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Hi, I'm brand new to this forum, but thought I'd share an issue that I'm experiencing with my 1985 380SL to see if anyone could provide me some insight and advice.

I purchased the car about 2 months ago from a private seller. The car has 71,000 verified miles.

The issue that I'm experiencing is that the car pumps out giant billows of white smoke from the tailpipe whenever I drive down a very steep hill or park the car on a very steep hill and then drive it.

I had the mechanic check the camshaft oil rails and replace the automatic transmission vacuum modulator based on suggestions I read on this forum. The oil rails were fine and no automatic transmission fluid was found in the vacuum lines.

The mechanic I took it to said that I might need to get a valve job to fully remedy the problem. Is there anything else I could look at before doing a full valve job? Has anyone had any luck solving an issue like this by just replacing the valve seals? Doing just the seals only will cost about 1/3 of doing the full valve job. At 71,000 I wouldn't expect the engine to need a full valve job.

Any insight or advice would be greatly appreciated! thank you!

-David

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1984 380SL
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Remember that 71,000 miles on a car that is 34 years old is becoming meaningless. Age and sitting around has done more damage to your girl than driving at this point!

White smoke could be leaking coolant from a slightly failed seal between the intake plenum and the block.

The only way to tell would be to check spark plugs. Number 7 should look like it has been steam cleaned.

Or remove the whole intake system down to block valley (very big job).

Doubly so, it could also be condensation in the exhaust.

A good test to try would be to park on a hill. Have someone else start it and see if it's steam.
 

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Discussion Starter #3
Remember that 71,000 miles on a car that is 34 years old is becoming meaningless. Age and sitting around has done more damage to your girl than driving at this point!

White smoke could be leaking coolant from a slightly failed seal between the intake plenum and the block.

The only way to tell would be to check spark plugs. Number 7 should look like it has been steam cleaned.

Or remove the whole intake system down to block valley (very big job).

Doubly so, it could also be condensation in the exhaust.

A good test to try would be to park on a hill. Have someone else start it and see if it's steam.
Your argument of age vs milage totally makes sense. So if the car is leaking coolant, how does the downhill aspect play into it? It only smokes when the car is nose downhill. I don't think it's condensation in the exhaust because it smells like smoke but I'll check again and see if I see water droplets collecting out of the tail pipe. Also the billows seem to hang in the air much longer than steam would.
 

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A couple more questions - Does hot or cold make a difference? Does it clear up fast once level or does it take some time? Lastly, does the car run fine otherwise or is there any hint of misfire? Perhaps when really steep nose down oil can pool up around the frontmost exhaust valve and leak past the seal into the exhaust port. This would definitley smoke if the stem seal was really bad, for example hardened and cracked. I'm not sure if oil can drain out under the front cam bearing into the chain opening.
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Discussion Starter #5 (Edited)
Does this happen both nose up/down or only one way? Does it clear up fast once level or does it take some time? Lastly, does the car run fine otherwise or is there any hint of misfire?
This only happens when I go downhill, not uphill. The car runs 100% perfect otherwise with zero misfires. It passed emissions and has very clear exhaust all other times. No smoke on acceleration or when idle. I haven't driven the car in the cold because I'm in Southern California.

When it does smoke, it takes about 2-3 minutes for it to clear up. The smoke billows are also enormous and seem to linger. When I say large, I mean comically large. As big as the car itself.
 

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This fits with a bad stem seal on one of the forwardmost exhaust valves, as the exhaust port is not affected by vacuum like the intakes and no significant oil will enter the combustion chamber (plugs stay clean and no misfire). The seals can be seen through the spring, if it is broken to the point it has fallen away this would be noticeable.

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Discussion Starter #7
A couple more questions - Does hot or cold make a difference? Does it clear up fast once level or does it take some time? Lastly, does the car run fine otherwise or is there any hint of misfire? Perhaps when really steep nose down oil can pool up around the frontmost exhaust valve and leak past the seal into the exhaust port. This would definitley smoke if the stem seal was really bad, for example hardened and cracked. I'm not sure if oil can drain out under the front cam bearing into the chain opening. View attachment 2608008 support.
Would you happen to have an image showing where the oil might be pooling up around the frontmost exhaust valve and leak past the seal into the exhaust port? What would cause the oil to pool in this area in the first place?
 

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Discussion Starter #8
This fits with a bad stem seal on one of the forwardmost exhaust valves, as the exhaust port is not affected by vacuum like the intakes and no significant oil will enter the combustion chamber (plugs stay clean and no misfire). The seals can be seen through the spring, if it is broken to the point it has fallen away this would be noticeable.

View attachment 2608009
This is very helpful, thank you. So I suppose it would make sense to check the spark plugs and see if they might provide any clues. I had the spark plugs replaced and all of the fluids flushed and replaced right after I bought the car.

Is simply checking the valve seals a big job? If the valves are worn or broken, is this something that can be visually diagnosed? Would doing a compression test help further diagnose this?

Thank you again.
 

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It seems to me if this were the problem the seal would be missing, but who knows. The part is plastic and with age can become brittle and crack. With the valve cover removed a careful inspection may reveal if the part is still intact, missing, or partially broken. The plugs would probably be ok, oil could only really enter the combustion chamber if the exhaust valve of the offending cylinder is by chance open when the engine is stopped. This would clear out very fast and may not show on the plug. The first step is to take a look at the seals. The exhaust is in front of the intake only on the forwardmost cylinders, so I think this scenario could only happen on these two.

Now I am worried as I also have an '85 380SL with 80k!
 

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So if the car is leaking coolant, how does the downhill aspect play into it? It only smokes when the car is nose downhill.
This will sound weird, but I had a very similar issue after I resealed my intake. At an angle, coolant from the left (passenger side) rear of the intake plenum would leak coolant in. It took a while, but it eventually sealed itself. I have no idea why it did this.

The other advise is also good.
 

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Discussion Starter #11
This will sound weird, but I had a very similar issue after I resealed my intake. At an angle, coolant from the left (passenger side) rear of the intake plenum would leak coolant in. It took a while, but it eventually sealed itself. I have no idea why it did this.

The other advise is also good.
Ah, this might be worth looking at as well. Did you get giant clouds of white burnt-smelling smoke coming out of your exhaust going downhill? If this doesn't seal itself like it did in your case, what would be the repair I'd need to make?
 

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1990 560SEC 334K miles as of 5/19, 1990 500SL (Euro Build) 67K miles as of 5/19
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My bad, I re-read you had tried the modulator replacement. However, my money is still on the modulator. Try getting on the highway at night (less traffic) and then exiting at the same rate of speed and then come to an abrupt stop at the intersection. I bet you will see the white smoke again in addition to downhill. I would suggest a different mechanic to check the modulator. I am not casting aspersions, However, just the other day, I have a friend who had transmission fluid changed on her '07 SL550. She recently started hearing a sound she didn't like. She brought it to me to drive and listen to. I am just a diy'er on my own cars. We found her oil dipstick on the hood rail. (the look on her face was priceless) I checked the fluid in her ABC pump(steering side) it was not registering on the stick. Then her strut side, it was very low. She said she asked them(the shop who did the tranny service) to check all fuilds 2k miles ago. I asked her who gets to do the work when the pump fails had I not caught it? ABC pump and struts are good money for shops for people who don't maintain their fuilds.. She said she would have gone to them. Point being a 2nd set of eyes is always the way to go when it comes to some things.
 

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1986 560SL with M120 V12 Engine, 1988 560SL Stock
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I had this problem in my 86 before I transplanted the engine. I was told that it was caused by broken camshaft oil fittings. I never fixed it as I just replaced the engine but I did find the cam oilier in the old engine were broken when I eventually dis assembled the engine.
 

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Discussion Starter #14
It is your transmission vacuum line sucking tranny fluid back into the engine and burning the white smoke. You need a new modulator!
I replaced the vacuum modulator and the mechanic didn't notice any automatic transmission fluid in the vacuum lines when he did.
 

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I had this problem in my 86 before I transplanted the engine. I was told that it was caused by broken camshaft oil fittings. I never fixed it as I just replaced the engine but I did find the cam oilier in the old engine were broken when I eventually dis assembled the engine.
Are you talking about about the camshaft oil rails?
 

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1990 560SEC 334K miles as of 5/19, 1990 500SL (Euro Build) 67K miles as of 5/19
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Also, re-check fundametals. Check your transmission fluid level. When its cold vs when it's hot. A small amount can change the level drastically. Reason I say is that I went to rebuild a tandem pump on my 500SL as I thought it was leaking due to age. Not! I found that the leak was due to not having a copper seal on each side of the output pressure line banjo fitting. One end of the fitting was agaist the pump with no copper seal and leaking. Wasn't the pump leaking at all. So recheck fundamental connections, vacuum lines etc. I bet its simple. Another example on your car. Get rid of the yellow and green check valve looking thing in your engine vacuum line that goes to your warm up regulator. My friends 380SL's WUR got replaced by a shop as they said it wasn't working as the cause of his high idle. I checked his OVP in the passenger foorwell and it was a 10amp fuse for 10 cents. $900 later from a shop who told him it was the WUR. He said it still wasn't running right after they put in the rebuilt WUR. I checked his fundamentals. The reason the WUR wasn't working was that the yellow and green round check valve looking thing is a foam pad that removes condesation from the vacuum line. It was petrified and no air could pass. I did a bypass with a rubber tube and it worked right. He never needed a new WUR is the point, he just needed to clear the blocked vacuum line. Things are usually more simple than you are told. It's hard for mecahnics to to pay the bills on 10 cent fuses on OVP's and 50 cent tubing by passes with15 minutes of billed labor.
 

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Discussion Starter #17
Also, re-check fundametals. Check your transmission fluid level. When its cold vs when it's hot. A small amount can change the level drastically. Reason I say is that I went to rebuild a tandem pump on my 500SL as I thought it was leaking due to age. Not! I found that the leak was due to not having a copper seal on each side of the output pressure line banjo fitting. One end of the fitting was agaist the pump with no copper seal and leaking. Wasn't the pump leaking at all. So recheck fundamental connections, vacuum lines etc. I bet its simple. Another example on your car. Get rid of the yellow and green check valve looking thing in your engine vacuum line that goes to your warm up regulator. My friends 380SL's WUR got replaced by a shop as they said it wasn't working. $900 later. Still wasn't right. Reason was that the yellow and green check valve is a foam pad that removes condesation from the vacuum line. It was petrified and no air could pass. I did a bypass with a rubber tube and it worked right. He never needed a new WUR is the point, he just needed to clear the blocked vacuum line. Things are usually more simple than you are told. It's hard for mecahnics to to pay the bills on 10 cent fuses on OVP's and 50 cent tubing by passes with15 minutes of billed labor.
Thank you! All the fluids were replaced as soon as I got the car. Would a blocked check valve on the vacuum line cause the car to smoke only when going downhill?
 

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1990 560SEC 334K miles as of 5/19, 1990 500SL (Euro Build) 67K miles as of 5/19
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I will look up the part for you that needs to be removed. So your WUR and other vauum is not impeded. Might not be this fix, but will help other operations. All fluid replaced?Recheck their levels yourself. Did you see what I wrote about the womans 550SL. lol You need to get a little more hands on and you will find that people you pay to do your work for $110/hour are farming out stuff to "trainees" and other grunts for 20 bucks an hour and detils really matter if you are OCD like me when it comes to the proper operation of these vehcles. This car will cost you a fortune if you don't learn about it now. Gas eats rubber, Now even faster with ethanol. The next thiing you will find is a warm start problem....this will be your accumulator. You can do that yourself. No URO, Bosch only. Fuel filter needs to be recent also. As I rfemember when I replaced my fuel filter on my 560SEC it plumed white smoke after I changed it.
 

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1990 560SEC 334K miles as of 5/19, 1990 500SL (Euro Build) 67K miles as of 5/19
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The part in your vacuum line i'm speaking about could be green and white. #2 in the picture Take it off and try and blow throgh each end. #1 is the WUR
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