Backfire p38

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Duncanincapable

Active Member
Posts
99
Location
Barnoldswick lancashire
Hi again 4 litre p38 petrol having done head gaskets and valve stem seals the rr has developed a backfire on one cylinder. As you stand at the front looking at the engine it is the left hand bank number three cylinder. Have changed plug and injector if I disable the cylinder either plug or injector it doesn't do it just runs rough and runs very rich. The backfire starts at once every 10 seconds or so but as the engine gets hotter it increases expedentially, I don't know whether this is significant but watching the exhaust the backfire interupts the "smoke" from both pipes. I am at a loss as to what has caused this and any suggestions no matter how small will be most welcome. I haven't got to the point of torching it yet so you can forget that for now!!! Thanks I'm off to bed and the nightmares that are going to follow
 
If it's backfiring, it's not containing the pressure.
So,
It's either injecting and/or firing at the wrong time

Or, more likely you have a sticking valve.
I say more likely as if it was timing related it would do the same all the time.
I'm assuming you took out the valves when you replaced the stem seals? Are you sure each valve went back in the same guide? My guess is that you have a valve guide that's slightly tight and with a different valve in it, it's sticking. As it gets hot, stem expands due to the heat and it gets worse.
It's only a guess but sounds feasible.
 
Wazzas explanation sounds highly possible! And very annoying for Duncan. I know it's possible to change the seals using the rope trick but would this help to free up a guide. Could be a broken spring or something else. So first job rockers off!
Jb
 
Wazzajnr said:
If it's backfiring, it's not containing the pressure.
So,
It's either injecting and/or firing at the wrong time

Or, more likely you have a sticking valve.
I say more likely as if it was timing related it would do the same all the time.
I'm assuming you took out the valves when you replaced the stem seals? Are you sure each valve went back in the same guide? My guess is that you have a valve guide that's slightly tight and with a different valve in it, it's sticking. As it gets hot, stem expands due to the heat and it gets worse.
It's only a guess but sounds feasible.
Click to expand...
Valves were done one at a time long way round but minimises mistakes in theory.
 
Duncanincapable said:
Valves were done one at a time long way round but minimises mistakes in theory.
Click to expand...
OK, so maybe a broken spring or a hydraulic lifter sticking open a bit.
I don't know the root cause but it sounds very much like a valve not closing properly.
Does it happen at all revs or only at idle?
 
Starts at when it starts to get warm from start up and then gets worse had rocker cover off and all seems to be working as it should if I can't sort this by tomorrow it will become a v7 I will disconnect the injector and run it as usual gone past caring now.
 
have you swapped plugs around ?

my P38, ran fine after the head rebuild, I even put new coil packs on. however the HT leads were not replaced as I got messed around being sent the wrong ones.. by the time the right ones were in my hand, the engine had gone back together. fast forward a couple of month.. running like a bag of spanners, replaced plugs and leads, running sweet as again. if you are confident the head rebuild was spot on, check the leads. the coil packs (on the Bosch/Thor engine - don't know about the gems) are redundant spark so if a coil pack goes, youll get a misfire off the corresponding coil on the correlating cylinder.
 
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