Sudden engine stop

[Bonymaenjack]

Good morning
I wonder has anyone experienced this and did they find out the cause
On a couple of occasions recently the engine (M20) has whilst driving suddenly stopped , no warning just stopped running , could it be a bad engine /body earth perhaps ?

[Speedtouch]

Could be a bad earth strap, possibly, or failing ignition switch, DME relay, or anything related to the electrical side of the ignition system.

[Brianmoooore]

Without details of how exactly it performed as it stopped running, it's difficult to say, but the most likely thing is a failing fuel pump.
Next time it happens, look at the tacho.

[Bonymaenjack]

Thanks both. I'll test the DME relay as a matter of course, the fuel pump is about 3 years old, I've not noted how the tachograph reacts ,(too busy worrying about stopping ) but I'm assuming if it does reset to zero then its possibly an ignition problem . Its happened driving up a hill so perhaps it is a mechanical movement of so sort to cause a disconnection?

[ChrisHC]

What does it take to get the engine running again?

[coopman]

This won't help, but I've experienced similar with an old Renault - generally after having been travelling at a steady speed and then slowing (e.g. for a sliproad or approaching a roundabout), the car would suddenly decide to turn itself "off". Fun/terrifying, and I never got to the bottom of it.

[Bonymaenjack]

What does it take to get the engine running again?

I can restart her normally on the key with no hesitation , tonight coming home after a good run at 60 70 mph approaching a roundabout when stopping to give way she cut out and I manged to bump start her as I hadn't come to a complete halt. and carried on driving , the engine was at normal operating temperature , when easing off the throttle she feels like shes running un even , Its just passed its MOT with the Co2 readings at idle slightly high but fine at higher revs .I've carried out a vacuum leak test with no apparent leaks , pressure tested the fuel delivery, system again as it should .A brand new AFM was purchased because I believed the old one had been messed about with with the seal missing .

[Bonymaenjack]

This won't help, but I've experienced similar with an old Renault - generally after having been travelling at a steady speed and then slowing (e.g. for a sliproad or approaching a roundabout), the car would suddenly decide to turn itself "off". Fun/terrifying, and I never got to the bottom of it.

Yes you're right , it doesn't

[Brianmoooore]

I can restart her normally on the key with no hesitation , tonight coming home after a good run at 60 70 mph approaching a roundabout when stopping to give way she cut out and I manged to bump start her as I hadn't come to a complete halt. and carried on driving ,

The next time this happens, look at the tacho. Has it fallen to zero, or is it gradually falling as the car slows?

[Bonymaenjack]

I can restart her normally on the key with no hesitation , tonight coming home after a good run at 60 70 mph approaching a roundabout when stopping to give way she cut out and I manged to bump start her as I hadn't come to a complete halt. and carried on driving ,

The next time this happens, look at the tacho. Has it fallen to zero, or is it gradually falling as the car slows?

Cheers Brian that's my intention , assuming if the techo falls to zero its an ignition problem. But sometimes when it happens I'm too aware of whats going on around me in traffic than to look at the gauges . Because of its intermittency I'm having trouble replicating the scenario in which it could may occur.

[Bonymaenjack]

Without details of how exactly it performed as it stopped running, it's difficult to say, but the most likely thing is a failing fuel pump.
Next time it happens, look at the tacho.

It happened albeit briefly tonight, travelling at 30 mph the engine just stopped and the rev counter died then restarted without turning the key , to be its an bad electrical connection of some sort , but where ?

[Bonymaenjack]

Without details of how exactly it performed as it stopped running, it's difficult to say, but the most likely thing is a failing fuel pump.
Next time it happens, look at the tacho.

It happened albeit briefly tonight, travelling at 30 mph the engine just stopped and the rev counter died then restarted without turning the key , to me its an bad electrical connection of some sort , but where ?

[Brianmoooore]

"But where" is a good question.
Do you have a OBC or an immobiliser? Both good candidates.
CPS? Usually temperature sensitive, getting worse as the engine heats up, but check the lead where it passes the water pump pulley.
DME relay? Either check by substitution or replace by wire links 30 to 87 and 87a.
Ignition switch has been mentioned. Easily checked by switching it when the problem occurs, but DON'T pull the key out!

[Bonymaenjack]

"But where" is a good question.
Do you have a OBC or an immobiliser? Both good candidates.
CPS? Usually temperature sensitive, getting worse as the engine heats up, but check the lead where it passes the water pump pulley.
DME relay? Either check by substitution or replace by wire links 30 to 87 and 87a.
Ignition switch has been mentioned. Easily checked by switching it when the problem occurs, but DON'T pull the key out!

Yes I have and use the fully functioning OBC and I've used the trick with the fuel pump you kindly shared. The switch used is for a rear demister and the fuse size fitted is 11 ( I know because I inadvertently test it every so often )
How to test if its the OBC ?

The CPS is recently new ( which is no guarantee of reliability) But I'll check it for cleanliness and any fraying which may have occurred since I've replaced it

I bypassed the C101 plug as the connections were failing so the connections have soldered joints with heat shrink and insulated

The DME test in Wiki I assume is to test it in situ if the car doesn't start and not the symptoms I'm experiencing?

"Bridge the DME relay by connecting pin 30 to both pins 87. This is to see if power to both the ECU and the Fuel Pump relay has been lost. If the car starts once bridged, it confirms battery power is getting to Pin 30, and that both 87 circuits are working (power to ECU and Fuel pump relay). It doesn't test the circuit that latches the DME relay, which therefore needs to be tested before condemning the relay"

No I wouldn't remove the key it wouldn't be in mine or other road users interest to do so

[BristolE30]

Had very similar situation on a friends 318i. Frayed CPS cable meant it was shorting out and cutting engine off.

Your new CPS cable could have got caught on a pulley or something

[Brianmoooore]

The immobiliser part of the OBC uses a relay to break the ignition feed to the engine, which is housed in a box that sits above the engine ECU. The copper tracks on the circuit board running between the relay and the socket on the box are not up to the job, and will become damaged if ever a short circuit has ever occurred on the circuits fed by them. You can open up the box and check the tracks (and relay contacts) or disconnect this immobilizer where it plugs into the socket with two green wires behind the glove box.
Unless the CPS was replaced in an attempt to solve this problem, the newer a part is, the more it is suspect. (bathtub reliability curve.)
I've never looked in the wiki for that test, but I recognise that text as adapted from something I posted long while ago. It's a test for the DME relay itself, as well as the circuit that controls it.

[Blanca]

Seems like a stupid question but do you have a lot of keys on the bunch? constant swinging from them can cause wear in the lock and switch the ignition off.

[Bonymaenjack]

Seems like a stupid question but do you have a lot of keys on the bunch? constant swinging from them can cause wear in the lock and switch the ignition off.

Nothing is a stupid question but no just the fob, ignition, steering lock & house key

[fixedwheelnut]

Both DME and Fuel pump relays suffer from dry solder joints on the board inside so they fail when getting hot as the resistance increases, also over bumps can effect it. Other suspects fuel pump itself , it could be a sensor but they tend to fail permanently.