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Engine misfires - eventually
SteveWallace - 25/1/15 at 01:39 PM

I'm running a 2.8 straight 6 BMW engine in my Viento and the other week I decided to wrap the exhaust manifolds. I didn't disconnect anything whilst doing it but now when I run the engine for about 10-15 minutes I start to get a misfire. This is probably just a coincidence (unless anyone can explain why not) as I've not run the car much due to the weather. The symptoms are:

I) When I run the engine from cold, it starts to misfire after about 10-15 minutes of idling/reving. It misfires when idling and when driving.

ii) I thought that this might be something that was triggered by the engine getting to temperature, but if I then turn the engine off for a couple of minutes, I can run it for another 10-15 minutes before it happens again. Obviously the engine gets up to temp again very quickly, so the problem doesn't seem to be related to that

iii) I'm thinking that its probably not electrical as its OK for a while and does the same every time

iv) My fuel system is a low pressure facet pump delivering to a swirl pot and then a high pressure pump delivering fuel to the injector rail. The tank vent is not blocked

My current theory is that the facet pump is no longer delivering enough fuel (maybe partially blocked filter), meaning that the swirl pot eventually runs down and starts to cause fuel starvation. This would explain why turning the engine off and on again returns it to normal for a while as the facet pump kicks in slightly before the high pressure pump does. I guess that I could test this by letting the engine run to see if it eventually completely stalls, but I don't want to damage it.

Its potentially a frustrating problem as I can only try one thing every 10-15 minutes.

Any advice from the wise people on here would be appreciated

inkafone - 25/1/15 at 01:50 PM

Temp sensor on engine not connected / faulty so ECU thinks it's still cold and over-fuels?

rusty nuts - 25/1/15 at 02:21 PM

i had problems with my Facet pump pre filter, the one that screws into the pump. In the end I fitted a straight union and a disposable fuel filter between the tank and low pressure pump , its now easy to see if the filter is dirty. Your symptoms dont sound like fuel starvation to me , Didn't you use the BMW ECU and wiring? it may be possible to get a fault code which may help? If not it may be possible to disconnect one injector at a time to identify which cylinder[s] are causing the problem


That reminds me , i never did sort out the misfire I had on the way to the Donnington show

[Edited on 25/1/15 by rusty nuts]

SteveWallace - 25/1/15 at 06:28 PM

quote:
Originally posted by rusty nuts
i had problems with my Facet pump pre filter, the one that screws into the pump. In the end I fitted a straight union and a disposable fuel filter between the tank and low pressure pump , its now easy to see if the filter is dirty. Your symptoms dont sound like fuel starvation to me , Didn't you use the BMW ECU and wiring? it may be possible to get a fault code which may help?


I did keep the BMW ECU so I may take it to a local garage and see if they can have a look at the fault codes, if any

The strange thing is that it takes 10-15 minutes for it to happen from when I start the engine, regardless of whether the engine is cold or hot, which is why I suspect a fueling problem.

Maybe a stupid question but is it possible that the lambda sensors are getting too hot? I wrapped the exhaust manifold all of the way to the sensors. I'm reluctant to take the wrap off to test this as it was a bit of a pig to put on.

rusty nuts - 25/1/15 at 06:33 PM

If the only thing that has changed is the exhaust wrap it may be worth taking it off if only to eliminate it