Intermittant fault with 316i

[victoruk2]

Hello. I have a 316i with the most annoying fault. When the car is cold it is fine. When it warms up it cuts out for a second and kicks back in again over and over. It is as if the ignition is being turned off and on repeatedly. I have changed the fuel pump because it was noisey and the ignition coil. I have checked all the pipes and wires on and around the inlet manifold (there are bloody loads of them) and they all seem fine. Can anyone offer any advice with this. Thanks, Victor.

[kam-325i]

Dizzy cap ?

[Brianmoooore]

Cutting out or stuttering because it's running rich? What colour are the spark plugs? What's the fuel consumption like?
Do we have another case of blue temp sensor failure?
Oh, and welcome to the zone!

[jonbuoy]

I'd go with the blue temp sensor as Brian has pointed out.
Change this and get back to the thread for a report

[victoruk2]

It's cutting out, not stuttering. Definatly electrical. Plugs are nice and gray, fuel consumpsion not brilliant, but acceptable. The temp thing sounds good. Where is it? Thanks all for the help.

[jonbuoy]

Under the inlet manifold, about centre on the block. They are not great to get too on the M40

[Brianmoooore]

If the plug colour is OK, then it's probably not the temp sensor.
Main DME relay is the most likely suspect. White relay under the wiring cover on the bulkhead.

[victoruk2]

Thats great. I'll have a look in the a.m. Thanks everyone.

[victoruk2]

I forgot tomention the car is automatic. I don't know if this makes a difference. Is there any way of testing the DME relay or shorting the connectors to see if that is the problem. It is Sunday and nowhere is open to get any parts and I need the car in the morning. I live in Middlesbrough if anyone nearby has a spare one I could borrow or buy. The blue temp sensor looks like a right bugger to get to.

[Brianmoooore]

Link together pins 30, 87 and 87a or b or whatever it's labelled of it's socket to bypass it.
Blue temp sensor's not easy to get at, but taking out the air filter/AFM assembly helps with access.

[victoruk2]

It's not the relay.

[Brianmoooore]

Does the rev counter respond to this cutting out?

[victoruk2]

Yes. It goes up and down as if the ignition is being turned off and on.

[Brianmoooore]

Yes. It goes up and down as if the ignition is being turned off and on.

If you're sure it's affected by the fault, rather than just reflecting the drop in engine revs when the fault occurs, then it is electrical.
Find a small 12V lamp and position it somewhere that you can see it as you drive. Earth one side of it, and connect the other side, with a piece of wire, to the + terminal of the ignition coil.
Is it affected by the fault?

[victoruk2]

I have just done about 20 miles round a round the park waiting for it to start missing. It must be the cold weather. Anyway, it started missing. I connected a volt meter to the coil (easier than a bulb) and the needle stayed steady at about 14 volts when the car was cutting out. I hope this is a good sign.

[Brianmoooore]

I hope this is a good sign.

No, it's not a good sign! If the meter dipped, then we would nearly have found the fault. All you've proved is one more thing it isn't!
Looking more and more likely it's the crank position sensor starting to go open circuit intermittently. Without connecting an oscilloscope to it, it's going to be very difficult to prove it except by substitution.
Fits the symptoms perfectly; they are often temperature sensitive, failing when hot.
Might be worth connecting your meter/bulb to fuse 11 to monitor the fuel pump voltage. If the ECU can't see CPS signals it will think the engine is stopped and cut the fuel pump, but the interruption may not be enough to cause this.

[victoruk2]

Could it be the high tension side of things?

[Brianmoooore]

Could it be the high tension side of things?

Not if it affects the tacho.

[victoruk2]

The tacho goes up and down but not off. The car is automatic so the engine won't keep the tacho pointer up when it is cutting out. The cut out is too clean (no chugging or sputtering) for it to be fuel related. I'll get some new HT leads and let you know how it goes. Thanks for now.

[Brianmoooore]

Won't be HT leads. Never replaced one - even on LPG. They last forever.

[BeemaBoy]

Could this not be a fault with one or two of the plugs? Not firing properly?

[BeemaBoy]

No, sorry...scrap that! Thought it occured all the time, but I see it does not when cold.

[victoruk2]

Just fitted new dizzy cap and rotor arm. No improvment. Connected volt meter to fuse 11, the needle stayed steady when it was cutting out. Is it a trip to the scrap yard!!!

[Nelly]

Maybe a trip to the scrap yard to pick up a stonking 2.5 and all the running gear for it to replace your problematic 1.6 but, stick with Brian's knowledge of these machines as he knows his 5hit on them - you'll get there in the end.

Unfortunately it's gonna be like the Medical world - "A process of elimination"

Neil.

[Brianmoooore]

. Is it a trip to the scrap yard!!!

Yes - to get a crank position sensor!

[victoruk2]

Is that the lead attached to no. 4 plug lead that goes to the front crank pulley?

[Brianmoooore]

Is that the lead attached to no. 4 plug lead that goes to the front crank pulley?

No, that's effectively the cam position sensor. Crank position sensor is a 15mm by 60mm aluminium tube mounted on the front of the engine, low down on the right hand side against the large toothed wheel attached to the crank pulley.

[victoruk2]

I have taken the CPS off. Is it a 3 pin plug at one end, about 2ft of stiff-ish wire and a small alluminium can at the other end? Is there any way of testing it? There is a small break in the plastic sheath close to the crankshaft end.

[Brianmoooore]

You've found it!
All that's inside the can is a bar magnet and a coil of VERY fine copper wire, connected to two of the pins, and this wire will eventually break. The third pin, if it connects to anything, is connected to a metal screen around the wires.
The resistance of the coil should be around 540 ohms, IIRC, and is connected to the two pins nearest the lead.
A dud one will have an open circuit coil, but yours (if it's the problem), is only open circuit very intermittently, and may not show up under test
The crack in the sheath may have let water in and caused corrosion.

[victoruk2]

The two pins nearest the lead show an open circuit. Are these expensive, and can you get them from a motor factors? Lets hope this is the problem.

[Brianmoooore]

Don't quote me on which two pins to test, but there should be a sub 1000 ohm resistance between two of them, which should stay exactly the same when you wriggle the wires about. Moving a piece of steel past the end of the can should affect the reading.
You'd only get the exact item from a BMW dealer or somewhere like GSF or Eurocarparts, and I don't think it's cheap, considering it's just a magnet and coil of wire.
Try one from a scrapyard. You could be unlucky and get another dud one, but most are likely to be OK.
That's one of the beauties of BMW - there are no weak parts where most that you get from a scrappy are likely to be as bad as what you're replacing.
Be very careful when installing it to clip it properly in place where it goes past the water pump pulley.

[victoruk2]

Iv'e ordered a CPS from BMW. £52!!! If that doesn't sort it, what else could it be. The car isn't worth a lot of money so I dont really want to spend much on it. I really appreciate your help, Thanks.

[victoruk2]

Iv'e just fitted the new CPS. It is no better. I think I will set fire to it

[Brianmoooore]

If it's any consolation and if you didn't get a resistive reading between any two pins of the old CPS, then it was nearly dead and you killed it removing it. New CPS was needed anyway.
Beginning to run out of ideas for what is wrong though. M40 is a fairly simple engine and management system.
Recap:
You've thoroughly checked for air leaks on the inlet side?
Blue temp sensor changed?
Main DME relay eliminated?
CPS changed for new?
12V supply to ignition coil OK?

Only really leaves the air flow meter and the ECU. AFMs wear out and cause hesitation but I can't see why it should be temperature sensitive.
ECUs are normally ultra reliable, but it's getting to the point when it would be nice to try a different one.
Must be another zoner near you somewhere that could lend you these bits to try.
Nothing like fault finding with a working example of whatever you're trying to mend at hand. You know you've nailed the fault when you can transfer the fault from one machine to the other.

[victoruk2]

Is there a zoner near Middlesbrough who could lend me an air flow meter and possibly an ECU for 316i M40. As you can see from this thread I'm having a bit of a do with mine. Cheers.