M51 lumpy running

Hi all -
after more or less a year of good running, I have a bit of a problem with my engine that I hope you can give me a mixture of abuse and excellent advice on

There's quite a long back story which I'll skip over for now and get to the point:
It's an '92 RRC with M51 engine transplanted from a 1996 BMW 325 td.
1) Idle is lumpy - can even see the revs rise and fall by 100 rpm sometimes.
2) right through the rev range power is inconsistent - it'll be good or a second, then 20% down for a couple of seconds and then good again. occasionally even seems to miss. It seems worst around 3k rpm, but not proven really.
3) Tested there weren't blockages / leaks in the fuel supply by running it direct off a can of diesel. It didn't make any difference to the symptoms but: I used clear pipes, and could see the supply pipe was just fuel, but the return pipe contents was very frothy.

based on little more than blind optimism and my last test there with the clear pipes, I'm hoping there's just air being drawn in and this is causing the problem.

Can anyone confirm/refute/suggest other tests?


thanks,

Jon

The FIP is not meant to suck fuel, it expects the fuel to arrive under low pressure from the lift pump, so is your lift pump OK?

Datatek said:

The FIP is not meant to suck fuel, it expects the fuel to arrive under low pressure from the lift pump, so is your lift pump OK?

Click to expand...

Thanks, yes, the lift pump is working, although it's not really a lift pump: in its former life it was a 3.9 v8, and still has the in-tank pump for that. At the mo, the in-tank pump is connected and runs, although when I got the car it was disconnected (and the car ran fine), and that's how its been for most of the time I've had it.

I guess the 3.9 efi pump is too high pressure? I've assumed it to be harmless, since the return flow out the top of the FIP seems pretty unrestricted, and so diesel just shoots round merrily in circles.
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what would the long term effects of no lift pump be on the FIP?
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I do have a 3psi 25gph petrol pump lying around I could put in line... hmm.

I'm pretty sure you're not supposed to use a petrol pump to pump diesel. Can't remember why but when I was looking for an inline pump for my mini digger I finallly got one in a ships chandlers. The guy had both but warned me to get the diesel one as he said the petrol one would fail sooner rather than later.

I guess diesel being thicker will work a petrol pump harder than it's expecting, but it seems to work fine at present.