1 second engine rattle on cold start up (M271)

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Add to your list a new crankcase breather, while you're there.... they get clogged over time.

Also check the vacuum hoses under the air filter box, they can get brittle and leak, and as you are finding out it's a pain to remove the air filter box so might as well check them now.

You could also consider replacing the thermostat, easy job and they're inexpensive, and they go after a while so will likely need replacing anyway in the not too distant future.
Great suggestions, much appreciated. I’ll add these to the list and continue tomorrow.
 
Yes, was just reading one of your historic posts about this 🙂.

Well, now I have begun I may as well do things properly. I’m making a list and going through and checking it over thoroughly. It’s a really nice car so i’d rather get to know it mechanically than to seek out a refund.

I’ll add these two bits to the list, any tips on the best way to check this? I was going to start by making a small cut into the sheathing of the cable just by the connector, but I am thinking the wicking would be under the wire insulation and not over.
Just unplug the large plug to the ECU (top right side of the engine . See if there is oil in there .
Yep , the oil is within the wires , not the sheath .
 
Just unplug the large plug to the ECU (top right side of the engine . See if there is oil in there .
Yep , the oil is within the wires , not the sheath .
Thanks, was nothing on the ECU plugs male or female side when I disconnected them, although I will check again and give them a blast down with electrical contact cleaner just to be on the safe side.

Have a Wahler thermostat on order.

Just heading out to disconnect the air filter housing. Not a very nice day out but not the worst either!
 
Thanks, was nothing on the ECU plugs male or female side when I disconnected them, although I will check again and give them a blast down with electrical contact cleaner just to be on the safe side.

Have a Wahler thermostat on order.

Just heading out to disconnect the air filter housing. Not a very nice day out but not the worst either!
This my airbox removal thread , it mentions socket sizes , especially for the fiddly one near the back
 
Thanks for that write up, I had a look at this breather pipe and to say it is in an awkward location is an understatement. I guess it does make sense to change it while I have access if it is known to fail.

I have removed all the air filter housing, made much easier with the long reach hose clamp pliers. All the electrics on top of the engine have been disconnected, and I have removed the cold air intake and the air intake silencer for the supercharger (Which I noted had some oil in it). The pipe at the rear of the engine that connects to the air filter housing also had some oil around its rim.

Next I need to disconnect the alternator connection to the main loom, which looks like I will have to unbolt the alternator to move it forward a few inches to gain access to it’s rear, although there is a metal plate at the rear of the alternator that may be able to move instead. After this ill be able to clear the wiring from the top of the engine and inspect the chain and gears.

No rush to get this done as I have some parts on order. Steady progress.
 
Thanks for that write up, I had a look at this breather pipe and to say it is in an awkward location is an understatement. I guess it does make sense to change it while I have access if it is known to fail.

I have removed all the air filter housing, made much easier with the long reach hose clamp pliers. All the electrics on top of the engine have been disconnected, and I have removed the cold air intake and the air intake silencer for the supercharger (Which I noted had some oil in it). The pipe at the rear of the engine that connects to the air filter housing also had some oil around its rim.

Next I need to disconnect the alternator connection to the main loom, which looks like I will have to unbolt the alternator to move it forward a few inches to gain access to it’s rear, although there is a metal plate at the rear of the alternator that may be able to move instead. After this ill be able to clear the wiring from the top of the engine and inspect the chain and gears.

No rush to get this done as I have some parts on order. Steady progress.
Good job buddy , when I inspected my chain I managed to get the rocker cover out with the wiring loom still in position .
 
Ah ok, I’ll give it a go without moving alternator then, although its connection does seem to hold the loom tight to the top of the rocker cover.
 
So, have been busy with work but I’ll continue this inspection tomorow. Just thinking that as the tensioner may well be drained of oil, how much play can I expect to find in the chain for it to be within reasonable limits? Have been searching for a while but have only found examples that are super loose, or very tight.

Also, the Mercedes cam magnet wire harnesses for blocking oil leaks seem overpriced at £45 each. Has anyone any experience with the cheaper variety on eBay/Amazon?

 
Have inspected the chain and it was quite loose with lateral movement, and could be pulled upwards with some force.

When turning the engine to TDC it became very tight, with very minimal moment.

On the underside of the engine cover, there is a bit of plastic that has worn as a result of chain contact.

Gears ⚙️ do not look too bad at all.

Will update here soon with images and would appreciate a second and third opinion.
 
Worn plastic on the engine/camshaft/rocker cover. On the intake side. The other side shows no markings.
 

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Some further notes from the inspection:

The chain was showing slack when the cover was first opened, with lateral movement and could be lifted up with some force. Once the engine was turned by hand, the chain became very tight, and I could only replicate the slack that I initially found when turning the crack to 1/8 a turn away from U/T (that mark on the opposite side to the TDC mark on the crank pulley).

Here is a video of the slack:

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The timing marks seemed ok, with the exhaust side half or a quarter millimetre late, and the mark on the intake a fraction early.

Pictures of the gears will follow when I can get the photos to a low enough size to upload here. The teeth on the gears did seem ok. My neighbour said they were not sharp, and they did seem to still have flat tops. Some wear was visible on the side of the teeth on the intake gear.
 
Images of the gear teeth:
 

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Timing marks and an additional picture of the engine cover:
 

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Worn plastic on the engine/camshaft/rocker cover. On the intake side. The other side shows no markings.

This means that the chain was running too loose, at least some of the time.
 
Thanks, I have been mulling over just trying to replace the tensioner, as perhaps this is the root cause. The chain is very tight whilst turning the engine until a certain point is reached where it slackens. However, I think as the chain has scraped the top of the cover, it has probably worn a bit by running intermittently without the proper tension.

Will have to see if I can borrow the locking tools and crimpers from someone!
 
The hydraulic tensioner on the M271 engine tends to develop a leak, and ir then no longer holds the oil pressure (and the tension).

In any event, the design is such that it will always cause some chain rattle at engine startup, i.e. until the oil pressure builds up.
 

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