1983 380 SL, ONLY cyl 8 fouls plugs and smokes after startup, idling at red light or any other idle posibility.

Removed all spark plugs and all other than cyl 8 look great, no dark deposits, if anything maybe running lean or hot.

decided to test the compression on all cylinders dry and this are the results.

1= 168
2= 178
3= 175
4= 170
5= 175
6= 174
7= 163
8= 200

I tested number 8 last and retested them all. How is that the bad one tested higher?
Is it that oil is leaking into the cylinder somehow and it test as if it was a wt test.
What do do next. Any ideas?

Thanks

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Thanks, Juan

I expect that it is higher because that hole is oily. The oil will bring up the compression, but usually that doesn't happen because the oil is coming around rings and a cylinder that is not sealing.

I think you might have a seriously bad intake valve seal on that hole.

You can verify this by doing a cylinder leak down test and listen for leakage on that hole. I expect that you will not hear any.

Good luck,

thanks Larry, i have been trying to figure out if a stem seal is the culprit or a guide.
your answer says it's a seal and i like your answer, better.

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Thanks, Juan

did you try doing a running compression test

Need a leakdown test.....This will let you find a bad piston ring set or leaking valve seats.
Need a reading of less than 12% for a health motor. Ideal is 5% or less.

As you apply air, you can hear where the air is leaking from.

From your discription, it sounds like a valve stem seal popped off.




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When I say a valve seal I should have said a valve stem that is not sealing. This could be the seal itself or it could be a worn valve guide and/or stem.

If the engine has lots of miles on it, the compression test numbers indicate that the bottom is okay. It would be a good time to pull the head and do a valve job with valveguide replacement.

Good luck

I am half way into replacing the stem seals on cyl # 8, it's the easiest of the possible jobs that could be the problem. I hope it cures it.

Thanks for the responses.

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Thanks, Juan