valve stem seal selective replacement?

[rix116]

I have a 74 450 SEL w/ 260,000 miles on it. I'm interested in replacing
the valve stem seals and I've read similar threads on this forum.
My mechanic says I might be "spitting in the wind", because if the valve
guides are also bad, there may not be enough improvement. He
suggests I inspect the spark plugs for "oilyness" (is that a word?)
and then to experiment replacing only the seals for the affected
cylinder(s) to see if there is an improvement.
This sounds like a good plan to me. I'm happy with the power and performance otherwise. I just want to dramatically lessen the smoking.
What happens is shortly after cold startup (one or two miles) there is
a very dramatic big clould of smoke (arrest me now officer, type smoke)
When the car is warmed up after that there is no visual smoking.
The car is a good second car for me that I want to keep on the road,
but it is definitely not worth doing a whole valve job on. Just want it better.
What do you guys think?

[joshhol]

Selective valve stem replacement doesn't sound like too bad of an idea. If you do it on the fouled plug cylinders, that will give you an idea if it will do any good. I'm sure you've read about the play you can feel if you move the valve stem in the guide. If you can feel any play, it is worn.

Just be really careful about not dropping a valve into the cylinder. If you do a search under my name, you will see about that mess.

[ctaylor738]

If you are going to the trouble of pulling the cover and getting out the spring compressor, you might as well do the whole side. With the spring off you can try to wiggle the valve stem, and the conventional wisdom is that if you can feel it move, you need new guides.

I just went through this on an M110 in a 280SL that is in rehab in my garage. It smoked profusely but the plugs were very clean so I'm not sure this is a good test.

I would also be curious about why it smokes only during warm-up. It seems to me that with thicker oil, it would be less likely to be pulled into the cylinders and burned. Did your mechanic have an opinion on that?

[rix116]

Chuck- the car doesn't smoke at first, but starts smoking about a mile or two from home. After it warms up, it seems to stop the smoking. I didn't ask my
mechanic about oil thickness (I run 20-50), but he did say that it may stop
smoking when it is warmed up because the valve stems or valve guides
expand, stopping the leaking of oil into the cylinder. This of course
may mean the seal replacement may not help much.
I'm also curious about your statement that your car smoked alot, but did
not have fouled plugs. This may be the case on mine too, because last time I changed the plugs I don't remember any that were fouled. But I need to
check them again to see. If I don't see any fouled ones, I don't know if I'll
do the selective seal replacement, but it is hard to live with the smoking
period durning warmup, and the car's value really doesn't justify a valve job.

[ctaylor738]

Maybe there is a window when the oil is warm enough to get past the seals and guides, but the guides/stems have not expanded enough.

My 2.8 did not smoke until it warmed up and then its worst smoke was at idle. The machinist thought that this was due to oil getting past the exhaust guides and being burned in the hot exhaust manifold.

You would probably be looking at $250-$275 a head for a rebuild plus $100 or so for gaskets and misc if you did work yourself and farmed out the shop work.

It doesn't sound like you are blowing enough oil to foul the plugs, but you might be able to tell something if you shut it off during prime smoke time and pulled the plugs.

[Tomguy]

My 4.5 used to smoke on cold starts in the summer (it hasnt smoked at ALL on startup lately, I think because it sat so long the valve stem seals were hard as rocks and they softened over time, but I'm still going to do them). It still smokes a bit if I back the air speed screw down a turn and let off the gas from a hot run (from 100 to no throttle, for example).

My theory was (and is) that after you shut it off and the engine isnt running, the way the heads on the m116/117s are designed, the oil sits in the head along the valves. If you park on level ground, this is exaggerated as the oil doesnt drain down the front where the timing chain is, or down the rear drain hole. The oil is above the level of the valve guides and bad seals will let some of it leak down the guide and into the intake/exhaust port(s), and also get on the top of the valve (and in the cylinder if it's open). This burns off shortly and once your engine is warm, the oil burned is so small it's not noticeable or, in the case of my engine I believe, the engine's movement and vibration keeps the oil from sitting in that spot to go through the guide and it drains out of the head instead. The thinner your oil is when your engine is hot, the worse the smoke will be on startup, because more oil will run down in the time it's off before the oil drains out.

That's my theory anyway. I might be totally wrong. But when my oil gets thin before I change it out, I notice some smoke on a cold start still (at least I did last time). Changing it got rid of the gas smell in the oil (for now, anyway) AND stopped the smoking.

Edit: Also, I believe the smoking from when I let off the gas, if I turn the air speed screw down, is that due to a larger manifold vacuum, oil is sucked down through the poor stem seals, through the guides, and into the intake where it's then sent to the cylinder to burn. I MIGHT have my air speed screw open more than it should be, but it keeps the smoking away!