gushogg-blake

I've been trying to track down where NVH is coming from in my E280 and have realised one element of it is the engine. It's a very low regular thrumming which I assume is from the tailpipe.

It used to idle silently but has had various engine issues since I changed the head gasket (and the head, with a used one from an R129, as the original was too corroded). I don't do much mechanical work and only have a basic understanding of things, so my mechanic has mostly been sorting these issues - primarily rough idle, which is now cured.

Another known issue is that the timing is off (retarded) by 5 degrees, but since this will have been the case before the head work I think it can be ruled out as the cause of the rumbling.

Could an engine issue cause this (something like misfiring messing with the timing of sound waves that the muffler is designed to deal with), or is it more likely to be an exhaust component? Any relatively easy tests I can do before taking it to a mechanic?

More details:

- Engine idles well at 600rpm and seems generally healthy, possibly slightly lacking in power, fuel economy OK
- There is sometimes a vibration/rattling from the back of the car; this has been particularly bad after a long drive
- I've just finished putting Kilmat sound deadening panels on the roof and sunroof as I thought all the flat metal areas might be resonating, but it hasn't made much difference to the exhaust related noise

gushogg-blake

After googling this (not sure why I assumed it was too complicated) I think this might just be an exhaust leak.

dolucasi

Though I had a '95 E320 with an M104 engine for 20+ years I have never worked on changing the head gasket. The dealer changed it under warranty when it was less than 4 years old due to a leaking head gasket. I have replaced the head gasket on my M103 though. And had to re align the camshaft with the crankshaft twice (once for Head gasket, the other for replacing the valve train) and know that the timing alignment can get tricky on a M103. Probably similar for an M104 as it will have camshaft and crankshaft timing sensors. On an M103 one would never know if this alignment is correct.

When you say the timing is off by 5 degrees, do you mean it is off at the crankshaft? You may want to double check the timing marks on the crankshaft (dampener timing marks) versus camshaft timing marks one more time by removing the valve cover. On an M103 the ignition control unit sets the timing based on the crankshaft position as it has no idea what the cam timing is. And if the timing is off at the crankshaft that would point to a problem in the ignition system electrically. Not sure how it is on a M104.

But again I have no experience in M104 head installation, just asking questions.

gushogg-blake

I can't remember the exact details now but my thinking is that due to chain/guide wear the crankshaft rotates slightly to take up the slack before moving the camshafts. So the camshafts are 5 degrees behind the crank. (The way we got to the 5 degrees is that it was about half a tooth further on a camshaft sprocket to get it back flush with the timing tool after timing it and then rotating the crankshaft a full revolution, and one tooth is 10 degrees).

dolucasi

Did you know that there is a rotation on the sprocket when you install it and you can go in between two teeth? On an M103 the sprocket is adjustable +/-3 degrees. So after you pick the teeth, the next is to rotate the sprocket +/- 3 degrees if needed.
That is how you fine tune the mechanical alignment on a M103. I assume it is also available on an M104.

So having the timing off by +/- 1.5 degree is acceptable to MBenz. Not sure about 5 degrees but I doubt you will even notice a 5% alignment issue. It will probably go unnoticed.

- Cheers!

gushogg-blake

Hmm I didn't see any mention of an adjustment in the walkthroughs I followed (mostly the Peach Parts guide). I think the M104 must be different. It has variable intake timing, not sure if that makes a difference.

gushogg-blake

Update: the join near the muffler was rusty so I took it apart and patched it with Holts repair paste; this seems to have worked but I think there is another leak nearer the front as well.

dolucasi

Next point to check is the resonator. It tends to develop a hole, and sometimes it is well hidden. Run the car on idle and feel for air flow around it.

- Cheers!

dolucasi

Could be, but I would check further into it. The folks over on BW are pretty experienced with M104 engines, perhaps they can help answer this question definitively.