M116 Oil Consumption, Spark Plug Fouling - PeachParts Mercedes-Benz Forum

JamesDean · #1 05-21-2016, 06:58 PM

Hi All,

The 420SEL has a bit of a drinking problem. It likes to burn a considerable amount of oil. Right now its roughly, 200 miles to a quart or so. It appears that the majority of this, if not all, is going out the #8 cylinder (driver's head, rear most cylinder).

Every weekend I end up changing the #8 spark plug and it restores the car's idle and power back to normal.

I'm only driving the 420SEL because my 300D is in paint still and my 190E has a idle/stalling problem that I am working to address.

Does anyone have any thoughts or ideas on what might be the cause of this? The next step we were planning on is a complete valve job....once funds are saved up.

The motor has 237K on it. I probably should pull the rocker cover and take a look and see if its anything obvious.....

97 SL320 · #2 05-21-2016, 08:16 PM

The only parts of the valve train that will cause excessive oil consumption are valve seals / guides. Valve seals can usually be changed with heads on the engine but I don't know the specifics of your engine.

There is a chance that oil is pooling at the back of the head, I'm not sure how large the drain back passages are.

I would not put a fresh valve grind on such a high mile motor. When you get good valve sealing, intake manifold vacuum goes up and so does oil consumption. And, . .with better sealing comes more power, that tends to get used . . . stressing high mile connecting rod bearings leads to failure.

There is also a chance that a piston ring has cracked or in extreme cases a piston. I'd do a leak down and compare it to other cylinders. A compression test won't give you a definitive answer.

JamesDean · #3 05-21-2016, 08:34 PM

At first my thought was valve seals but the fact that the #8 plug is fouling so quickly makes me think that whatever the issue is its related to that cylinder and not something common to all. Granted...maybe they are all worn and #8 is just far gone.

I was under the impression that the M116 bottom end was typically good for a while longer than 250K? Maybe not true with a fresh valve job.

I will pull the rocker cover and inspect, maybe see something obvious. After that probably a leak down test?

BWhitmore · #4 05-21-2016, 08:54 PM

Worn valve stem seals will usually evidence themselves by puffing out blue smoke after idling. Keep an eye on your exhaust after idling at a stop light.

duxthe1 · #5 05-22-2016, 01:27 AM

Those bottom ends are rock solid well into the several 100Ks. The valve guide are a weak spot and will cause excessive oil consumption. Valve jobs on that engine is fairly expensive but typicaly good for 150K miles.

97 SL320 · #6 05-22-2016, 07:50 AM

In a case like this I'd tend to either find another engine or do a freshen up on the existing one. Check car-part.com , there has to be a salvage yard that has one or two floating around. There is a large import yard in Ohio you might want to avoid or at least be prepared to be extra cautious when making the deal.

I'm guessing that this car is one of many "spares" and does not see heavy use. If a good used isn't in the offing, I've put together lots of "junkyard rebuilds" when the expected remaining life / yearly use does not justify a full $$$ rebuild.

The concept is to make sure the crankshaft and bearings are in spec as rod bearing failure is the number one engine failure point. Many times new bearings will take care of this. I forgo having the connecting rods resized on the big end if the bearings look evenly worn and or if the crank is on the small size. The bores tend to stretch top to bottom and pinch across the rod bolts. If the crank is slightly small this isn't a problem. Rod small end bushings get changed if they have then.

Pistons are reused if not in too bad a shape ( unless they are sub $ 20 per bore then they get new. ) New piston rings are used.

Badly worn valve guides are changed, one on the high side of the spec are left in, same goes for worn valve stems. The surfaces are ground and new valve seals installed.

Timing chain guides are replaced and chain sprocket sets if not too much $. The oil pump cover is ground flat as this is the largest cause of low oil pressure once you get past worn bearings.

The result is a good used engine that you can have confidence in. Sure, it won't last as long as a full mega $ rebuild, but it doesn't have to since the car probably won't outlast the engine.

nulu · #7 05-22-2016, 11:30 AM

Ive done several 420 , 560 sels all ran poorly around 150-200 k miles, they all ran like new after a valve job and new guides, all went over 250,000 miles with no issues, my brother in law has a 420 sel now with over 475,000 miles, all these engines got a valve job new guides and timing chains, oil use was normal after work done, these bottom ends are over built, all engines were cared for and serviced on time

duxthe1 · #8 05-22-2016, 11:23 AM

Man I gotta disagree. The 116 117 engines are bulletproof on the bottom end. I've seen them run so low on oil it locked up the cams in the heads and the bottom end would still turn after pulling the heads off. If you keep oil and filters in them them they are damned near indestructible.

The top end is a weak point in the 116 /117.(discounting the chain and rails) The way they are designed the valve is pushed sideways slightly every actuation and it wears the valve guides. Fresh heads last about 150K miles before they wear the guides. New seals can buy a little more time but the guide wear is inevitable.

The 119 motor is a 117 block with 4 valve heads. The valves are actuated with buckets and no side loading is incurred. Those motors are indestructible if you keep the timing drive in decent shape.

97 SL320 · #9 05-22-2016, 09:49 PM

Quote:

Originally Posted by duxthe1

Man I gotta disagree. .... . .

The top end is a weak point in the 116 /117.(discounting the chain and rails) The way they are designed the valve is pushed sideways slightly every actuation and it wears the valve guides. Fresh heads last about 150K miles before they wear the guides. New seals can buy a little more time but the guide wear is inevitable.

Yea, I'm used to other brands where the entire engine is starting to wear out at that point and just a valve grind leads to problems.

It also makes a difference what climate the car is used in. Sub zero temps and hills are way harder on a car than sunny sky's and level ground.

rocky raccoon · #10 05-23-2016, 06:39 AM

The M117 in my 560SL went from one qt/200 as yours to less than one qt between 6000 mile oil changes after valve seal replacement.

JamesDean · #11 05-23-2016, 12:25 PM

Hmmmmmm I think the shop was steering us more towards the complete valve job rather than just a valve seal replacement. I think they felt that it was a better solution and would have a better chance at addressing the issue. The seals may or may not.

rocky raccoon · #12 05-23-2016, 01:39 PM

Valve seals can be a moderate DIY whereas a "valve job" leaves the door wide open for additional and expensive shop labor.

Frank Reiner · #13 05-23-2016, 02:07 PM

Quote:

Originally Posted by JamesDean

Hi All,

The 420SEL has a bit of a drinking problem. It likes to burn a considerable amount of oil. Right now its roughly, 200 miles to a quart or so. It appears that the majority of this, if not all, is going out the #8 cylinder (driver's head, rear most cylinder).

Every weekend I end up changing the #8 spark plug and it restores the car's idle and power back to normal.

I'm only driving the 420SEL because my 300D is in paint still and my 190E has a idle/stalling problem that I am working to address.

Does anyone have any thoughts or ideas on what might be the cause of this? The next step we were planning on is a complete valve job....once funds are saved up.

The motor has 237K on it. I probably should pull the rocker cover and take a look and see if its anything obvious.....

JD:

Do the other plugs come out free of oil, or nearly so? If so, an inspection of the stem seals/valve guides of #8 may be in order. The heads on the iron block M116/117 engines, especially the earliest ones, were prone to loosening of the valve guides in the head. The heads on the aluminum block engines have been largely free of that problem, however it is a possibility. A loose guide will allow quite a bit of oil to pass thru, independent of the seal. If neither of the guides of #8 are noticeably loose, then new seals may help the problem.