2017 GLC250d engine failure

[Damoiam]

Hi
My 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 & 4 showing around 11bar and cylinder 2 showing 15ba, all well below the minimum 22bar needed.

Car has 135k miles with full history, and this has not happened quickly but got progressively worse over the last 6 months.

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

 

[ajlsl600]

Has it been over fuelling and washed the bores you changed injector for?
If that's the case there was a quality addative to fuel that during combustion re etches the bores if comp loss thru valves very limited options. Get shot of or lift head and fix it
I have some experience that says sorting top end on well worn motor often stuffs the bottom end in short order esp turbo motors

 

[Damoiam]

I only changed the injectors last week because of the bad starting assuming at that mileage they were due. But fitting a full set of new injectors made no difference, hence I went back to basics and did the compression check and found the above. (But it doesn’t smoke one tiny bit so it could be loosing compression at the top and not due to rings or bore wear)
what I’m struggling with is cause? I accept its fxkt and I either get rid, fit a new engine in it or pull it all down but if that reveals the block is dead as well it’s a lot of work for nothing

 

[ajlsl600]

Take it for a good thrash at higher rpm load the motor, watch water temps. Then at same temps as previous run Yr comp test again see any diff.
I don't know Yr motor but how are valve clearances set? Hydraulic tappets or? I suspect hydraulic, in which case not much to do.
These modern engines that have 7, 9 odd gears and keep rpm, s right down. Don't in my view promote the avoidance of bore glazing I often hold my gears down to keep rpm higher when on short runs (which I try to avoid anyway) my idea being keeping system cleaner is easier if rpm up a bit. Tho I know such practice ain't great for mpg.

 

[Damoiam]

Update: Car went into a Merc dealership this week and they spent 3hrs with it in the workshop and concluded……nothing! They have no idea why it has such bad starting. So I have worked with a local Indy and we have discovered that ONLY cylinder 2 actually has enough compression to fire on cranking. (manual compression test not star, shows 15bar). With continued cranking the engine gathers and gathers and will eventually start, no smoke, no misfiring and will tick over perfectly and immediately once it runs. It does pretty much the same process wether hot or cold, but is slightly worse when cold.
Manual compression test shows zero on number 4 when cranking. Now I don’t understand how it can be zero on cranking but without a misfire.
So we then 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 if revved or driven high into the rev range it pulls like a train and does not smoke one bit.
Any ideas anybody?
(Also can anyone explain the operation and purpose of the additional butterflys that are in the inlet manifold please?)

 

[Damoiam]

Just to add: worn cam chain tensioner causing valve timing to be out or a jumped tooth was a consideration but the engine is as quiet as it should be, Indy said that if the tensioners go the chains rattle horribly, also a jumped tooth would mean it would run like a sack of crap if it would run at all?

 

[ajlsl600]

Valves not closing due to carbon build up stem, guide. Or faulty clearance settings, (where adjustment exists)
Either way Yr looking at head off to resolve.think butterfly u refer to is part of egr system. Plenty on here about those search, read
If I ever, fhn, have to remove inlet I will be fixing that fu in the open position and looking for the resistor solution
Idiotic design in plastic manifold that cokes up. And leads u by nose to dealer for a £££ replacement.

 

[Damoiam]

If it was carbon build up on the stems would they not stay stuck open all the time? I too am minded to point towards the hydraulic tappets sticking the valves open then once oil pressure comes up they release but knowing how the hydraulic lifters work that should be the other way round, it really makes no sense!

 

[ajlsl600]

Not much.... but if car starts then maybe the valves free up till next time however as said any resolution is going to involve head off

 

[davidsl500]

Is it worth trying a terraclean in case it is carbon build up?

 

[mioba]

Carbon build up on a 2017 car - no way surely.
OP are you running it on chip fat

 

[Botus]

Running VEG fat fuel causes piston rings to gum up and the pistons to smear themselves up the bores (assuming not the steel ones that crack) which tractor engine is it that got a USA class action led recall due to cracking pistons?

The additive ajlsl600 suggested sounds interesting

piston failure defect in vehicles with the M274 engine

Mercedes-Benz Class Action Lawsuit Over Piston Failure Defect ​

Consumers filed a class action lawsuit against Mercedes Benz in the Central District of California on November 7, 2022, alleging a piston failure defect

www.legalscoops.com

 

[alexanderfoti]

First thing I thought when reading this, was that the timing is out. Fairly easy to check, seems very odd that all the inlet valves would leak randomly.....

I'd be popping the valve cover off and checking the timing!

 

[mercmancdi]

First thing I thought when reading this, was that the timing is out. Fairly easy to check, seems very odd that all the inlet valves would leak randomly.....

I'd be popping the valve cover off and checking the timing!

Here here //:Happened to me on a vw when belt slipped , dealer even tried putting fluid in the fuel to cure smoking , before finding out it was timing.

 

[alexanderfoti]

Here here //:Happened to me on a vw when belt slipped , dealer even tried putting fluid in the fuel to cure smoking , before finding out it was timing.

I do understand what their indy is saying, the 651 engine rattles and rattles when the chain is worn or the tensioner has failed, but this is clearly an outlying case.

 

[Damoiam]

I can now update due to a complete engine tear down: ALL exhaust valves are leaking severely with one valve head actually cracked. Blow by on pistons, especially number 4, is also significant during a leak test. (filled each cylinder with fuel and timed to empty). Cylinders are also severely glazed.
Took the head to an engine specialist and he said instantly that the cause was because the combustion temps, especially on number 4 had got too hot and had burnt the edges off the valves and seats causing the combustion loss. He has suggested that it is fuel that has caused it, either from bad injectors or bad fuel quality.
I have added a picture of one exhaust valve (plus close up) on cylinder 4 which is the really obvious one, but they are all terrible. Nothing obvious on pistons that I can see, happy to post pics if anyone wants to see them. Engine specialist has said that I should replace the pistons and not just the rings as he says if the valves are burnt the pistons will be damaged also. ( He is a friend of mine who I trust and is not doing the work or supplying the parts so has nothing to gain from the advice)
interested to know anyones thoughts please.

 

[mercmancdi]

I can now update due to a complete engine tear down: ALL exhaust valves are leaking severely with one valve head actually cracked. Blow by on pistons, especially number 4, is also significant during a leak test. (filled each cylinder with fuel and timed to empty). Cylinders are also severely glazed.
Took the head to an engine specialist and he said instantly that the cause was because the combustion temps, especially on number 4 had got too hot and had burnt the edges off the valves and seats causing the combustion loss. He has suggested that it is fuel that has caused it, either from bad injectors or bad fuel quality.
I have added a picture of one exhaust valve (plus close up) on cylinder 4 which is the really obvious one, but they are all terrible. Nothing obvious on pistons that I can see, happy to post pics if anyone wants to see them. Engine specialist has said that I should replace the pistons and not just the rings as he says if the valves are burnt the pistons will be damaged also. ( He is a friend of mine who I trust and is not doing the work or supplying the parts so has nothing to gain from the advice)
interested to know anyones thoughts please.

That’s sick , and to be honest the mileage is not overly big , I would love to know what started this off , anything to do with these nox sensors or any of these other stupid ideas being forced on engines to help them run cleaner. I know a few folk had bad fuel some destroyed there injector pumps others were lucky and got a knock in there engine ticking over which is a tell tale sign of bad fuel , but at this point you usually can resolve it by filling with good grade fuel or tiny drip of oil added to you next fill . Keep is up to date of how you get on and if you ever find the real cause.

 

[ajlsl600]

Twas a while ago. But fairly sure it was one of oil companies that made that glaze busting addative