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  #1  
Old 11-28-2013, 11:53 AM
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Join Date: May 2012
Posts: 65
Wet Compression Test vs Leak Down Test

My 380sl has plenty of power, but it frequently emits a puff of greyish white smoke on exit ramps after hwy driving. So I decided to do dry and wet compression tests to see whether the smoke may be caused by leaky valve stem seals or rings

The results of both tests, listed below, showed that the dry readings on the right cylinder bank were all higher than they were on the left bank, probably because that head had to be shaved when I did the valves app. 70k miles ago.

Cyl_Dry Wet

1 _160 _180

2 _160 _180

3 _160 _175

4 _160 _190

5 _180 _200

6 _175 _180

7 _180 _200

8 _185 _200


Considering that the difference between the wet test and dry test on most of the cylinders was near or slightly greater than the acceptable 10%, I was wondering if anything could be gained by doing a leak down test, other than to determine how much the rings in each cylinder are leaking.

Thanks very much in advance for your usually well-informed opinions

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  #2  
Old 11-28-2013, 12:23 PM
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Join Date: Dec 2010
Location: upstate SC
Posts: 79
I would take a look at #6. You only gained 5psi difference from dry/wet. Somethings going on in that cylinder. Maybe Just do a leakdown on that one first..... fwiw
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  #3  
Old 11-28-2013, 10:54 PM
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Join Date: Jan 2012
Posts: 7,534
I don't use wet comp tests just dry and leak down. Compression is a quick way to find a weak cylinder, leak down tells you if the leak is rings, valves or compression to coolant.

Valve stem seals tend to smoke at idle / acceleration from an extended idle, slightly worn rings on deceleration ( closed throttle , high intake vacuum ), very worn rings at idle.

There isn't a direct way to test valve stem seals. Given your engine has time on it, changing valve stem will help. Parts cost is low, labor time consuming but not difficult.
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  #4  
Old 12-02-2013, 12:37 AM
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Join Date: Jan 2004
Location: Jacksonville, FL
Posts: 474
White smoke on coast down can also be caused by a bad transmission vacuum modulator. Remove the vacuum hose from the modulator to check for the presence of transmission fluid. Mark
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  #5  
Old 12-03-2013, 09:13 AM
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Join Date: May 2012
Posts: 65
The modulator was replaced 2 years ago when I had the transmission rebuilt. So I didn't check. But I guess that would still be a good idea. Thanks for reminding me

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