p38 4.0 popping.

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mattfletcher

New Member
Posts
147
Location
walsall, west midlands
Hi guys, got a slight problem with my p38. You can start it and it will run on petrol for a couple of minutes, you go to rev it and there's loads of black smoke out the exhaust and it revs rough like its misfiring. Then it won't idle and just cuts out. Then if you start it on gas, idles just above 500rpm, you can give it a few blips of the throttle and all is fine, albeit seems to hold back slightly on the rev counter, as if its going up in stages. Left it running on gas for about ten minutes, give it another rev, and bang! that echoes down the street and it cuts out?
Anyone got any suggestions on what the fault could be, I really want to get it on the road and get some use out of it, but at this rate it looks like I'm going to have to turn it into a big bonfire! Blasted thing, I just don't get it any more, runs fine to start with, looks promising then all of a sudden disaster strikes. Been trying to get it to an MOT station for the last month but it's no good as it is.
Any suggestions or advice is much appreciated. Thanks Matt
 
take each plug out in turn, there will prolly be one that is oiled up, if there is its either rings or stem seals,

thats if the black smoke is oily stuff!
 
No Gav it looks more sooty to me but I'll take the plugs out and check. It does sound like there's no compression if you try and start it after it's cut out, and sometimes it will lock up. How hard is it to do the rings? Is it engine out or can you do it in the car?
 
hi matt if its mis firing that will make it back fire (un burnt fuel in exhaust) but it sounds like a valve burning out you will have to check thre commpressions which will ignite un burnt fuels in exhaust hope this helps
 
You know what? I had a feeling cracked block was coming. Will a compression test bring this fault up? Or is a lack of compression going to mean valves, pistons etc? Can you get the cracked block repaired or is it a case of a new block? How much do decent blocks go for? Anyone fitted a 3.9 efi to a p38?
 
3.9 wont fit witout modification, i looked at this but it proved more than enough hassle! if its sooty then could it be overfueling? possible temp sender issue? prolly not with lpg doing it too but it could be totally unrelated!before you go looking at expensive stuff i would go for a few cheaper options!
 
Well so far I've changed the crank sensor, cam shaft sensor, coolant temp sensor (I think, the one with the brown plug on), mass air flow sensor and the spark plugs. Thinking of changing the plug behind the temp sensor, the one with the single bullet connector on it? And a compression test over the weekend, tried to remove the thermostat (should it be at the front where the top rad hose enters the engine?) because the engine was throwing the water back out the header tank. And also the heads look to have been removed before as they are both clean as a whistle and the valley gasket also looks new. This is also making me wonder if someone has put the engine back together properly. Either way there's something in the back of my mind thinking the heads are going to be removed in the not too distant future.
The cause of the problem is not a major problem as I'm quite mechanically minded, but its the bloody finding of the problem that's really starting to get on my nerves.
 
another thought, i don't run gas and have no experience so this may sound idiotic, what happens if it doesn't correctly or fully switch between gas and petrol?
 
if it is popping back through the inlet mani fold points down to valves burnt out when compresion testing do a dry test first then a wet test if you have low compresions with a dry test then to a wet test by puting a little bit of oil down the plug hole to see if the compresion rises if it dont rise be head work req but if it does means pos bust piston ring.
But compresion testing may not pick up on a knackerd block. you can have liners fitted into your block but it aint cheap. There is an alternative if you go on good old e bay there is some stuff on there (carnt remember what is called) that i put in my mates rrover cos of a cracked block and two years latter its still going well. but type in on ebay over heating range rover and you will find it its exelent stuff its for america but it works
 
I was looking on ebay last night and come across some stuff, looks like industrial rad weld type of thing. That was going to be my next question whether that stuff works? The one I saw was Ironrite I think and the seller was saying it is good for cracked blocks etc. and all the industrial engineering companies use it. £25 a bottle. Trouble is it said you need to use the car for 2-3 days to let it work itself round. Mine will only run for 20 minutes and its blowing the coolant back out the header tank.
 
trust me matt it is worth it just go the long way round to let it work in but you have to flush the cooling system thoroughly for it to work. it is quite expensive but it does work if i was brave enough to run a v8 (and my misses wont let me) and it was doing what yours is i would cos i have used it before in my mates 4.0 v8 and that was 2 years ago and still going well
 
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