M40 Engine misfire issue on highway, Help.

[blueray0512]

Model: 1989 316i M40B16 manual

• 60 miles /2500-3000 rpm
• warm engine

misfire situation
1. Rev drop down to zero in moment.(I keep rolling, no step on brake)
2. any electronic or mileage gauge is normal, maybe because engine still rolling.
3. gas up no response,
4. in this status, if I don't step on clutch, keep Rolling, have a chance rev up to normal working, some times rev cannot reworking through Rolling.
5. if rolling to restarts fail, I stop the car, just"one"turn ignition switch can starts engine smoothly.

Parts status:

pause sensor 2017.12.30-168140 Km
Spark plug 2018.10.29-178016 Km
distributor cap 2019.05.16-185083 Km
distributor rotor 2019.05.16-185083 Km
Ignition wiring kit 2019.12.13-

Current Mileage- 205635 Km

This issue already annoyed me a long time...not every day, occur in month or couple months,
Please help me or give me suggest, if need any more information Please let me now, So much Thanks.

[twright]

Fuel pump relay?

[tomstin]

Does it make any difference if the fuel tank in full or low? In these old cars its not uncommon to have debris in the tank. With high volume going through the pump at highway speeds, the filter sock clogs. Once the engine quits, the fuel volume drops and the debris falls off the pickup sock. Would be more prevalent in a low fuel tank versus a full tank.

With the tank low, pull the cover, remove the pump and inspect the inside of the tank.

[blueray0512]

Fuel pump relay?

Hi, I already replace at 2019 year, sorry for missed.

[blueray0512]

Does it make any difference if the fuel tank in full or low? In these old cars its not uncommon to have debris in the tank. With high volume going through the pump at highway speeds, the filter sock clogs. Once the engine quits, the fuel volume drops and the debris falls off the pickup sock. Would be more prevalent in a low fuel tank versus a full tank.

With the tank low, pull the cover, remove the pump and inspect the inside of the tank.

Hi Tomstin,

Thanks for your suggest, very detail, I will check my fuel tank,

So engine misfire status, rev gauge also not working? even have speed and keep engine rolling?

[roguetoaster]

So, when it stalls only the rev counter drops to zero? If so I would suspect the crank position sensor, even if you replaced it already. You could test it and shake the wiring where it goes in to the sensor at the same time in case the connection is loose.

[blueray0512]

So, when it stalls only the rev counter drops to zero? If so I would suspect the crank position sensor, even if you replaced it already. You could test it and shake the wiring where it goes in to the sensor at the same time in case the connection is loose.

Yes, just a moment, drop to zero and die.

I every day must through many broken road, in shack or bumps condition never occur misfire,

misfire date( every day driving same mileage)

2021/1/11
2021/1/13
2021/1/20
2021/1/28

in early, when misfire I can keep the rolling to restart engine, but recently always must stop to manual restart, seems worsen.

ECU relay or motronic is same have suspicion??

still suggest replace pause sensor first?

I don't want to scare driving in every day...Please help, Thanks.

[roguetoaster]

Since the car thinks the engine is not rotating for a short time it can only really be power to the DME, or the DME itself, or the crankshaft sensor.

I suggest electronic testing and visual inspections while the engine is off, or moving connections around while the engine is running to replicate the fault. I do not suggest just replacing parts, but you certainly can if you would perfer to do so.

[twright]

Is this just a fuel issue?

Please put a new fuel filter on it.

If that doesn't work, put a fuel pressure gage on the car and drive it. If the fuel pressure doesn't stay above 10 psi while driving, there is your problem. A low pressure could be the fuel pump, fuel filter or a collapsed fuel line.