shardul

iTrader: (1)

I was helping a friend out with his CLS63 this weekend. The car had a misfire on no 5 and was throwing a low fuel pressure code.
We check the railed pressure with DAS and it was within spec.
We also did a compression test via DAS and with a gauge, no 5 was at 60psi which very is low.
Injector for that cylinder would not respond to DAS when we tired to turn it off with the engine at idle.
So my question is if the injector has failed will it cause low compression in that cylinder as to where the combustion chamber pressure is being forced back into the fuel rail?

christsay

If that was the cause of the low compression on #5 that would be the best case and also easiest to remedy. However seems unlikely...

Could be if the injector failed open is that it's been dumping fuel into #5 and washed the walls and the ring seal has been compromised.

What did the #5 plug look like when you removed it to do the compression test, vs the other cylinders would be most useful?

What are the compression numbers on the other cylinders?

Can you smell any fuel from the dipstick?

Are you setup to do a leak down test?

-chris

shardul

iTrader: (1)

Plug looked fine when compared to other cylinder
Compression in no 6 cylinder was 180 psi.
Washing the cylinders is also very possible.
We did not do a leak down, that would be the next best thing to do.

Not related but worth noting that on the M157 when you change injectors there is a 4 digit code on top of the injectors that has to be entered into the ECU coding via DAS in order for them to function correctly.

shardul

iTrader: (1)

Well now thinking about it the high pressure fuel pumps deliver a rail pressure higher that 180 psi so it may be hard for the compression to blow back into the rail.

KLR CLS

Well, sounds like Occam's Razor situation...low compression and misfire on one cyl....simplest answer is likely a dead cylinder. good luck, but I think the head is coming off whether its rings or valves.

shardul

iTrader: (1)

That is what I told the owner of the car.

TECHNICIAN

The fuel injector has zero bearing on compression. Washed down cylinder walls absolutely can though

shardul

iTrader: (1)

That is what I though. I just wanted to make sure.

Luke-BITURBO

Was the car stock out of interest

shardul

iTrader: (1)

No it is not stock. ECU calibration and down pipes.

shardul

iTrader: (1)

Exhaust valves on no5 had damage.
I was speaking a few local MB techs and they said they have seen a lot of M278 with this issue. Likely cause is the proximity of the turbine housing (heat generated) to no1 and no5 cylinder.

Silverp30wagon

How many miles and what year?

shardul

iTrader: (1)

2014 about 35k

Silverp30wagon

Great. Something else to worry about on my 2012. Haha

ItalianJoe1

Haven't seen a LOT of them, but we have had a few exhaust valve failures. It is usually the end cylinders too. I'm sure it's an exhaust reversion issue as you speculate.

I would think with the downpipes it would be better but the tune was probably very aggressive.

Curious if the car was ever tracked?

shardul

iTrader: (1)

The car was street raced. and had a set of downpipes.

Maybe a good set of turbo blankets will help mitigate this issue.

ItalianJoe1

No way it's radiant heat. More like exhaust backing up in the manifold heat due to the fairly small turbos and the high demand placed on the system by the tune/usage. Radiant heat would start to warm the head first, which is cooled by coolant. The valves already run much hotter as they deal with the exhaust flowing over them, they rely on contact with the seat/guide to dissipate heat in them. The turbos can't directly heat them.

It may be a valve guide or seat issue causing the valves to burn. I assume the exhaust valves are sodium filled, if not, an upgrade may be possible then.

shardul

iTrader: (1)

I was speaking to a Doug Wolff (if anyone follows him on IG, he knows a lot about these engines) he also said proximity of the turbos to no1 and no5 is causing the exhaust valves to get burnt.

Mikeki7

What would be the solution to this issue? Isn't there already a heat shield next to the turbos. How old was the coolant and oil when the issue occurred? My tuned 90k m157 hasn't had any of those problems yet. Also does he have turbo back downpipes? Hopefully the installer did not remove the heat shield.

shardul

iTrader: (1)

Yes there is a heat shield apparently that is not enough. This particular car had 40k miles on it oil changes etc were done on a regular basis. It has down pipes not turbo back ones.

I know that all M157 and M278 will not have this issue but we have seen a lot of them starting to develop this problem.

Who knows you car may be fine for another 90k miles.

b0kix

So failed injector or valve? I have seen injectors to fail on the M157

ItalianJoe1

I think it may be a guide issue that's not presenting until the valves burn. Similar to the LS7 issue, but less catastrophic. I've seen some 278s have valve problems on stock, untuned cars. Heat in that application shouldn't be an issue, or it would be in every car.

6speeddave

.

shardul

iTrader: (1)

Valve issue and also injector issue