Misfiring

[Mitchen]

Had a hectic day of driving today, drove from East Sussex through London and on to Zausts place in Welwyn to get some audio work done.

As I was coming off the motor way, I started noticing a slight misfire, felt like the auto gearbox changing gear, although I was driving 80mph at the time so obviously wasnt. As I came onto normal residential roads and driving around 30mph, it was clear the problem was obviously misfiring when the revs were high.

Me and Zaust were looking for air leaks in all the normal places but couldn't see anything. Driving back it was just continuous misfires when the revs started reaching a high amount.. It also knocked the rev counter needle down whenever the misfire happened.

Was due to have cambelt service next week, but if a fix for this is going to involve refurbing injectors etc. Then that may all have to be put on hold.

[Brianmoooore]

Rev. counter response suggests an electrical fault early in the spark control circuitry, so crank position sensor, ECU or a problem with the electrical feed to the ECU.
CPS faults tend to be temperature sensitive, so try the car again when cold.
Electrical feed fault is most likely to be the DME relay. Easiest way to check this, other than by a known good replacement, is to remove it and use wire links to connect pins 30 and both 87s of its base together, and drive the car like that.

[Mitchen]

Thanks Brian. Just took it for a quick spin in sports mode with the high revs and a cold engine, and it seemed ok! May just be this CPS fault that you speak of. I've read that the CPS is near the front, of the engine, but what's it near?

[kam-325i]

its right next to the crank pulley, bolted to the block..

[maxfield]

If you look next the front crank pulley you should see it

[Mitchen]

Brilliant, cheers guys.

I take it this is where the DME relay is kept? In the housing between the coolant and the steering fluid. But which one is the DME? They're both the same relay as well.

[Speedtouch]

The white one:

http://www.e30zone.net/e30zonewiki/index.php/Relays

[Brianmoooore]

The white one:

http://www.e30zone.net/e30zonewiki/index.php/Relays

Why is it sitting so low down? Something odd there!
Unless you have a cat. in the exhaust and a lambda sensor, one of those orange relays shouldn't be there.
You may get an insight to the condition of the CPS by, a) a visual inspection of the wire to it, checking that it isn't cracked near the sensor, or has been savaged by the water pump pulley, or b) driving the car until the fault occurs again, then quickly unplugging the sensor (under the diagnostic socket) and measuring the resistance between two of the three pins.

[Mitchen]

Ok well I've actually broken down now on my way to work. The car was driving, and then soon as I hit the normal hammersmith traffic, it started jumping and the temp, revs, and speed dials just kept jumping and it's cut out completely. Now whenever I start the car it jumps for a couple of seconds and dies. Currently waiting for the engine to cool down to see if it does the same on cold.

[Mitchen]

Just phoned a friend of a friend who's a mechanic and turned the engine over with him listening, he thinks the head gaskets gone as be said he doesn't hear any compression.

[Grrrmachine]

Don't jump to conclusions. Look for mayonaisse under the oil cap or cappucino in the coolant. Get a compression test done on all six cylinders - you won't have lost compression on all of them, so it could be your mate's talking out of his arse.

CPS failure is heat-related in many cases - the car works fine from cold but fails when hot, and won't restart until all the heat sinks away. Do it too many times and it fails at cold too. The way to confirm is to TEST for spark and fuel - CPS knocks out both. If you have one but not the other, the problem is elsewhere.

[Mitchen]

The oil looks fine, although the coolant does have a dark ish shade to it. Currently waiting for recovery. I'm thinking the worst but guess time will tell.

[Grrrmachine]

If you've got a piece of wire (or a paperclip or even a twisted-up kitkat wrapper) you could try bridging the DME relay. If that's the fault, it'll be enough for you to limp home.

[Brianmoooore]

If the DME relay is the problem, and you can bypass it, then you won't just be able to limp home, you'll be able to drive the car normally.
Head gaskets don't fail catastrophically across all six cylinders at the same time. The most you'd lose is two, which still leaves you with as many working pots or more as 90% of the cars on the road.
Worst case scenario is a broken or jumped cam belt, which would result in zero compression, but this wouldn't be the original problem, since the engine doesn't do temporary recoveries from this!
Chances are that the CPS is at fault - have you been disturbing the area around the water pump?

[Mitchen]

Ok from looking at it now, I've got a water hose that runs from the stat, and it's leaking water over the CPS cable. I think I may have found the culprit.

[Mitchen]

So got a new CPS through from ECP today, not genuine but needs must.

Turns out the cable which as you all know runs very close to the fan belt, had come out of it's home and was scraping the fan belt as it was turning. Coupled together with a leaking water pipe directly above, it's not a good look.

Thanks to everyone that helped me diagnose this and get it resolved very quickly.. Exactly why I love the zone.

[Speedtouch]

Good to hear you found the problem.

You could probably repair the cable and keep that as a spare.

[Falamon]

My CPS sensor housing is a little cracked, if water gets inside the housing could I see a similar issue to the above?

[Brianmoooore]

if water gets inside the housing could I see a similar issue to the above?

Almost certainly!

[Falamon]

I'll seal it up again later to avoid this problem then, think the tape fell off while doing the water pump.

[Brianmoooore]

think the tape fell off while doing the water pump.

Tape?? Does this mean its previously been bodged?
If you're going to seal it up with tape, at least use self amalgamating tape, that will do the job properly, and will never fall off.

[Falamon]

I'm finding plenty odd jobs, bodges and horrible rounded things all over it I'll look at the tape but would rather fit a new piece of housing to be honest.