Author Topic: Blue smoke when accelerating - Valve stem seals?  (Read 132 times)

NormW116

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Blue smoke when accelerating - Valve stem seals?
« on: 13 March 2019, 07:55 AM »
Bit of an issue with my father's 1977 280SE - when he is driving about 60 or 70 mph +, take his foot off the accelerator for a bit, then back on he gets a fair bit of blue smoke coming from the exhaust. I wondered if this would be valve stem seals, or could it be something else potentially?
Many thanks in advance for any replies.

daantjie

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Re: Blue smoke when accelerating - Valve stem seals?
« Reply #1 on: 13 March 2019, 08:56 AM »
Indeed this sound like valve stems.
A good test I like to do is to pick a long downhill, then go down it with throttle backed off with your foot off the pedal completely, so you are creating high vacuum.  Then at the bottom of the hill you floor the pedal.  If you see huge plume of smoke out the tail pipe, for sure you have some back breaking valve stem seal replacement in your future :o
Daniel
1977 450 SEL 6.9 - Astralsilber

NormW116

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Re: Blue smoke when accelerating - Valve stem seals?
« Reply #2 on: 14 March 2019, 04:56 AM »
Thanks. Sounds like that's what it could well be then. Any idea if the head needs to come off to do that job? Is it worth placement anything else while we are in there?

rumb

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Re: Blue smoke when accelerating - Valve stem seals?
« Reply #3 on: 14 March 2019, 05:57 AM »
On the car it's a fiddly job. You can get a air hose that screws into the spark plug hole that should hold the valve in place as you use a spring compressor to remove the 2 valve stem holders. Strike the top of spring  with a hard plastic hammer will jar things to loosen the pesky retainers. use a fine pick and magnet to help remove them. Too much pressure on valves will make them drop into cylinder, so always have that piston at top. Alternate is to push rope in spark plug hole enough to hold valve up. Then remove spring and old valve seal, put new ones on using the little plastic condom cap to prevent scoring of new seal. with the spring removed you cant really tell if guides are bad unless they are totally buggered.

Before deciding do a wet and dry compression test to see what is leaking, valves or rings.

I used this method on my  6.9.  On my 300se I had the head off to replace head gasket so I just had machine shop replace seals for me. On that car it worked extremely well, stopped use of oil between changes almost completely.

'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

daantjie

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Re: Blue smoke when accelerating - Valve stem seals?
« Reply #4 on: 14 March 2019, 09:45 AM »
For me at least the "rope trick" did not work at all, it was hella frustrating and I was worried that the rope would somehow bind or break.
Further to what Robert has said, a good tip which worked for me to release those pesky keepers is to give it a good soak with carb cleaner and to let it sit for a while.  Then whilst you put slight tension on the valve by way of the valve spring compressor (an absolute must for this job by the way), give LIGHT taps on the end of the handle of the compressor with light hammer (must be metal hammer not "soft hit")  This sends a small shock down to the valve and this worked very well for me to break the keepers loose.  But indeed if you do this with motor in the car it is murder on your back :o
Daniel
1977 450 SEL 6.9 - Astralsilber

NormW116

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Re: Blue smoke when accelerating - Valve stem seals?
« Reply #5 on: 15 March 2019, 06:38 AM »
Cool, thanks again guys