misfire

[boomer]

When cruising on part throttle, around 2.5 - 3k, after the engine warms up I can detect a very faint hesitation/stutter - almost like one cylinder is dropping in and out.

Its a 2l 6 pot and there is a slight blow at the exhaust manifold close to the bulkhead (snapped stud) - could this be the problem or should I be heading down the electrical path?

The engine's got 174K on it and needs a service.

Appreciate any help on this

Cheers,

Angus

[Brianmoooore]

Unlokely to be anything to do with the exhaust blow.
Do the service first, before fault finding, but plug or distributor cap is where I'd look first.

[E30BeemerLad]

i had similar symptoms with a blown inlet manifold gasket. I had a lumpy idle too though

[boomer]

Cheers fellas,

We'll see what happens after the service

[tonyppe]

i have a snapped stud where they swapped the engine.
it only blows on cold and i dont have any problem on running from hot or cold.
does your idle chase and not sit at one spot?
possible a bad HT lead may be causing it, but when its been HT leads for me, they normally break down at high revs.
can you drive through it?
if you can it could be a mixture problem, can anyone verify this?

when i first got mine i had an air leak on the induction side, which caused a bad idle. but only made driving at cold a little problem cos it gave me a flat spot. but i wasnt bothered really as i hate reving or driving hard when its cold.

[boomer]

As I said the the misfire/stutter only appears at 2.5K to 3K revs when running on part throttle. Put your foot down and it mostly dissappears only to return when you ease off again.

The blow from the exhaust manifold reduces to a tick when warm, although it comes back under moderate and above accelleration.

I'm not aware of any inlet blow (suck??) and it idles perfectly - no hunting like the other one I have.

My inital thought was electrical gremlins - I intend to replace the rotor arm/dizzy cap anyway.

I know the BMW leads are said to be indestructable but how true is this?

[E30BeemerLad]

I would go with Brian's suggestions and then see what the score is

[fannyj]

I have the same prob

[Templ8e30]

I know the BMW leads are said to be indestructable but how true is this?

They're not !, I had my e30 on a dyno last year and the leads were really knackered. They have a resistor in the caps that breaks down giving massively out of spec resistances which in turn gives spark duration and voltage variations losing power and torque.

I replaced mine with Magnecor electrosports leads and they made a big difference to economy and quality of idle and also gave better responsiveness.

Cheers,

Iain T

[boomer]

I know the BMW leads are said to be indestructable but how true is this?

They're not !, I had my e30 on a dyno last year and the leads were really knackered. They have a resistor in the caps that breaks down giving massively out of spec resistances which in turn gives spark duration and voltage variations losing power and torque.

I replaced mine with Magnecor electrosports leads and they made a big difference to economy and quality of idle and also gave better responsiveness.

Cheers,

Iain T

Were these for a six or 4 pot motor?

[tonyppe]

i bought magnecor leads for my other car.
they are £10 per lead for the ones from spark plugs to dizzy.
the king lead is £50, so thats £110 on HT leads for the 6s, apparently guaranteed for life, and are 10mm thick (they do 8mm ones cheaper too)

i broke one of my leads by pulling it. a rookie mistake, but i managed to repair it.

[Templ8e30]

Were these for a six or 4 pot motor?

It's an M20 2.7 6 pot.

I got them from here http://www.bbperformancetuning.co.uk/

Their website doesn't seem to be working very well at the mo, I got the 8mm Electrosports leads for £92.39 inc Vat.

I've found them to be a very good high quality set of leads and would definately buy them again.

They do 2 styles for the M20, one with (more expensive) and one without the pulse sensor on number 6 plug lead. I bought mine without as it is only used for diagnostics. The engine runs perfectly without one.

Cheers,

Iain T

[tonyppe]

so is that 90 quid for 6 leads or 7?

[Brianmoooore]

.

I know the BMW leads are said to be indestructable but how true is this?

HT leads have to be perfect for LPG powered engines. (Takes much more energy to initiate a spark in a LPG/air mixture than a petrol/air mixture).
I have tested many sets of E30 HT leads, all original, and have never found one even slightly out of spec.
If you think you may have a lead problem, then swap them for a set from a scrapper. The chances of finding two dud sets is extremely small.
C**p like Landrovers will hardly run on LPG without changimg to Magnecor leads, but, for a BMW, don't waste your £90 on them.

[Brianmoooore]

as it is only used for diagnostics. The engine runs perfectly without one.

Cheers,

Iain T

This only applies to early engines. Most use the pulse as a cam position sensor, as the ECU can't determine whether a piston is TDC on the firing point or mid scavenge.

[sean_318i]

I have tested many sets of E30 HT leads, all original, and have never found one even slightly out of spec.[/quote]

I have a similar problem on my 325i touring it misses between 3 and 4k rpm when accelerating but not when coasting. i tested my leads with my new expensive multimeter and found a difference of about 320 ohms highest being 6.08k ohms and lowest 5.83k ohms the coil lead was 1750 ohms and No. 6 lead was off the scale possibly broken. Are these resistances correct is this how you test them or is there a better way. No. 6 cylinder is firing at idle.
The car has done 167k miles now so maybe its due a new set.

[Brianmoooore]

I e
I have a similar problem on my 325i touring it misses between 3 and 4k rpm when accelerating but not when coasting. i tested my leads with my new expensive multimeter and found a difference of about 320 ohms highest being 6.08k ohms and lowest 5.83k ohms the coil lead was 1750 ohms and No. 6 lead was off the scale possibly broken. Are these resistances correct is this how you test them or is there a better way. No. 6 cylinder is firing at idle.
The car has done 167k miles now so maybe its due a new set.

6k ohms is about right. Sounds like someone has stretched No 6
and broken it, so it's very likely to be the cause of your missfire.
167k leads I would regard as nearly new. My wifes LPG fuelled touring (lead condition extremely important on LPG) has done about 240,000 on the original leads.

[sean_318i]

Yep i replaced No. 6 today and it cured the problem straight off it revs cleanly with no hicups. i found this one hard to diagnose cos it ran sweet everywhere else except those revs and the plug was firing. probably just wasnt getting the voltage at maximum compression pressure cos it had to jump two gaps one at the plug and one inside the lead. is the 320 ohm difference any thing to worry about as now 4 leads hawe 5830 ohms resistance and two have 6080 ohms aswell as the coil lead being 1750 ohms.

[Brianmoooore]

Wouldn't worryabout that small variation at all.

[boomer]

Well,

Done the plugs (old ones looked fine) and the air filter.

But the old filter had some oil contamination - and there wire traces of oil in the boot after the AFM.

Where could this oil be coming from and is it a problem?

[Brianmoooore]

Well,

Done the plugs (old ones looked fine) and the air filter.

But the old filter had some oil contamination - and there wire traces of oil in the boot after the AFM.

Where could this oil be coming from and is it a problem?

Oil is coming from the crankcase vent system, and, as long as it is not excessive, is perfectly normal.

[boomer]

Thanks Brian,

But what would you call excessive - there was enough to drip onto the top of filter and soak through a section of it.

[Brianmoooore]

TBH, I don't think I've ever seen it get back into the filter before, although there can be quite a bit in the folds of the air hose.
You don't park uphill, do you?

[boomer]

Nope,

The misfire has reduced with the change of plugs and filter, I'm waiting for GSF to get a dizzy cap in before doing them at the weekend.

What would cause crank case over pressure?

[Brianmoooore]

What would cause crank case over pressure?

Not what you want to hear - clapped out bottom end. Rings/bores ro be presice.

[sibzilla]

couldn't a blocked breather hose also cause it? just a nicer possibility, possibly. (was the case on my last car)

[boomer]

What would cause crank case over pressure?

Not what you want to hear - clapped out bottom end. Rings/bores ro be presice.

Wouldn't it be smoking a bit then? Coz there is no sign of the blue haze.

Doesn't the breather hose return to the inlet side? or does it vent to air?