Momentary engine cut out on 320i

Post Sun Aug 17, 2014 8:03 pm

Hi All,

Re: E30 320i (M20) on 'E' reg 1987.

Struggling a bit with a weird intermittent fault..... Driving along normally and the engine suddenly cuts out completely for around 1 to 2 seconds then continues fine! It only happens once every few hundred miles and is fine again. The symptoms are that the rev counter drops to zero and effect is the same as switching off the ignition momentarily. There is no warning lights or backfire. By the time you have worked out what is going on, the engine gets going again. I suspect it to be electrical rather than fuel or fuel pump related.

I assumed it to be the crank sensor and installed a brand new one but it has happened again since.

Any ideas?

Jason.

Post Sun Aug 17, 2014 8:54 pm

This may be an electrical 'spike ' causing the engine ECU to momentarily shut down. Not easy to analyse as it is so random and of short duration. When this occurred on my 320i I eventually found it was caused by interference from spark plugs,although they all 'looked OK'.So my advice is: try new spark plugs first.

Post Sun Aug 17, 2014 9:29 pm

Based on the rev counter activity, it'll either be the DME relay dropping out, or the CPS failing. If it's not consistent (as you say, once every few hundred miles) I'd be more inclined to blame the DME relay.

Post Sun Aug 17, 2014 10:41 pm

Grrrmachine wrote:I'd be more inclined to blame the DME relay.

This is the starting point, but it could also be a problem with an immobiliser, or even the ignition switch itself.
Check that the lead from your new CPS is clipped as it should be, and hasn't been savaged by the water pump pulley.

Post Sun Aug 17, 2014 11:27 pm

Thanks all,

Having changed the CPS sensor I took extra care to ensure the cable didn't chaff on the adjacent pulleys and belts, so I am pretty certain it isn't that, (the old one was intact anyway).

You have all raised really good points and I think that perhaps the list of probabilities is:
1. DMU relay
2. Plugs
3. Ignition switch (I will need to wait for it to fail completely to prove this one, so will leave it for now).
4. Immobiliser - This crossed my mind too. It could be linked but I would have thought it would kill the car completely rather than just for a second or two.

Thanks again everyone, I really appreciate the help!

Jason.

Post Sun Aug 17, 2014 11:43 pm

Paceracing wrote: 4. Immobiliser - This crossed my mind too. It could be linked but I would have thought it would kill the car completely rather than just for a second or two.

The average aftermarket immobiliser uses cheap and nasty relays, whose contacts can become pitted and intermittent.
If you have a 13 button OBC in place of the analogue clock, the relay for the immobiliser built into this is a bit under specified, and has been known to give problems like you describe.