E30 engine cutting out - any advice?

[davidk]

Hi All,

I wondered if anyone could advise on an engine issue I have had for a while on my M20 (325i touring 1989). It cuts out every now and again, just for a second or two and then comes back fine.

It seems to be random in when it happens - i.e. whether engine hot or cold, accelerating/decelerating

I am going a bit crazy as I have replaced
- Ignition coil, HT leads, dizzy cap and rotor arm
- Fuel pump, filter, lines
- throttle position sensor
- Had ECU checked (replaced one of the sensors)

I have gotten so frustrated that I actually had a friend video it doing it, as its intermittent (and inevitably it didnt do it when I took it to the garage!).

It feels electrical (some sort of loose connection) as it just cuts out and then comes back again immediately.

Things I have not done yet
- Checked all of the grounds
- checked AFM
- it has got a dodgy clifford alarm/immobiliser/remote locking thing, which isnt working but I have not touched yet (a bit scared to - wires everywhere!)

Any thoughts anyone has would be really really welcome - I am starting to pull my hair out!
Cheers!

[magpie]

put a direct 12v feed to the fuel pump with an in-line fuse so the pump is buzzing away then go for a drive and see if i does it . you will have to splice the pumps live feed wire to do it .this by passes the cars electrics, if it still does it then you can cross that section off the list of possibility's .

[davidk]

Thanks I'll give that a try!
David

[Brianmoooore]

The very first thing to do is to observe the tachometer when it cuts. Does the needle drop like a stone, or track the falling revs?

[davidk]

Hi there - it drops like a stone, resulting in a big shudder if you are accelerating at the time. I have noticed that it tends to do it more when accelerating hard, but it still does it at other times (perhaps its just more noticeable as the shudder is stronger when accelerating). I have checked the fuel pump electrics and ground points around it and it isnt that.

However, I have come across a couple of other potential things while I was checking all the groundpoints the bentley manual was telling me. It has a replacement exhaust and there doesnt seem to be an oxygen sensor attached anywhere, which is strange. Could it be this that causing the problem?

[davidk]

ps - thats a great pic of tourers you have there brianmooore! Are they all 325is?

[Brianmoooore]

ps - thats a great pic of tourers you have there brianmooore! Are they all 325is?

Only the red one in that pic. is a 325i, but there are a dozen more not in that pic. A few 325s amongst the others, including a 24 valve one.
If the rev, counter drops like a stone, then you have an electrical fault which cuts the low voltage part of the ignition circuits.
By far the most likely suspect is the crank position sensor, especially if the fault gets worse with under bonnet temperature. Next on the list would be the DME relay, which can be checked by unplugging it and linking together pins 30 and both 87s of its base with pieces of wire, then, after that the ECU is probably the next suspect.
Other causes can be an intermittent connection somewhere in the power feed to the ignition circuits, which is best checked by connecting a small 12 volt bulb between the ignition coil + terminal and earth, positioned somewhere that you can see it when driving, then checking if the bulb goes out when the fault occurs.

[davidk]

Thanks a lot - this is really helpful. Ill give the DME relay check a go as I have already replaced the crank position sensor and see how that goes.

By a 24 valve one - thats an M50 conversion right?

[Millsand]

I had a similare problem which turned out to be the main power relay/fuse. (One of the large fuses in the little fuse box with three fuses in it, just in front of the big fuse box in the engine bay, unsure if that is the same one that Brian is talking about). The diagnosis was to turn the engine on and tap the relay with a pencil, if it cuts out when you tap it the relay needs replacing.

[Brianmoooore]

I had a similare problem which turned out to be the main power relay/fuse. (One of the large fuses in the little fuse box with three fuses in it, just in front of the big fuse box in the engine bay, unsure if that is the same one that Brian is talking about). The diagnosis was to turn the engine on and tap the relay with a pencil, if it cuts out when you tap it the relay needs replacing.

Only fuses are in the fusebox. The thee items under the cover are relays, and the relay to which you are referring is the DME relay.
My 24 valve E30 is fitted with a M50B25 engine. It was while looking for some info. while building it that I discovered the zone.

[BATTS91]

Id go with what Brian said ,Crank position sensor . Does the oil light come on when it cuts out ? Indication a loss in oil pressure . If so you can rule out an ignition switch fault for sure.

[bigwinn]

+1 on CPS

I had the same but more serious as it would cut and not restart,

the CPS had worn as it had been moved on a previous cambelt change and was rubbing on a pulley, then worn through

Stu

[318E30is]

You never know it could be as simple as an earth strap, it could be intermitent as it is able to arc over to continue the circuit and when you accelerate hard it pulls too far away from any metal to arc and cuts the engine. Sounds simple but my car was doing this and then failed to turn on at all it was the earth strap. Most importantly its free to fix.

[davidk]

Thanks everyone - I did the wire trick with the DME relay Brian suggested (joining pins 30 and the two 87s) - and hey presto, perfect running! Thanks very Brian - Have ordered a new relay and on its way to me now...

Was getting to the end of my tether as had replaced everything else (which all needed replacing tbh - the previous owner hadnt looked after it very well), but this was the last thing which had eluded... Very happy!