Busy working through the long list of issues on the 500SEC... My 1984 500SEC smokes a bit, mostly under heavy acceleration. Its blue oil smoke for sure and I have noticed some oil usage. When I changed the plugs I noticed the tips on all were nice and clean with minimal deposits except the one from cylinder number 4 which was showing signs of oil/carbon fouling. If the plug wire is pulled from number 4 de activating it the car does not smoke when driven. I did a dry compression test (warm engine) and they all range from 175-185 psi (number 4 was 185psi) which is very good. I removed and tested the injector from number 4 and it makes a nice spray pattern as well. The motor pulls very strongly and does not feel down on power at all. It is a bit embarrassing when doing full throttle kick down overtaking manoeuvres with blue cloud following you... Any thoughts? Oil rings in number 4 stuffed?
How can I tell if it is valve seals? I tried the trick of gearing down going down a hill fast, and continue down the hill on trailing throttle and then accelerating at the bottom of the hill - almost no smoke. It doesn't smoke at start up either. It needs a good boot to the accelerator to smoke. I cant see much blow by from the breather pipe as well.
No suggestions? Compression good, almost no blow by and very good power BUT it smokes, uses oil and fouls plug #4. To be honest the rest of the car is not worth rebuilding the motor for (at current values) so I'm just looking at economically extending the life of it a bit. It is just too fun to drive, reliable and peppy to scrap.
Same thing happened on my SEC right at 155K (miles) and it was the valve seals...great way to fog for mosquitoes!
I was able to do the do the valves seals with the engine in the car and I will say its not fun and requires at least 1-2 additional people to help rotate the engine and help with the valve spring compression while fishing out the old seals. My seals were hard and brittle, so not surprised they were by-passing oil into the exhaust.
Hope that helps and if you decide to tackle this at home, let me know if you have any questions.
I'd also say likely the seals, the PO of mine had a problem with plug fowling also. He opted to have the heads rebuilt though at "his shop" (I think his Daddy was paying the bill on it, I've got the repair ticket on it). This was at 175K miles btw
Just out of left field ,,, but is the gearbox filled to the correct level. Over filling can cause fluid to be dumped onto the exhaust, especially on steep hills with hard acceleration.
Valve seals for sure.
If the heads were re-furbished perhaps one seal has worked its way loose from not being seated properly and is allowing oil down the guide. (that is assuming oil rings are good and no scoring of any cylinder walls? - can be checked with piston down and camera through the spark plug hole)
I did the valve stem seals , timing chain and guides 7 months ago. Quite a job. I dropped the sump to replace the oil pump pick up, oil pan gasket and to fish out the broken chain guides. While open I had a look at the bores from underneath, they looked good (including suspect cylinder number 4). I have been using the car regularly since and it doesn't seem to foul the plug on cylinder number 4 as badly and it doesn't smoke under normal gentle driving and performance is good. If driven very hard and redlined I can still notice tailpipe smoke and it still uses a bit of oil. Sounds and feels healthy so I just keep topping up the oil and I don't look in the rear view mirror if I hammer it . The car is rough so the smoke sort of goes with the "Rat Rod " theme
In my experience (which is mostly not Mercedes gas engines*) valve stem seals show up on deceleration and do not contribute to full throttle smoking other than immediately after decel. If you mat the throttle from cruise and get smoke it's most likely the oil control rings. Oil in the cylinder will also give you false high compression readings as it tends to seal the rings.
Another possibility is crankcase pressure forcing oil out the breather and into the intake manifold, but, again, this points back to the rings.
As mentioned before the car has almost no blow by/crank case pressure issues and compression test comes up good on all cylinders even suspect #4. New Valve stem seals have not helped the oil consumption much and it still drinks oil and will visibly smoke if redlined. I have just completed a 2000km round trip and it used about 4 litres of oil... other than this it drove well and still feels and sounds strong. Spark plugs burn nice and clean except (still) for cylinder #4 which shows fouling. If the car is driven fast with sustained revs above 3500rpm for long periods it eventually fouls plug #4 and starts running on 7 cylinders until the #4 plug is changed. I have heard that with the M116 & M117 motors the oil control rings become weak after the engines have overheated badly with the rear cylinders #4 and #8 often the worst off ?
Top up Oil and the occasional new #4 spark plug seem to keep this beast going strong.
A heavier weight oil might reduce the symptoms. If I remember correctly, the original piston rings were made by Glyco and are cast iron.
I see online replacements in CroMo, a much harder metal. I don't think you have to be an expert to assume the alusil bores wouldn't last long.
As an aside, when we took apart a M117 GenI 500 engine, we noticed some moron mechanic had installed one bank of pistons backwards.
The pistons have arrows and are all supposed to point forward.
Sounds like the oil control ring in the four hole , once your revs kick up the crank starts slinging oil from the pan around the bottom end of the motor with some force and the bad ring lets it in the cylinder.
Not an expert on rebuilding 117 motors but you might be able to pull that one piston from below and swap in a new ring . I bet the rings cracked.
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