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Shen Long

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If timings chains being out causing the camshafts to hold valves open in the two middle cylinders with low compression then it would be the same for all cylinders.
That sounds like a good point.....
 

Skylarking

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If the engine has been ingesting dust, which cylinders get the most dust will depend on inlet plenum design. The back of the valves and valve seats will be sandblasted which will impact sealing. It probably won’t be equal impact in all cylinders as the more susceptible cylinders will be worse off. Such may explain the low comp on two cylinders, or it may not… You need to do some checking…

The other issue is the engine is consuming oil which could be related to rings which could also be damaged by dust getting into the combustion chamber. Or it could be related to blocked PCV valve or poor design resulting in oil being sucked into the inlet which then gets burnt in the cylinders. Or it could be a combination of both... More things to check..

The best way way forward is to look into the inlet ports see if the valves are sandblasted. Also look into the combustion cylinders via the spark plug hole and check for burnt Valves, scored cylinder walls and such. You can use a borescope (#10 cheap on eBay) for such which will answer such questions about obvious mechanical damage.

Then it’s also worth doing a leak down test of each cylinder at TDC on comp stroke but also listen to where the air is leaking to, be it intake, exhaust, oil inlet or radiator cap, as that give hints as what’s not sealing (intake valve, exhaust valve or failed head gasket… Such requires a compressor and a leak down test kit. Finally do a dry comp test then a wet compression test which gives more info about the condition of the piston rings and valve seats. A compression tester and a few spoons of oil is needed for this test…

All a tedious to do but it will give good info on the engine‘s condition.

The thing is, to do good comp test, you need to ensure the timing is indeed correct and the cam chain hasn’t jumped some teeth… The best way to check is to look at timing marks but depending on design and access that can be a difficult.

I’m sure there are lots of ways to check timing is correct, and I haven’t done it on a V6. but in one problematic I4, I’ve removed all plugs and used a hollowed out plug with a sliding rod within to indicate piston position. I then rotated the engine by hand to ensure piston #1 position was at TDC on comp stroke, then I check crank pulley was also showing zero on timing mark, it was not… Thus I found rolled crank key that was the cause of power loos due to retarded timing… Tedious but it resolved why the timing marks weren’t aligning as they should….

As to the loss of power which comes and goes, that sounds like an issue with yje thriller position sensor of accelerator pedal position sensor… check DTC’s when it occurs and act accordingly…

Depending on what is found after a look and a few tests, that will define what needs to be fixed mechanically. The rest is normal modern electronic/sensor related PITA issues…

Such will answer lots of questions and provide the info you need to decide what is the best way forward…
 
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