GLC 250D low compression

[Damoiam]

Hi
My 2017 GLC250d has been getting harder and harder to start, tried all sorts including new injectors and no improvement. 3 hrs at main dealer workshop last week and they have no idea. Anyway I have finally tracked it down to extremely low compression on cylinders 1 & 3 showing under 9bar, cylinder 2 showing 15bar and cylinder 4 not even registering compression. All well below the minimum 22bar needed. Compressions tested manually the old fashioned way not electronically with Star (dealer said star didn’t pick it up)

We even used 2 different compression gauges as we didn’t the believe the results, but both were consistent.

When it cranks over it attempts to fire, it gathers and gathers and eventually it will start (very much like an old diesel with only 1 glow plug working). As soon as it does start (can be several attempts of cranking timeout) it runs as smooth as anything, no misfire, no juddering and no smoke, it drives absolutely fine and pulls like a train. (Although it will sometimes stall when pulling up if still cold) Even if you drive it hard and keep it revving it does not smoke one bit, so I’m sure it can’t be piston rings or warn bores.
The starting is the same wether hot or cold but is worse when cold.
We discovered that on cranking only cylinder 2 is firing, hence the gathering behaviour of the other cylinders. I believe that cylinder 2 is the only cylinder with enough compression to ignite.

Car has 135k miles with full history, has always had genuine Merc oil and parts. It has not happened quickly but got progressively worse over the last 3 months.

Now I don’t understand how cylinder 4 can be zero on compression when cranking but without a misfire when running. (All cylinders are well below what they should be)
So we, did a leak down test (just on cylinder 4 as it’s the worst) and it would appear to be leaking out the air intake so clearly on the inlet side. It can’t be a just a burnt valve as it would be obvious when running, plus that would mean all inlet valves are burnt as all compressions are so bad.
My conclusion is that something is holding the inlet valves open (to varying degrees across the cylinders) but only during cranking, once it fires up it runs absolutely fine, and drives fine as well, has plenty of power and no smoke.
I thought maybe timing chain had perhaps stretched or jumped and put the cam timing out but it runs perfectly quietly as soon as it has fired up, plus I think a jumped tooth on timing chain it wouldn’t run at all.

Anybody experienced anything similar or have any idea on cause? (My last Merc with the 651 engine did 250k miles and was still going strong when I sold it)

Any help appreciated

 

[grober]

Hydraulic tappets slow to pump up? or wrong horizontal camshaft adjustment

https://diysprinter.co.uk/reference/Introduction_OM_651.pdf

 

[ToeKnee]

Hydraulic tappets slow to pump up?

If the tappets were slow to pump up wouldn't that make the valves stay closed longer rather than stay open?

 

[Bellow]

My conclusion is that something is holding the inlet valves open (to varying degrees across the cylinders) but only during cranking, once it fires up it runs absolutely fine, and drives fine as well, has plenty of power and no smoke.
I thought maybe timing chain had perhaps stretched or jumped and put the cam timing out but it runs perfectly quietly as soon as it has fired up, plus I think a jumped tooth on timing chain it wouldn’t run at all.

If the timing had jumped - all cylinders would be affected.

On another forum, a guy describes hydraulic tappets as ''oil filters'' on account of their very small exit route for oil being prone to catching any debris in the oil.
Have to wonder if yours aren't a bit choked up and the blockage relieves with a combination of thinning heating oil and/or, the more vigorous force applied to them with the higher rpm running entails.
When you did the leak down test, did you check for valve clearance? Is there a possibility that during the leak test pressure leaked past the rings into the PCV system to appear in the inlet manifold?

 

[Damoiam]

To be honest I wasn’t present when the leak down test was done so unsure if PCV was disconnected but what you say makes sense. I do know that with the oil filler cap off during cranking there is barely any pressure as I tried it, I also tried it with the dipstick out and again barely any pressure.
I would expect if there is that much leak past the rings then it would smoke really bad when running.
Have not checked valve clearances yet so that is going to be my next job to see if I can detect valves sticking open.

 

[Bellow]

To be honest I wasn’t present when the leak down test was done so unsure if PCV was disconnected but what you say makes sense. I do know that with the oil filler cap off during cranking there is barely any pressure as I tried it, I also tried it with the dipstick out and again barely any pressure.
I would expect if there is that much leak past the rings then it would smoke really bad when running.

I see your thinking but, it's possible the oil control rings are OK but the compression ones not. Also, the smoke thing when running is more a petrol engine phenomenon where a closed throttle creates cylinder vacuum and pulls oil up past the rings. Diesels aren't throttled (if at all) to the same extent. Petrol engines can consume considerable quantities of oil without it showing as smoke due to the cat. Not sure if diesels are the same but in case they are...
Has it been using oil?

Have not checked valve clearances yet so that is going to be my next job to see if I can detect valves sticking open.

First port of call for sure.

 

[Damoiam]

I see your thinking but, it's possible the oil control rings are OK but the compression ones not. Also, the smoke thing when running is more a petrol engine phenomenon where a closed throttle creates cylinder vacuum and pulls oil up past the rings. Diesels aren't throttled (if at all) to the same extent. Petrol engines can consume considerable quantities of oil without it showing as smoke due to the cat. Not sure if diesels are the same but in case they are...
Has it been using oil?

First port of call for sure.

Thanks for your insight. Car has not been using any oil and I checked it this morning and is if anything very slightly over the full mark. (Could that be additional diesel creeping past the rings on cranking when its not firing?) I maybe over analysing here!What I have discovered is that if I clamp the PCV pipe closed so that it can’t vent crank case pressure back into the inlet, on cranking it blows the dipstick out and forces oil up the dipstick tube by the end of a cranking sequence.
That to me says too much crankcase pressure on cranking and it’s ring bypass. I’m not sure how much crankcase pressure to expect from one of these engines?

 

[Bellow]

Thanks for your insight. Car has not been using any oil and I checked it this morning and is if anything very slightly over the full mark. (Could that be additional diesel creeping past the rings on cranking when its not firing?) I maybe over analysing here!What I have discovered is that if I clamp the PCV pipe closed so that it can’t vent crank case pressure back into the inlet, on cranking it blows the dipstick out and forces oil up the dipstick tube by the end of a cranking sequence.

That to me says too much crankcase pressure on cranking and it’s ring bypass

Inclined to agree.

.I’m not sure how much crankcase pressure to expect from one of these engines?

I don't have much experience with diesels so can't say definitively but it does sound excessive.
Next question I suppose is whether the rings or bores are at fault - or both. And why?