M116 Chain Tensioner

[TheMadRacoon]

Is it just a matter of undoing the two mounting bolts to remove and replace the chain tensioner on a 3.5L M116?

Besides changing the gasket between tensioner and head, do I need to do anything else.... any special anti-seize on the mounting bolts given the head is light alloy?

Do I need to pre-fill the tensioner with engine oil? I guess it is self bleeding because I understand there is an oil return orifice.

Anything else?

Two and a bit years ago I had the top chain guides inspected, as best can be when the rocker covers were off, and they appeared to have been relatively new, same with the chain.

Lately I've heard a sound that comes occasionally which I thought might be sticky valves. A couple of days ago I heard a sound on start-up which, although I'd never heard before on any car, I felt sounded like chain slap. Oh-oh. I thought I would start with the tensioner and an oil change (overdue by time but not mileage).

Fingers crossed!

[CraigB]

I think what you said is fine and no special things to do - see what others say. I would pull the rocker covers off and have another look at the guides for what it is worth. I think you will have the alloy guide with plastic cover, so shouldn't have the same disaster as the M117 when they break and throw the chain off.

[TheMadRacoon]

Hi Craig, thanks for the advice.
Are the replacement guides also alloy with plastic covering, or just the originals that came with the car?

[CraigB]

I think you can get both. If you still have the metal ones I would keep those and put the other bit on. I have a feeling i have seen the alloy ones too. I just like the idea that if plastic cracks up you will hear it on the alloy and get a chance to fix before pistons hit valves.

[JohnH]

Perhaps consider changing the chain as well - they do stretch over time and should be replaced every 160,000 or so.....

[CraigB]

Good point - you can measure the stretch though and i think that is more important for its effect on engine timing. Not heard of chains actually breaking usually. Cam sprockets can wear too.

[TheMadRacoon]

Thank you all.

A check of my oil level showed it at the lower mark on the dipstick when cold. Topped up to upper mark, and the noise - which lasted 2-4 seconds the first time I heard it, is no longer there. So I think the tensioner was not getting oil quickly enough before and it took those few seconds to build up pressure.

A few days later I removed the driver's side rocker cover. I should have re-read some earlier posts, especially Mercmad's instructions on checking for chain stretch, but all I did in the available time was have a quick look and take some photos for the record.

The upper chain guide is discoloured, mainly yellow, though darker on the edges maybe due to oil not being wiped away. I don't see grooves worn into it - it is flat and not gouged. The chain has about 4-5mm slack, though I did not rotate the engine to TDC - I was having a feel at this stage. Sprocket looked reasonable.

The inside of the top of the head looked clean, not sludgy. There are not scrape marks from the chain on the inside of the rocker cover or around the edges of the casting on the top of the head.

Putting it together again and restarting, again no noise like chain slap. So my aim is to change the oil & oil filter, save up some $$ ---- hello, customers, please pay your invoices ------- and have them professionally inspected and chain stretch measured in a few months; I won't attempt replacing the chain and guides myself.

[CraigB]

I wouldn't think topping up the oil would make a change, because the pick up is way down in the oil. It looks ok there, but probably more important what is under the LH rocker cover which has the guide just before being able to jam between chain and sprocket - or at least that is where all the M117 problems are. What oil grade are you running? Wondering if wear in the tensioner and oil draining out etc, if running a thinner more modern oil may be worse than say running just 20w50, which i think is what they are supposed to run.

[TheMadRacoon]

I'm note sure what oil I'm running as I have the local indie (but authorised MB service agent - not part of a stealership) service the car.

I will monitor closely. I don't need it as a daily driver - in fact the last 3 months I've used it about 3-4 times as I have moved out of our office and work from home now.

[CraigB]

Might be worth asking because if he is buying drums of lighter oil for the bulk of late stuff he does, probably not the end of the world but might make that difference. I found it interesting that a M117 500 is listed as taking 20w50 but the basically identical 560 is 10w40 ( I think). Same for my range rover but the engine builder said to run the 20w50 in what is essentially the same engine - he said they went lighter in search of fuel economy - not sure about that but it is lighter.

[AMG]

That guide needs replacing. do them asap.

[TheMadRacoon]

Thanks Joe. Good to get a firm opinion on this.

[TheMadRacoon]

If a picture paints a thousand words then this video is priceless...hopefully a good reference for those doing it themselves:

https://youtu.be/7DQdGlSLMLM

Perhaps, though, he should have covered the open cambox before grinding off the old chain (he didn't find a master link).