318i engine performance after +/- 150kms. Coil?

[moncon]

Hello, I am sitting at a roadside garage waiting for a tow truck

My 1988 318i M40 starts and runs well, and then after about 100-150kms the engine starts coughing badly, and I lose power. The water temp is fine (new thermo three months ago), so I thought it was the fuel filter so on Saturday I changed that and also the spark plugs, but it happened again today. I fitted a new timing belt about 60,000kms ago.

Also, the SI Board is not working correctly - the speedo works for 100 metres and then stops working, and the service lights would either be on, or off, or some of them on, or others off, and so on. I put new batteries in, and checked the solder and various components, they all look okay (to me). Now the service lights are working properly but the speedo is the same: works for the first 100 metres or so, and then goes dead. The economy gauge is also not working. I had a look under the diff and that sensor and its wiring all look okay to me. All the dash lights, fuel gauge, rev counter, are working.

Perhaps the two issues are related?

[Brianmoooore]

Speedo fault is not related to the SI board, and will most likely be either wiring at the diff. (it breaks internally - you won't see anything), or dry joints on the speedo PCB.
Economy gauge can't work if the speedo doesn't.
Your current problem is most likely a failing crank sensor, although there are other suspects as well. What does the tacho. do when the car starts to fail?

[moncon]

Hi, thanks for the info about the speedo/ I'll have a look at the speedo PCB, and have another read on the Wiki about the diff and the wiring there, I hope that's what the cause is.

I can't remember now exactly what the tacho itself was doing when the engine started coughing/missing. I was pressing the accelerator and the engine was revving, but there was a loss of power. At first, the engine starts missing/coughing just a little bit while I'm driving along, and then it begins to happen more often and with worse effect, until finally there is hardly any power. As I change down to pull over (its a manual), it gets to the point where there is virtually no power.

[Dlb26]

Had similar fault on another car. Would start and run great for approx 20-30 mins then gradually splutter to point of having no power, as soon as it cooled it was fine. Eventually traced to coil breaking down with heat. At point of replacement also found out I was running with wrong one! Never had a problem since.

[moncon]

Had similar fault on another car. Would start and run great for approx 20-30 mins then gradually splutter to point of having no power, as soon as it cooled it was fine. Eventually traced to coil breaking down with heat. At point of replacement also found out I was running with wrong one! Never had a problem since.

Thank you - I'm certainly no expert, but if the coil is heating up it would affect the voltage to the distributor, no? I suspect the coil is faulty. Will order a new one and report back.

[moncon]

I tested the coil's resistance between the two primary posts and it is 0.5 ohms which is correct, however resistance between primary 15 and the king post is 5400 rather than the 5000 specified in the wiki (I tested several times to be sure of the contact). Is that a fault in the coil?

[Brianmoooore]

Probably means it's a different make of coil. The resistance goes down, if anything, when a coil goes faulty, as turns short circuit together.
Random parts replacement with new is an expensive way of fault finding.
In your OP you were waiting for recovery. Is the car running now?

[moncon]

Probably means it's a different make of coil. The resistance goes down, if anything, when a coil goes faulty, as turns short circuit together.
Random parts replacement with new is an expensive way of fault finding.
In your OP you were waiting for recovery. Is the car running now?

Hi Brian, yes it runs very well, until it doesn't ha ha. It starts well, turns over fine, powers up hills and down dales, no problems at all until about 100-150 kms into a drive, and then it starts to cough/miss just a little bit, and then progressively more often and more violently, until finally there is no power (but it's not stalling at all). The coil which is in there now is Bosch.

[Brianmoooore]

What colour are the tips of the spark plugs? Take one out and have a look.

[martauto]

Dont shoot me down for this please as I had a similar situation in an escort many years ago but is the petrol tank
vented properly ?
Mine created a vacuum in the tank and after a random while, stopped the flow of petrol.

Mart.

[moncon]

What colour are the tips of the spark plugs? Take one out and have a look.

I changed them just last Saturday, because this engine power issue arose for the first time last Friday - the old ones were not horrible, a bit of blackening and white deposits but they have been in there quite a long time (my bad). So I put in new sparks and a new fuel filter but on Monday it all happened again same as on Friday.

[moncon]

Dont shoot me down for this please as I had a similar situation in an escort many years ago but is the petrol tank
vented properly ?
Mine created a vacuum in the tank and after a random while, stopped the flow of petrol.

Mart.

Short answer: I have no idea
How did you check for that?

[martauto]

Dont shoot me down for this please as I had a similar situation in an escort many years ago but is the petrol tank
vented properly ?
Mine created a vacuum in the tank and after a random while, stopped the flow of petrol.

Mart.

Short answer: I have no idea
How did you check for that?

It was when I went to put some fuel in it , I heard a whoosh of air going into the tank when I took the cap off.
Just a long shot.

Mart.

[moncon]

Although the symptoms are not exactly the same, this thread suggests it might be an issue with fuel pump:

https://www.bimmerforums.com/forum/show ... ucking-e30

Thoughts?

[moncon]

It was when I went to put some fuel in it , I heard a whoosh of air going into the tank when I took the cap off.
Just a long shot.

Mart.

Hi, no I have nothing like that, but thank you!

[Brianmoooore]

Dont shoot me down for this please as I had a similar situation in an escort many years ago but is the petrol tank
vented properly ?
Mine created a vacuum in the tank and after a random while, stopped the flow of petrol.

Mart.

So my experience wasn't unique, then! Mine was on a brand new Escort, on its first long run. Flattened the tank to about half capacity before the engine quit.

[Brianmoooore]

If the plugs aren't black, it looks like we can rule out the engine faltering as a result of an over rich fuel mixture.
The observations of the tacho would really help in diagnosing further, but the CPS, DME relay, fuel pump, and even the engine ECU are prime suspects.
DME relay can be checked by removing it and using two short pieces of wire to connect together 30 and both 87s of its socket an driving the car. This is the equivalent of having the ignition turned on, so remove the links after you have stopped the engine.

[moncon]

Update: I installed a new Bosch ignition coil and took the car for a long drive, 300kms, and the coughing and power loss problem has gone away. I guess the coil was over-heating and not conducting properly (it looked to me like the coil I replaced was the original).

Over the past month the tachometer has started twitching around every now and then. It's not consistent: sometimes when the engine is cold (but not always), sometimes when it's at 3000 revs (but not always), etc. It was happening a little bit before I changed the coil, and it's happening with the new coil in there too. I plan to replace the CPS this weekend.

[Brianmoooore]

Try the DME relay bypass I described in my last post.

[moncon]

Thank you, I had forgotten about that, I’ll give it a go this weekend.