L322 4.2SC misfire after engine rebuild

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onebounce

Member
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36
Location
Surrey
Hi all, looking to tap into the brains trust for thoughts on how to solve the latest challenge…

A brief recap:
1. I’ve owned car for about 10 years Engine has done about 200k, I rebuilt the gearbox but failed MoT due to excessive emissions. Investigations confirmed some exhaust valves leaking. Stripped heads and confirmed all exhaust valves and seats had serious pitting and one was broken.
2. Removed heads, skimmed, valves, guides and seats all done - vsr due to long term running on lpg.
3. Refitted heads, rebuilt supercharger, fitted new timing chains, belts and water pump.
4. Engine started up fine and ran nicely 😁 when parked but on test drive serious pinking/knocking and associated power loss.

Thoughts on possible culprits ?

1. Induction leak, running over lean ?
2. Ignition timing disturbed somehow?
3. Engine / cam timing set incorrectly ?

Thanks all

Andy
 
LPG is the work of the devil... Killed more good engines than a few in my experience.
Any DTC's?
Smoke test the inlet to identify any induction leaks, then do the same for the exhaust, injection & spark timing is by CKP/CMP sensor input to the ECM, did you replace the camshaft dephasers?
 
Graculus said:
LPG is the work of the devil... Killed more good engines than a few in my experience.
Any DTC's?
Smoke test the inlet to identify any induction leaks, then do the same for the exhaust, injection & spark timing is by CKP/CMP sensor input to the ECM, did you replace the camshaft dephasers?
Click to expand...
Thanks - no, didn’t replace them, replacement is new one on me, can you expand ?
 
Mark Piercy said:
Injector issues or wiring to them. You may have to back over what you have done 🤔
Click to expand...
Interesting, could be… I pulled one and from a quick visual test it was firing an okay spray when turned over and engine ticks over and revs on no load fine, just a problem when under load. Next steps ?
 
onebounce said:
Thanks - no, didn’t replace them, replacement is new one on me, can you expand ?
Click to expand...
We always replaced them if the timing chains were being replaced. More to the point, what, if any, DTC's are there?
Have you carried out a compression test since the rebuild?
 
Did you check and adjust the valve clearances once the replacement valves were fitted?

1734028990261.png
 
Graculus said:
Did you check and adjust the valve clearances once the replacement valves were fitted?

View attachment 331641
Click to expand...
Yes, all inlet reset to 0.2mm, exhaust to 0.25mm, plus or minus. Couldn’t afford new buckets, so ground the valve stem tips to get the correct clearance (used my lathe with a very fine grinding wheel). Took several iterations, needless to say became a dab hand with the valve spring compressor !
 
Graculus said:
We always replaced them if the timing chains were being replaced. More to the point, what, if any, DTC's are there?
Have you carried out a compression test since the rebuild?
Click to expand...
Good point, will get a (generic) diagnostic reader and see what it says. No compression test not (yet) done - thanks will add that to the list :)
 
I've seen oval bores on the 4.2 due to extended use of LPG. Not sure if it's entirely down to LPG's 'drying' properties or the typical usage pattern of LPG-equipped vehicles.
 
Just to update… found a faulty coil pack, replaced and engine now running nicely, without any misfire :)….

But now overheating and cannot seem to properly bleed the system as some hoses are cold, some empty, despite filling the tank and running the engine to top it up. Water pump and thermostat have been replaced. Electric water pump (for the SC) running fine. Thinking it might be a gunged up radiator, as engine sat for nearly 2 years with coolant hoses disconnected ? But welcome suggestions before removing the radiator(s) to check if they are flowing coolant okay.

Thanks
 
Vacuum fill first, it won't bleed properly the way you're doing it.
Why were the hoses disconnected?

Did the DTC read point you to a coil pack?
 
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