starts going on, like the speedo lights start flashing. I guess this can be expected from time to time since my car is a 1992 with 250K miles and is parked outside in the elements.

Has anyone had similar symptoms? If so, do you how I can narrow down the problem?

[glen-sj]

I had the same symptom, and it turned out to be a bad fuel pump. But you said you replaced the fuel pump. So I would look into the fuel pump relay and the Crank Position Sensor.

[Dave M]

If you're confident the pump is working (do a search on how to verify), get 'old school' and pull a plug/coil to check for spark. If you have spark get a multimeter and test the Crank Position Sensor. The specs are in the Bentley or here if you ask. The speedo guage lights flickering makes me think something like a fusible luink might be at fault (need to know where your batt. is to find teh link).

More info = more answers. Keep us updated.

DAveM

[bissellh]

I had the same symptom, and it turned out to be a bad fuel pump. But you said you replaced the fuel pump. So I would look into the fuel pump relay and the Crank Position Sensor.

I did replace the fuel pump, but is was a used one. After my car stopped running, I swapped out the fuel pump relay with a spare I had. That did not help. Maybe I better jump the fuel pump relay socket to see if the fuel pump winds up. I bet my repair manual has information on how to do that. it probably also has information on the crank position sensor test.

[bissellh]

If you're confident the pump is working (do a search on how to verify), get 'old school' and pull a plug/coil to check for spark. If you have spark get a multimeter and test the Crank Position Sensor. The specs are in the Bentley or here if you ask. The speedo guage lights flickering makes me think something like a fusible luink might be at fault (need to know where your batt. is to find teh link).

More info = more answers. Keep us updated.

DAveM

I know how to jump the fuel pump, check for spark and I think I can find crank position sensor testing information in my repair manual. I am not sure what the fusible link is, but my battery is under the back seat on the passenger side. Is there a fuse in there I need to check. Maybe the inverter I wired to my battery is causing my problems.

[bissellh]

If you're confident the pump is working (do a search on how to verify), get 'old school' and pull a plug/coil to check for spark. If you have spark get a multimeter and test the Crank Position Sensor. The specs are in the Bentley or here if you ask. The speedo guage lights flickering makes me think something like a fusible luink might be at fault (need to know where your batt. is to find teh link).

More info = more answers. Keep us updated.

DAveM

I just went out and jumped posts 30 and 87 on the fuel pump relay. On my 1992 525i E34 it is the middle one, the blue one in the set of three in between the engine and ABS ECU's under the cover in the front right area of the engine compartment. At least I hope that is the fuel pump relay. I did not hear any sounds from the fuel pump or any sounds of fuel running through the lines. However, I squeezed the rubber fuel line hoses that connect the rail to the metal lines. I could barely squeeze the one at the front of the engine because it is full of pressure. While I could fully compress the one at the back of the engine. Does anyone know which is the supply and which is the return? I imagine the front is the supply. Could this mean my fuel rail or regulator is clogged? Perhaps I dredged up some stuff in the fuel tank while changing fuel pumps. Actually I pulled up some metal from the fuel tank with a magnet and scooped out some dredge with a fine filter while I was swapping fuel pumps. Does this pressure difference mean anything?

I am planning on relieving some of that pressure tomorrow by allowing some fuel out and trying this test again.

[whiskychaser]

I just went out and jumped posts 30 and 87 on the fuel pump relay. On my 1992 525i E34 it is the middle one, the blue one in the set of three in between the engine and ABS ECU's under the cover in the front right area of the engine compartment. At least I hope that is the fuel pump relay. I did not hear any sounds from the fuel pump or any sounds of fuel running through the lines. However, I squeezed the rubber fuel line hoses that connect the rail to the metal lines. I could barely squeeze the one at the front of the engine because it is full of pressure. While I could fully compress the one at the back of the engine. Does anyone know which is the supply and which is the return? I imagine the front is the supply. Could this mean my fuel rail or regulator is clogged? Perhaps I dredged up some stuff in the fuel tank while changing fuel pumps. Actually I pulled up some metal from the fuel tank with a magnet and scooped out some dredge with a fine filter while I was swapping fuel pumps. Does this pressure difference mean anything?

I am planning on relieving some of that pressure tomorrow by allowing some fuel out and trying this test again.

The fuel pump relay is the middle one of 3 in the e-box. May be worth checking you actually have a voltage at the pins and then at the pump before you start ripping the pump out again. The fuel goes in the front (radiator) end of the rail and the pressure regulator is at the back. The hoses are pretty solid - especially if they are old -so a finger and thumb pressure test wont really do. To save you looking it up, you should have 51psi fuel pressure with engine off and jumper in place. Im not going into why but this will drop to about 43 with the engine running. HTH

[glen-sj]

When you performed the jumper test on the pump relay. Did you also checked to see if any voltage at the fuel pump?

[outomoneyracer]

My 1992 BMW E34 525i with M50 non vanos engine will not start. The symptoms: The check engine light does not illuminate at all preventing me from doing a stomp test. The engine does turn over, but does NOT fire or start.

The last time I drove it, some lights on the speedo were flickering. The ABS and check engine light flickered. The engine actually cut out momentarily
on the highway, but started up again without having to pull over.

Just to make things more complicated, here are some things I did to the car recently:
1. replaced fuel pump and fuel level sensor. This seemed to impove engine performance. The engine had been stuttering while pulling away from stop lights. The fuel pressure must have been low. The gas gauge had been flaky. The new fuel level sender seems to have improved this behavior, but I have not been able to run the gas low in the tank in order to test this since the car will not start anymore.
2. I wired an electic inverter directly to the battery under the back seat so I can charge my electric golf push cart battery on the way home from the golf course. It has automatic shut off in case the battery gets low. Plus, I have one of those voltage regulators bolted on there. I hope I did not damage any of my electronics with this thing.
3. I switched out the master and slave brake cylinders recently and got some air in the ABS unit along the way. I have not had time to take it in to get this resolved yet. I hope the ABS is not sending some sort of kill signal to the ECU.

These 3 things probably have nothing to do with anything. I suspect that my ECU/DME/Motronic is bad, but I am not sure. My car is so flaky, I usually think the engine harness needs replacing when something quirky starts going on, like the speedo lights start flashing. I guess this can be expected from time to time since my car is a 1992 with 250K miles and is parked outside in the elements.

Has anyone had similar symptoms? If so, do you how I can narrow down the problem?

My 530 stumped me for couple of days when it wouldn't start. I tested and knew I had no spark and thought I had gas but was wrong and chased my tale. Make sure if you have gas or not going to the motor. Easy way to tell if you have juice going to fuel pump is turn on key and chek for voltage at the fuel pump fuse under the hood. If no juice to feul pump fuse and no juice to coils(no spark) then its probably the main relay located in box at right rear of engine compartment. $25 at the dealer

[shogun]

I had a similar case on an E32 750 last Saturday. One side of the V12 was not running.
I by-passed all the fuel relais etc and used the battery post under the hood and connected a test probe directly to fuse 22 and 23, which are the fuses for the 2 fuel pumps on the 750. I could hear both running, but one was making much more 'noise' and I could even feel the fuel running thru the hose in engine room and back thru the return line. The other pump also might a smlight humming noice, but was much to weak to produce the power to pump the required volume and pressure.
That is the second time now that I helped someone out with bad fuel pumps, and in both cases they bought used ones and in both cases the used ones were bad!