advise needed low compression on one cylinder 280sl

[StevenF]

I brought my car into my mechanic because my car was smoking (white smoke) when I would accelerate.  I was told from a few different sources that it is most likely the valve stem seals causing the problem.  Fast forward-- my mechanic performed a leak down test and determined that one cylinder has low pressure.

From what I understand , it is not a small job to repair the low compression issue because you have to remove the engine.  Here are my questions:
1-can I drive the car with low compression on one cylinder --in other words to I have to get it fixed? Currently, the car drives well and I had the car up almost  to 100mph on the highway
2-my point of bringing the car is to fix the smoking issue---do I just fix that?

What do I do?

[twistedtree]

White smoke when accelerating could be oil or antifreeze being burned.  Smell is the best way to tell the difference.

Bad valve stem seals typically will NOT smoke on acceleration.  The classic test is a long deceleration followed by reapplication of the accelerator.  If you have bad seals, the vacuum created during the deceleration will suck oil past the seals and when you hit the gas again it burns off with a puff of smoke.  Ideally you do the test with someone following you to watch for the smoke since it's hard to see from inside the car unless it's really bad.

Do you know what the compression values were on the cylinders?

It sounds like you might be getting questionable diagnostics from your repair shop.

[stickandrudderman]

It sounds like your shop are unable to explain the findings satisfactorily, which means you should find another shop.

[StevenF]

Thank you for your response
The Compression test yielded 215-225 per cylinder and the leakdown test showed 32 on cyl1 30 cyl 2 33cyl 3 32 cyl 4  31 cyl 5 and 10 on cyl 6

I have 2 mechanics tell me I do not to rebuild the engine and one says I need to....
what do I do?

As a reminder, I went in because the car was smoking a puff of white smoke ONLY at a traffic light and then after accelerating ZERO smoke.  All three mechanics are saying that I need to check the valve seals (which I agree).  If the seals are bad then replace the seals.  If the guides are bad then change those---but do I have to rebuild because of the leakdown test?

[stickandrudderman]

A cylinder leak down test is only the start of the investigation. The next step for a competent shop is to explain to you what the next step is in making further analysis. They/you need to know whether the leak is at the valves or the rings and since you are paying them they should be guiding you. If they are not then clearly they cannot be trusted and you should find someone more competent.
Going from "cylinder 6 is leaking pressure" to "should I change the valve seals" is like going from  "should i check my tyre pressures" to "I need a complete suspensions overhaul" just because your car pulls to the left.
The cylinder leak test may or may not be connected with your smoke issue but in any case continuing to drive a car with a known compression issue is only asking for a bigger bill further down the line.

[StevenF]

maybe I wasn't clear:  according to all three mechanics the smoke has nothing to with compression....

[stickandrudderman]

How do they know when the cause of the loss of compression has not been identified?

[StevenF]

what would you do....I am a first time 280sl owner

[Garry]

Steven,

I think Stickandruddermans starting point with a leak down test would be a good place to start the investigation as he suggested to get some indication on what might be occurring.

Garry

[StevenF]

The Compression test yielded 215-225 per cylinder and the leak down test showed  reduced pressure of 32 on cyl1 30 cal 2 33cyl 3 32 cal 4  31 cyl 5 and 10 on cyl 6

I have 2 mechanics tell me I do not to rebuild the engine and one says I need to....
what do I do?

As a reminder, I went in because the car was smoking a puff of white smoke ONLY at a traffic light and then after accelerating ZERO smoke.  All three mechanics are saying that I need to check the valve seals (which I agree).  If the seals are bad then replace the seals.  If the guides are bad then change those---but do I have to rebuild because of the leakdown test?

I recently heard that 30% leakage in these cars is not abnormal....

[WRe]

Hi,
 some information about a leak down test: https://www.youtube.com/watch?v=ofSiTGeLXvc .
...WRe

[Shvegel]

Speaking as a professional mechanic and former instructor of mechanics for BMW I don't think you have found the right mechanic yet.  the right mechanic would do the compression test then squirt some oil in the offending cylinder and check again.  This would isolate the problem to the cylinder block(short Block) or the head.  If the reading changes with the oil it is the short block and if the reading doesn't change it is in the head. 

For what it is worth you car is experiencing exactly what mine did and the result was a valve guide that had come loose in the head which is fairly common.  If the problem is in the head change all the valve guides.  If memory serves the new guides have a step that keeps the from slipping down in the head.

[Benz Dr.]

Over 200 PSI per cylinder seems on the excessive high side to me. However, I fail to see a problem here, aside from some smoking coming off of idle which is of course valve guide problems.

[Shvegel]

After reading Dan's post I reread you compression numbers.  My advice would be to pull the head, replace all the guides and seals and do a proper valve job.  MAKE SURE that the machine shop replaces the entire bronze guide rather than bushing the inner bore of the existing ones.  These engines have an unusual issue in that the Bronze guides can actually come loose in the cylinder head and move up and down with the valve.  If your machinist isn't familiar with this they will want to do what "They've done a thousand times without any problems." the guides are available in oversize outside diameters to compensate for the wear in the aluminum cylinder head.

Air cooled Porsche engines have this weird mystique when it comes to machine work that only certain shops know how to do it right.  I resist this thinking whenever possible because I think a good machinist is a good machinist but in this case if I were you I would pull the head and send it to someone who has done this before.  Ask Joe or anyone else who regularly works on these engines who a good shop is.  It isn't that most shops can't handle this it is more that you have to be able to educate them as to what you want and trust that they will do it.  If all else fails I have a guy in Minnesota that is aces.

As for driving it as it is.  If one of the guides has worked loose you are now wearing the aluminum bore in the cylinder head.  If you wear it too much you will need a new cylinder head.  Not fun.  I would drive it home and park it until you are ready to attack it.

[Benz Dr.]

If a valve guide is loose it will smoke a lot all of the time. This one appears to be giving a puff of smoke off idle so my bets are worn valve guides.

 The best engines produce about 175 - 180 PSI. 215 is very high and I'm wondering if there's a lot of deposits on the piston crowns or in the chambers. Since the readings are fairly even I'm also wondering if the gauge is accurate.

 I've had valve guides lined with excellent results. 

[Shvegel]

My exhaust guide the dropped out behaved exactly like the OP describes.  Puff of smoke off idle.  The longer I idled the worse the puff. 

[Benz Dr.]

My exhaust guide the dropped out behaved exactly like the OP describes.  Puff of smoke off idle.  The longer I idled the worse the puff. 

That makes sense. Vacuum is highest or at least very high from 18 - 21 inches at idle so there's a lot of draw on the valve guides. Driving down the road at 100 KPH,  you might get 18 - 19 inches of vacuum but any oil leaking into the cylinder would mix well enough with a much larger air volume so you really won't see it. The throttle linkage is designed so that the fuel mixture will go leaner during mid range driving.