Hi all,

Apologies for the long rambling post, but this is a difficult problem to describe!

As some of you may know, my P38 has been off the road for a while. In attempting to put it back on the road I've had to battle becm problems and other little nasties, none of which were present when I took it off the road. My plan was to repair what I thought was a leaking head gasket while I wasn't committed to using it daily, but as with a lot of things life got in the way and as it happens, it turns out the head gasket symptoms appear to actually have been caused by a leaking diaphram in the vapourizer for the lpg system but that's another story...

Anyway, it's almost there, but I still have a problem that only cropped up when I had the cats replaced, although I'm starting to think that was just coincidence now.

Basically, it starts fine and appears to run fine until you actually start driving it. Then the fun starts.
It will suddenly lose power, although kicking down usually allows me to keep going. After a short while it seems to sort itself out and the power comes back - it suddenly surges forward as though the accelerator has been floored. This happens quite frequently. Coming to a stop when it's misbehaving has been very problematic as it won't idle and just splutters to a stall, unless I keep revving it (I've not quite got the hang of left foot braking so this is a real pain especially now I've moved to traffic jam central). Weirdly it will restart immediately although it still struggled to idle.

Interestingly, this problem doesn't seem to exist when running on lpg, but for reasons mentioned earlier, I can't use that currently but it at least narrowed the problem down to the fuel system (well, that's my current theory).. I thought originally that the 2nd hand cats might be knackered.

Today I replaced the fuel filter - it clearly hadn't been done for a very long time. Interestingly, this appears to have dramatically improved the situation - it seems to be able to continue idling now when I come to a halt, but it is still randomly losing power (and returning it again with a surge).

Could it be that the replacement fuel filter (yes, I admit it - I bought a britpart one - surely they couldn't bugger up a filter, could they?) is iffy? It did seem to be slightly shorter than the one I took off.

The only other thing I can really come up with is the fuel pump, but is it likely to fail in this way?

It doesn't appear to be a breather problem in the tank as there is no hiss when I open the filler cap.

Is there anything else it might be and can anyone think of any meaningful diagnostics I can try?

I'm a bit reluctant to put it into my garage as they're now a long way away and I'm not sure I trust it on a long run in it's current state (and they couldn't find anything wrong last time they looked at it).

Any suggestions are welcome!

Guy

Hi, I have known similar issues to be caused by the maf sensor especially if it's been sat, another thing it could be is lambda if you have had cats, for 40 quid on ebay you can get a code reader which will help you find the culprit or I can read codes for you if you are local?

richieb said:

Hi, I have known similar issues to be caused by the maf sensor especially if it's been sat, another thing it could be is lambda if you have had cats, for 40 quid on ebay you can get a code reader which will help you find the culprit or I can read codes for you if you are local?

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Thanks for the suggestion. I hadn't considered the maf. I think it isn't used when running on lpg so it's a possibility, although why would the new filter have improved things?
The lamdas were replaced with new when the cats were replaced as apparently the connectors were different despite them coming from a gems vehicle. I was a little unconvinced at the time.

It still feels like a fuel problem to me though. When it happens, it's quite smooth - as though someone has gently applied the brakes.

kooky_guy said:

Thanks for the suggestion. I hadn't considered the maf. I think it isn't used when running on lpg so it's a possibility, although why would the new filter have improved things?
The lamdas were replaced with new when the cats were replaced as apparently the connectors were different despite them coming from a gems vehicle. I was a little unconvinced at the time.

It still feels like a fuel problem to me though. When it happens, it's quite smooth - as though someone has gently applied the brakes.

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In tank pump should be easy to tesy so yhat would be one less thing. Lambda sensors a good call, especially if not genuine items. Does petrol need a stronger spark?

Does it do it with a full tank, half a tank, low fuel level? Or does it not matter?

sometimes I have changed something and the vehicle has behaved better but it still has not cured the problem, I think a fuel pump either works or it doesn;t unless the power to it gets interrupted while driving but if that was the case kicking down and thrashing it would make no difference as kicking down requires more fuel,
if it was ok before the lambda's were fitted I think they are your fault, and if they have messed with your wiring then that's an even bigger alarm bell to me, the lambda sensors will be giving the ecu the wrong info so fuel will be interrupted etc, that's definitely where I would be starting

I understand this is a petrol engine, but I've just had similar symptoms (powerloss, then stalling when idling unless revved) with my 300tdi when I overfilled the diesel tank. Fixed it by removing the filler cap and driving until the level dropped. There was no hiss when I took the filler cap off. Could there be some crap in the tank that's being sucked up intermittently, or a hole in the fuel pipe inside the tank that's letting in air when fuel is low (not sure if this would affect a petrol, but it does a diesel). Or water contaminated fuel (eg from condensation). The fuel filter change may have helped, maybe change again after a while.

I've not put much fuel in it. If I'm going to have to drop the tank then I probably don't want to fill it up, especially considering how little use it's getting. It's done about 40 miles in the last year. Hmm.
I was thinking that perhaps it might be crud in the fuel system, but not sure where else to look. I've got the old filter but nothing seems to be visible in it.
I think the new lamdas were genuine - they certainly cost enough. The wiring looks correct so they haven't butchered anything to get them to fit.

Water contamination is possibility, I suppose - it would explain the surging nature of the problem although not why it's so eager to restart after it's spluttered to a stop at a junction. It literally always starts and restarts on the first touch of the starter. It has been stood on and off for about 3 years, although it was run up fairly regularly. The problem is that it's almost undrivable as it is so using it and hoping the problem will clear isn't really feasible. If I still lived in the country it would be manageable, but with the stop start nature of my journeys now the thought of driving anywhere in it is enough to bring me out in a cold sweat!

Weirdly, I have never got it to repeat the symptoms when running it on the driveway. Surely water in the fuel would appear at any time not just when under load? Same with the fuel pump.?

Any ideas how I could test the fuel pump? The problem is that the problem is intermittent so measuring fuel pressure when stationary isn't really going to tell me anything. Or can this be done when in motion? I was toying with the idea of buying a Nanocom, but not sure if any of the live data will actually help diagnose this.

How old is the fuel the modern stuff goes of and your left with brown varnish crap, have this problem with my classic car.

brianp38dse said:

How old is the fuel the modern stuff goes of and your left with brown varnish crap, have this problem with my classic car.

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As above I'm thinking old fuel if I don't use one of our school bikes for just a month the carbs glue up, if the fuel is old drain it in to a jam jar and examine if its cloudy its got water in it if not and its old drain in down stick a couple gallons of optimax in it and give it a try

I never kept much petrol in it as I always ran it on lpg. Pretty sure what was in there has been flushed through by now.
What came out when I changed the filter looked clear and golden and smelled ok.

kooky_guy said:

I've not put much fuel in it. If I'm going to have to drop the tank then I probably don't want to fill it up, especially considering how little use it's getting. It's done about 40 miles in the last year. Hmm.
I was thinking that perhaps it might be crud in the fuel system, but not sure where else to look. I've got the old filter but nothing seems to be visible in it.
I think the new lamdas were genuine - they certainly cost enough. The wiring looks correct so they haven't butchered anything to get them to fit.

Water contamination is possibility, I suppose - it would explain the surging nature of the problem although not why it's so eager to restart after it's spluttered to a stop at a junction. It literally always starts and restarts on the first touch of the starter. It has been stood on and off for about 3 years, although it was run up fairly regularly. The problem is that it's almost undrivable as it is so using it and hoping the problem will clear isn't really feasible. If I still lived in the country it would be manageable, but with the stop start nature of my journeys now the thought of driving anywhere in it is enough to bring me out in a cold sweat!

Weirdly, I have never got it to repeat the symptoms when running it on the driveway. Surely water in the fuel would appear at any time not just when under load? Same with the fuel pump.?

Any ideas how I could test the fuel pump? The problem is that the problem is intermittent so measuring fuel pressure when stationary isn't really going to tell me anything. Or can this be done when in motion? I was toying with the idea of buying a Nanocom, but not sure if any of the live data will actually help diagnose this.

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Well if it is a GEMS and you want to plug the Nanocom in you're welcome to pop up. Guess I could meet you halfway at High Wycombe if you're still in Sandhurst.

kooky_guy said:

I never kept much petrol in it as I always ran it on lpg. Pretty sure what was in there has been flushed through by now.
What came out when I changed the filter looked clear and golden and smelled ok.

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But you don't have a problem on LPG????

biketeacherdave said:

But you don't have a problem on LPG????

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Valid point. Back to in-tank pump, fuel itself, a blockage ...

biketeacherdave said:

But you don't have a problem on LPG????

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Lpg appeared to run fine, but unfortunately it blows all the coolant out so can't be used at the moment.
The coming and going nature of the problem did make me think blockage which is why I changed the filter, but that only improved the situation. It's still there. Any other likely choke points for blockages in the fuel line?
Can a pump fail with these symptoms?

kooky_guy said:

Lpg appeared to run fine, but unfortunately it blows all the coolant out so can't be used at the moment.
The coming and going nature of the problem did make me think blockage which is why I changed the filter, but that only improved the situation. It's still there. Any other likely choke points for blockages in the fuel line?
Can a pump fail with these symptoms?

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Yes a failed pump can cause low fuel pressure mines a Thor so no fuel filter, did you say its fine on tick over ???? If so and cuts out when revved or driven deco sounds like fuel starvation is failed pump\blockage caused by old stale fuel. I have to completely strip the carbs on my bikes when the fuel goes off

I reckon its a blockage in a main fuel line. Sounds like a blocked jet in the float chamber in the good old days of carbs and when you were pulling lots of fuel through then a wee bit of gunge would get sucked into the jet and then fall out when the revs went down. Check the suction pipe for the fuel pump? Or its water or gunge in old fuel. I was always told that to avoid condensation then you should keep the tank full and if storing for long time then to fill up during the winter so fuel is winterised.

Pull the fuel pipe at the back of the inlet manifold and check for fuel there, if all else check out pull the inlet manifold off and clean the injectors directly, simple to do I did mine when I did my hg.