M104 -Broken Valve??

[richard300]

Hi - I have had a misfire on my Mercedes S280 (M104.944) that i have struggled to resolve. Changed the plugs, leads, bought a new coil and swapped them around.... But its still there.

When i changed the plugs i found the front 3 were dirty and the rear 3 were wet. I also had someone think it could be an electrical fault (loom isn't disintegrating) with power going to the plug that feeds the coil on Cylinder 4.

I was all set to send the car off to an auto electrician when someone recommended a good mechanic - I gave him the car and he initially thought it was the fuel injector on cylinder 4, but later ruled this out...

Now, at the same time that the missfire started, i noticed a 'chuffing' noise coming from the rear manifold, or somewhere around there and based on this noise, the mechanic has told me i 100% have a cracked valve.

Does this sound right? should i still explore electrical issues?

 

[Bellow]

A simple compression test will prove/disprove the cracked valve theory.

 

[grober]

Sounds like your cylinder head gasket might be gone. The compression test Bellow recommended would possibly confirm.

 

[richard300]

Thanks chaps - the auto parts store local to me, sells a compression tester for $50 (I'm currently living in Ausralia). So from what i can gather i need to remove the plugs and disable the fuel and ignition system before cranking the engine?

On the M104 what is the best way to disable those systems? And what should i be looking for when performing the test?

 

[hotrodder]

01 General Engine Work - M104 01-0100 lists compression pressures, 01-0110 shows the starter harness connector to bypass the ignition switch

I don't get the connection between a chuffing noise from the vicinity of the exhaust manifold (typical of a cracked or leaking manifold) and a cracked valve. A cracked exhaust manifold doesn't explain a misfire though

Personally instead of buying a compression tester i'd look for a workshop that can do a leak down test... a compression test will show if any cylinders have a problem but a leak down test will help narrow down whether the problem is with inlet or exhaust valves, rings or head gasket. 01-0150 is the relevant pdf in the link. https://en.wikipedia.org/wiki/Leak-down_tester

 

[bolide]

When i changed the plugs i found the front 3 were dirty and the rear 3 were wet

Wet with what? Water?

Nick Froome

 

[Vlad]

Head gaskets on a lot of M104's tend to blow at the back.....have a look to see if you have oil or water running down the back of the engine block.

A quick test for the head gasket would be to start the car from cold and if you immediately have pressure in the top hose, its highly likely your head gasket has blown.

A quick sniffer test will also tell you very quickly whether your head gasket has gone.

 

[richard300]

Wet with what? Water?

Nick Froome

Fuel..

 

[richard300]

Ok, so had a leak down test performed on the car and the results were apparently good - 95% on all cylinders.

Garage said that (bearing in mind the car had sat overnight without being started) when they pulled plug 4 out, it was wet with fuel...

They said they think it might be a fuel injector, but I know that switching the injectors over makes no difference to the misfire on cylinder 4.

Think the problem might be electrical (although the loom appears to be in great condition)... as when I put a test light, on the electrical connector to the coil pack on cylinder 4 it stays constantly on. if I do the same on the other 2 connectors (Cylinder 2 & 6) the light pulses?

 

[grober]

On the M104 the coil packs are linked as follows. 1-6 2-5 3-4. If swapping the coils round doesn't change things and you get that constant 12v on coil 4 that suggests the ignition switching transistor in the ECU has gone open circuit.[burnt out] That said slightly puzzled that the plug on the same coil plug 3 isn't wet also since it should not be firing either. Sometimes you get problems with the extender under the coil breaking down with the coil perfectly sound. There is a thread here where I published details how to repair the ignition transistors on these ECUs.

 

[grober]

http://www.mbclub.co.uk/forums/electronics/144774-how-repair-your-ecu-ignition-part-1-a.html
http://www.mbclub.co.uk/forums/electronics/144776-how-repair-your-ecu-ignition-part-2-a.html

 

[k-works]

Head gaskets on a lot of M104's tend to blow at the back.....have a look to see if you have oil or water running down the back of the engine block.

Lost an engine due to the head gasket blown at the back. I was young and stupid. :fail

 

[richard300]

So, the situation goes from bad to worse - Garage said car had 95% compression from leak down test, then i have to wait a week to get the car back - I am still trying to find out why.
Went there to Kick off, and notice my car has most of the front plastic under tray hanging down - Do they need to access the bottom of the engine for leak down test?

The garage then tell me, that 4 days after doing the leak down test, they did a compression test and there is only 25% compression of cylinder 4.

I take the car, which previously had a notable misfire BUT now it barely drives at all, as if missing on multiple cylinder and doesn't even have enough power to get up my driveway??

Is it possible to cause further damage from doing a leak down/compression test?

 

[grober]

On this occasion when you had a pre-existing problem its very unlikely. The compression test is simple and difficult to get wrong the leak down test while more precise and can identify things like leaking valves or rings is not so easy to do correctly. Pretty sure from what you have said your cylinder head gasket is on its way out- the various tests may have hastened its demise slightly but not caused it. It is however important you trust the garage doing work on your car. If you lack confidence in the present garage then it might be best to take it to an engine rebuild specialist instead. The head will possibly have to be skimmed flat or even welded depending on damage /warp-age What are the present lot saying to you?

 

[richard300]

Sadly this ended up being cracked valves in cylinders 3 and 4.... was luck to get $2000 AUD for it in the end..... Divs over here run everything on 91 even though 95 and 98 is readily available