M40 Misfire, connected to MPG gauge operation.

[joew23]

Hello,

Recently my 316i (1990, M40) has developed a misfire. The ignition seems to drop out and the car stutters, especially under acceleration.

the strange part is that the MPG gauge is intermittent. when it is working the car runs fine, when it doesn't work at all the car runs fine. but it stops and starts working as the misfires occur, typicaly just for a split second.

is this the crank sensor failing? i cant see what else would affect ignition as well as the MPG gauge as i am guessing that is where it gets its reading from?

thanks in advance.

[Contours]

I have a car of similar vintage and the MPG gauge has been working as it should since replacing the service indicator batteries. My car was also fitted with a fairly crude but effective immobiliser switch which cuts power to the coil which allows the car to crank but not start. The contacts on this switch can corrode over time so may be worth looking at if your car has this switch.

[arrisbmw]

I,ve got an m4016b in mine too. so will be interested to see how this progresses. I believe you can do a basic check to see if the crank sensor is working . " remove the fuel pump fuse 11 , then crank the engine over watch the rev counter the needle should flicker indicating the sensor working.
I am sure there is a better test involving removing the ecu plug , and a multi meter.
someone on here will know. how to perform this.

could be one of the basic service items , plugs , leads , dizzy cap, rotor arm, even the coil.
bad earth somewhere !

[Brianmoooore]

The MPG gauge takes inputs from the road speed sensor in the diff. (which also works the speedo.) and the fuel rate output from the engine ECU. This output is derived from the injector pulse circuitry, which has the crank position sensor as one of its major inputs, so you could well be on the right lines.
Any noticeable drop in the tacho. reading when the fault occurs, other than just tracking the falling revs?
A failing DME relay will also give similar symptoms to an intermittent crank sensor, and is a lot cheaper and easier to check. Just pull it out, and run the car with wire links joining pin 30 of its socket to both 87s.
Don't leave the links in when you're parked.
I did have one case of a M40 where changing the crank sensor and the DME relay didn't cure the stuttering misfire, but changing the ECU did, but this is unusual.

[joew23]

Thanks for all the input so far. The car does have a functioning SI indicator and did have an immobilser but was taken out many years ago as I found the remnants, not connected to anything. The car ran perfectly until recently.

None of the other gauges have a problem. Just the mpg. And it never misfires unles there is a momentary loss or start of operation of this gauge. Or sometimes the gauge doesn’t work at all and the car works perfectly.

If it’s idling the car will misfire many times, and each time the mpg gauge will start to fall (stop working) then cut back in again.

I’ll try some of the tests suggested and get back to you guys, Thanks!

The car was an auto but I rencently did a manual conversion and rebuilt the M40. If that helps at all, but I’m pretty sure all is fine. Although the fuel pump relay did give up in the time the engine was out, guess the Dme relay mentioned could be going the same way?

[arrisbmw]

what does dme stand for. and what does this relay do, ? what power does it supply to what component ?
thanks just interested.

[Brianmoooore]

DME = Digital Motor Electronics.
Relay supplies power to the injectors, to the bulk of the engine ECU, to the fuel pump relay (but doesn't switch it on), and on late E30s, to the ABS ECU. It is controlled by the small part of the engine ECU that remains active when the engine is off.

[arrisbmw]

thankyou.

[arrisbmw]

thankyou.

[joew23]

Ok, unfortunately I’ve been really busy and haven’t had the chance to do any proper tests. I noticed today while the car was idling, that whenever the misfire occurred, the relay was switching. Does this further confirm that this is the relay at fault? As I am sourcing a replacement now.

[Brianmoooore]

This throws doubt on the relay being at fault. A relay fault is more likely to be with its switching contacts or the hinge falling apart, rather than a coil problem.
I'd be carefully looking at the path the green wire from the ignition switch takes to the ECU and coil. If you had done the relay bypass I described, this would, depending on results, either confirm the relay's guilt, or lead you to the green wire's path, as above.
Any immobilisers fitted, including the CODE one included in a obc?

[joew23]

So i did all the checks to diagnose the issue. as it turned out, unplugging the sensor from the HT leads solved the issue and the car runs better than ever! I have no idea why this should be as i am guessing this sensor allows the DME to advance ignition?

[Brianmoooore]

It may be running better with the sensor disconnected, but it cannot be running correctly, since the ECU needs the signal from this sensor to run as intended!
Which ignition lead is the sensor on?

[arrisbmw]

following on from this, I have the same car still auto, i discovered the other day where the sensor should be plugged in ( under the alloy inlet manifold just below where the rubber bellows meets the throttle body ) its not easy to get too. I have plugged it in now. but don,t really understand how it works iirc its around ignition lead 4 on 4 pot and 6 on 6 pot engine.
my q is how is the information used from this , obviously something to do with when No 4 plug fires. at so many degrees btdc on its compression stroke ?

i know when i purchased the car someone had just cut the cid lead leaving the old plug in situ, rather than replacing it with the new one when they fitted new ignition leads.

[Brianmoooore]

The sensor is the equivalent of the cam sensor on more modern engines. It tells the ECU which cylinder out of 1 and 4 is on its firing stroke after both reach TDC together, and allows the injectors to be fired in pairs, rather than all together.

[arrisbmw]

thanks for the info, does that mean it would save fuel.

[Brianmoooore]

thanks for the info, does that mean it would save fuel.

Saves a small amount of fuel and emissions. Doesn't affect power, since all injectors fire together anyway under full throttle.