W126 6 cylinder diesel engine

[Beirut Brit]

Hi again
Anyone have any experience of these turbo diesel engines? Would appreciate 'things to look out for' advice when buying as i have found a 300 SDL which is the six in line turbo diesel.


thank you in advance - especially to the folks before who all chipped in about my previous queery about the 500 which can the diesel engine.

best


MJ in Morocco

[t walgamuth]

In a diesel a smooth idle is a very good sign of good compression which is the most important thing to have a good running engine.

[rumb]

I would suggest a compression check on any older vehicle being purchased as that is about the only measure you can take to judge wear.

The W126 chassis is stout. rust areas are on front frame behind wheel and bottom of rear suspension A arms. Interior trim is hard to find. It's a fairly easy car to work on.

I had a 300SD years ago. ran smooth.

[Beirut Brit]

Originally Posted by t walgamuth

In a diesel a smooth idle is a very good sign of good compression which is the most important thing to have a good running engine.

Indeed, that is the general rule of diesels. This one doesn't idol so smoothly though but then again it doesn' misfire exactly. I think the owner has negelcted it for many years with not changing oil etc.

When it revs up though there is no smoke so i at least assume that compression is ok and no oil is passing rings etc.

I wonder if there is a gunk/sludge build up in the sump which is affecting the turbo. Or that the turbo is failing, so, effectively you have a non turbo engine.

Perhaps a flush might do wonders to make it smoother.

Or, then again perhaps a weepy injector.

thanks again for your views. Much appreciated.



M

[Beirut Brit]

Originally Posted by rumb

I would suggest a compression check on any older vehicle being purchased as that is about the only measure you can take to judge wear.

The W126 chassis is stout. rust areas are on front frame behind wheel and bottom of rear suspension A arms. Interior trim is hard to find. It's a fairly easy car to work on.

I had a 300SD years ago. ran smooth.

Yes, thanks. i would guess that rust issues are not relvant here as temp here is more or less like Arizona. I was really hoping to hear whether there was a classic fault on these older diesel engines which you have to look out for.

best

M

[jsb357]

I'm almost sure you need to look at the cylinder head stamping number

numerical position 7&8 need to be 17 or higher

see sample below

MERCEDES BENZ
R603 016 17 01

[Diseasel300]

As mentioned above, the cylinder heads can crack and pressurize the cooling system on the 3.0L diesels in these cars. The original stamping was a 14, which many consider undesirable. Myself and many others on this forum run or ran cars with hundreds of thousands of miles on them with the original #14 head, so make of that what you will. The engine itself is quite durable and is more refined with a better power band than the 5 cylinder OM617 that came before it. Keep the cooling system serviced and the oil changed and it is no less reliable.

Other faults that are common are common to all of the Gen II W126 cars. Usually electronics issues which can be resolved with replacing capacitors and touching up solder joints. Be aware that the radiators for these cars have been NLA since 2012. There are a couple of all aluminum aftermarket ones available nowadays though.

[ah-kay]

Originally Posted by Beirut Brit

Hi again
Anyone have any experience of these turbo diesel engines? Would appreciate 'things to look out for' advice when buying as i have found a 300 SDL which is the six in line turbo diesel.


thank you in advance - especially to the folks before who all chipped in about my previous queery about the 500 which can the diesel engine.

best


MJ in Morocco

Which year? The 91 or later w126 has "bent rods" problem. In any event, it would be a 30+ years old car, so expect problems. Lot of them.

[t walgamuth]

Originally Posted by rumb

I would suggest a compression check on any older vehicle being purchased as that is about the only measure you can take to judge wear.

The W126 chassis is stout. rust areas are on front frame behind wheel and bottom of rear suspension A arms. Interior trim is hard to find. It's a fairly easy car to work on.

I had a 300SD years ago. ran smooth.

Agreed with rumb.

A compression test is a good idea. A low compression cylinder will always be less noticed at higher revs as it doesn't have time for the gasses to get away around the leaky parts head? Valves?

Of course the leak could be an injector too, so that can be checked by isolating the missing cylinder and swapping the injector with a known good one.

[Beirut Brit]

thats very useful to know. Thank you.
But if the head was cracked, surely that would be easy to spot with a creamy sump oil, no?

And can we take it for granted that a 14 head WOULD crack if the water ran dry in the cooling system just once? Is it that vulnerable?

MJ

Originally Posted by Diseasel300

As mentioned above, the cylinder heads can crack and pressurize the cooling system on the 3.0L diesels in these cars. The original stamping was a 14, which many consider undesirable. Myself and many others on this forum run or ran cars with hundreds of thousands of miles on them with the original #14 head, so make of that what you will. The engine itself is quite durable and is more refined with a better power band than the 5 cylinder OM617 that came before it. Keep the cooling system serviced and the oil changed and it is no less reliable.

Other faults that are common are common to all of the Gen II W126 cars. Usually electronics issues which can be resolved with replacing capacitors and touching up solder joints. Be aware that the radiators for these cars have been NLA since 2012. There are a couple of all aluminum aftermarket ones available nowadays though.

[Diseasel300]

The head usually cracks between intake and exhaust valve. The most likely cause is overheating at some point in the car's life, though there is still no definitive answer. The result is usually pressurized cooling system from exhaust gasses being forced into the water castings.

[Beirut Brit]

Heads are alumimium?

And the pressurised cooling system. Is there an obvious way of spotting that?

thanks again

Originally Posted by Diseasel300

The head usually cracks between intake and exhaust valve. The most likely cause is overheating at some point in the car's life, though there is still no definitive answer. The result is usually pressurized cooling system from exhaust gasses being forced into the water castings.

[Diseasel300]

Yes, aluminum head. Usually the pressurized cooling system is found shortly after the engine is started and pressure builds quickly in a cold engine. Another clue is firm cooling hoses after the engine has stopped and allowed to cool down completely.