Failed compression on a warm engine

1.4 Petrol 2012 Picasso C3. Started off with cam position sensor P1340. replaced sensor. got about 100 miles before EML came back on. This time saying Misfire cylinder 2. Changed all 4 spark plugs. and swapped some coil packs around. Still coming back with cylinder 2. Did a compression test. Passed on a cold engine. Failed compression on a warm engine. Mechanics suggesting we remove cylinder head and send off for re-sealing. Quote coming in for £1500 with no guarantee it will fix it. car only cost £3000.

Any other ideas guys?

Hi

Probably cheaper to drop a replacement engine in there.

Not good. Possible:

Cracked head.
Carbon build up on the valve face, seat or stem (causing valve to stick).
Broken piston compression ring/badly scored cylinder/gunked up compression rings/damaged piston crown.


The 1500 pound should cover repair of the head and remove/install. Cracked head might not be repairable.
Damage to the block, like a scored or worn cylinder or piston troubles would make the engine uneconomical to repair.

You best hope is a sticky or leaking valve.

I take it that they did not do a "leak down" test? You need to know what is causing the compression leak. The cylinder can be pressurized and depending on where the air leaks out, give an idea of potential causes.

Strange, compression usually fails on a cold engine but passes on a warm engine.

The mechanic needs to do more diagnostics and pin down the actual fault before he pulls the head off. You might be wasting your time and money.

Does it misfire all the time or randomly?

wow! thank you for such an in depth reply. very thoughtful. i am truly grateful.

the misfires are random- but i think they usually happen at the lower revs.
when i ask the engine to go- it goes.
it starts off being quite driveable. starts first time. idles a bit dippy.

after the engine management has been cleared- we get a few miles before it comes back on again.

my dad thinks i should reset the ecu by removing battery and draining memory- just to see if any different new fault codes become foremost.

Hi

You won't clear codes by disconnecting the battery

Use a compatible OBD2 reader to delete the codes from the memory.

winkerbunhead wrote: ↑Sun Oct 23, 2022 8:53 am
the misfires are random- but i think they usually happen at the lower revs.
when i ask the engine to go- it goes.
it starts off being quite driveable. starts first time. idles a bit dippy.

If it's intermittent then it might NOT be some of the very bad fault's I mentioned. It might be something "gummed up".

You could try an "upper cylinder lubricant" like Morey's or Lucas Fuel Treatment. See if it will free up any sticky parts. It goes in the fuel and as the bottle says lubricates components in the upper cylinder, like the valves. The results aren't instantaneous but it's relative cheap and worth trying.

Maybeanother clue here, but straight before we had misfire on diagnostic, the diag was reading error code P1340 cam position sensor. i understand there to be 2 sensors of which both got replaced. this code went away - but this new cylinder misfire thing started within 2 weeks of the new sensors. THing I dont understand is, How can there be a misfire AND loss of compression? i didnt think both were possible.

winkerbunhead wrote: ↑Thu Oct 27, 2022 6:27 pm THing I dont understand is, How can there be a misfire AND loss of compression? i didnt think both were possible.

For an engine to "FIRE", you need 4 things. Compression, Ignition, fuel and oxygen.

Each time a cylinder fires the crank shaft is accelerated a little bit and between firings the crankshaft slows down. You feel it as vibration but the engine ECU sees it as each cylinder firing.

The definition of a "MISFIRE" is: "The crank shaft did not accelerate at the moment that the engine management unit expected a cylinder to "fire" and produce power".
If the cylinder 'fires' before or after the engine ECU expects it...that's a misfire.
If the cylinder doesn't fire at all...that's a misfire.

So anything that causes a cylinder to not fire correctly (for any reason) is called a misfire. If you have poor compression, that might not allow the air/fuel mixture to burn and produce enough power for the engine ECU to detect the acceleration of the crank shaft at that point. It might burn the fuel but not produce power.
If the loss of compression (the ability of the cylinder to hold pressure) is borderline, the cylinder might produce power most of the time but now and then be unable to support combustion and you get the occasional misfire.

Having said all that, how an engineer "looks" at things might be very different to the way you and I look at things. For example: the only reason for the CAM position sensors is to ensure that the camshaft(s) are correctly aligned to the crank shaft. If they were out of alignment that could mean at best, the engine is less efficient, at worst, could cause damage to the internals of the engine! BUT, is it possible that the CAM position sensors are somehow used in the engine timing? I don't know, I wouldn't have thought so, but I'm not the engineer who designed it and I am not privy to any sort of relevant documentation that might help...sorry. So while it's plausible that the camshaft sensors are somehow involved, any conclusions I might draw from that are tenuous at best!

Can you feel if the car misfires?