Diesel In cylinder pressure HELP

RYM6746 · Post by **RYM6746** » Tue Feb 22, 2022 5:05 pm

Hi guys I have a 2012 volkswagen transporter 2.0L diesel engine code CAA. Long story short cam belt broke another garage replaced the belt without stripping head (Still runs evenly on 4 cylinders). Its never run quite right since. It logs a MAF sensor code and the actual and nominal are off at certain rev range (ECU not happy with the amount of air that the engine pumps. There is no cam VS crank fault being logged and the timing pins align perfectly. That said camshaft sensor is on the camshaft pulley (everything behind is not being monitored). The camshaft pulley key way is in tact.The crankshaft sensor is on the flywheel (kind of prooves that keyway on crankshaft pulley hasn't slipped) There is a gear system on the back of the exhaust cam that drives the inlet cam that needs locking tools to check/setup which we dont have (strong possibility this is where our problem lies). So in with the pressure transducer (Something definitely isnt right). I have also noticed that the average compression peaks are higher after the WOT, Something changed but what?? Can anyone explain what exactly this waveform is showing me I certainly am not an in cylinder expert (especially with diesel engines) Any help is greatly appreciated, Thank you for your help in advance. RYM

muttnjeff · Post by **muttnjeff** » Tue Feb 22, 2022 6:43 pm

RYM6746, no expert myself, from what I think I know, you are looking at an exh valve leak on the cylinder you were testing..

Possibly you have a timing issue, since you indicate no timing codes, no mechanical timing issue that you've found and your MAF code indicates a air flow problem.

Post compression stroke the wave never develops a vacuum pocket prior to exh opening, more or less the WF is extremely sloped and "lazy" in appearance, there is a slight hump around 360* which to me should be the beginning of intake however the intake is considerably late ~450*.

Would be good to add cam timing and see what your timing vs. cyl pressures are doing..

I have attached a great doc. describing the details of cylinder pressure diag.

PicoKev · Post by **PicoKev** » Tue Feb 22, 2022 8:38 pm

RYM6746 wrote: ↑
Tue Feb 22, 2022 5:05 pm
I have also noticed that the average compression peaks are higher after the WOT, Something changed but what??

You have a volume change due to a higher charge rate, pressure is naturally proportional to volume hence the higher peak pressure.

I can't see you WF as I am viewing on a phone with terrible service.

However a thought for consideration is does this engine have those awful hollow camshafts with the pressed on lobes that have a propensity to rotate on the shaft when pistons and valves come together?

With fun diags like this one sounds, I do sometimes find that repeating the test in each cylinder and comparing the results can reveal a great deal.

If you can capture the cylinders injection event as a marker against peak compression (note I do not use the term TDC here) that can also help to identify bent rods as well. Again if you can rinse and repeat on each cylinder and compare you can sometimes reveal the area of concern.

Kev.

RYM6746 · Post by **RYM6746** » Wed Feb 23, 2022 11:02 am

Thanks for the replys guys. The engine runs evenly on four cylinders so unless all 4 exhaust valves are leaking evenly i don't think that can be the problem. It lacks power in the lower rev range (Until the turbo kicks in) it fails a VE test at only 60per cent all the way through the rev range. Everything really points to the valve timing and the in cylinder waveform definitely shows a problem in that cylinder at least (I would love to do all 4 cylinders but the struggle to get those glow plugs out isn't worth the pain). I was hoping someone would be able to look at the waveform and give an in detail explanation of the data. I can see a problem but I can't say for sure what the cause of the problem is. If you conclude a sealing problem all 4 cylinders must have the same sealing issue ish as there is no misfire. Thanks again for your help

muttnjeff · Post by **muttnjeff** » Thu Feb 24, 2022 3:28 am

Not sure how you know or don’t know it’s running even on all 4 cylinders, please explain what testing has been done to support this, I suggest again adding a timing sync to your WF, would also be able to rule out timing by checking a a correlation of the cmp and ckp .

RYM6746 · Post by **RYM6746** » Thu Feb 24, 2022 10:33 am

When i say it runs even on all cylinders i mean it idles with no abnormal behaviour it pulls away with out any misfires all 4 cylinders are working evenly. I have pinned the cambelt up with timing tools. We do not have the timing tools to pin both camshafts that Is why the in cylinder pressure transducer has been used. I am not asking for help on further diagnosis. There is clearly a problem in the cylinder i am asking if someone can explain in detail the the fault that can be seen in the cylinder waveform. There is a problem with the ability of that cylinder to pump air. Can anyone tell me what the problem is with confidence???? Is it a leaking exhaust valve?? Is it a leaking inlet valve?? Is it cam timing?? Is it a large leak causing us not to be able to use cam timing in this waveform??

Interrupt · Post by **Interrupt** » Thu Feb 24, 2022 8:27 pm

I've never done diesel in-cylinder(so this is question, in the interest of learning. Hope this isn't considered jacking this thread/Post).
I would imagine that the wave form looks completely different because of the lack of vacuum? I take it, it has EGR? if so It should have a throttle plate if this is disconnected so it is closed and injection disable would you get a cranking waveform that looks more like a petrol engine? maybe this would take some of the mystery out of what we seeing here.

I have looked at some other diesel waveforms on this forum and they seem to have an anomaly that is present in yours (see attached circled in green). I'm really curious to know what causes that???

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