Compression and leakdown #'s

sballard · 12-13-2015, 09:26 AM

Background: The car is a '72 911T. When I first bought it, the engine was very strong and there was minimal exhaust smoke. I was driving it one day and I downshifted while slowing for a stoplight. There was a huge puff of blue smoke out of the exhaust. It's smoked quite a bit ever since. I once read that some owners will put new valve seals in when selling cars with bad valve guides to cover up this problem and I wonder if this happened to me.

This weekend, I did a compression test and leakdown test (my first attempt). here are the numbers:

Compression: 140-145 lbs across all cylinders

Leakdown: 1 - 21%; 2 - 17%; 3 - 10%; 4 - 10%; 5 - 10%; 6 - 11%

I could hear some hissing through the exhaust on most, so I assume that a valve job is in order. Are the compression values low enough to suspect rings as well?

Steve@Rennsport · 12-13-2015, 10:46 AM

Need more information,...

1) How many miles on the engine?

2) Did you hear any leakage from the oil tank?

sballard · 12-13-2015, 12:54 PM

89,000 on the speedo, but I don't know how accurate that is.

Unfortunately, I was focusing on the intake and exhaust and forgot about the oil tank. I will repeat the leakdown and report back.

sballard · 12-15-2015, 01:06 PM

I repeated the compression and leakdown tests on the engine cold. Here are my findings.

Cyl 1: 135 lb compression; 20% leakdown

Cyl 2: 145 lb compression; 12% leakdown

Cyl 3: 145 lb compression; 13% leakdown

Cyl 4: 142 lb compression; 11% leakdown

Cyl 5: 142 lb compression; 12% leakdown

Cyl 6: 145 lb compression; 12% leakdown

I detected air leaking from the exhaust and the oil tank breather hose on all cylinders. I only detected minimal air leaking from the intake of Cyl 6.

BK911 · BK911's Garage

Squirt some oil in the plug holes, rotate engine a few times, and re do leakdown. If better, rings are suspect.

sballard · 12-16-2015, 06:51 AM

I've read about using oil to distinguish between rings and valves in a compression test but not a leakdown test. Since my leakdown numbers are not that bad, wouldn't I likely get a bigger effect if I did this with a compression test?

sballard · 12-17-2015, 10:33 AM

Today, I squirted some 10w40 into Cyl 5 since access was easiest to this cyl. Compression went from 147 (before oil) to 175 (after oil). Two more sequential measurements gave me 165 and 155.

Are these values bad enough to merit a full rebuild (rings and valves) now?

dad911 · 12:26 PM

Quote:

Originally Posted by sballard

Today, I squirted some 10w40 into Cyl 5 since access was easiest to this cyl. Compression went from 147 (before oil) to 175 (after oil). Two more sequential measurements gave me 165 and 155.

Are these values bad enough to merit a full rebuild (rings and valves) now?

Wayne's article about when to rebuild.
Pelican Technical Article: Time to Rebuild? - Part I

IMHO it is probably borderline.....but it's easy for us to spend your money. How is oil consumption?

Are you going to DIY? How is the rest of the car(suspension, rust, transmission)?

Do you want to keep car original, or if it's a 'outlaw' like mine, you may want to swap.....

sballard · 12-17-2015, 01:16 PM

Oil consumption has been hard to judge in years past because the engine leaked so much oil. I dropped the engine awhile back to deal with the leaks. It turned out that most of it was leaking from the rocker shafts and tightening them cut the oil leaks substantially. Soon after that, I took the car off the road to fix the rusty rockers and front pan. The car hasn't been driven much recently as a consequence and I've finally had the time to pay attention to the engine again.

When I was dealing with the rocker shafts, I noticed that one of the exhaust valves was seriously "coked up" with carbon, so I've suspected for a while that the valve stems were badly worn. I was just hoping that the rings were in good enough shape to overlook this time.

I've read both of Wayne's books cover to cover, and I once took a course once in rebuilding 350 Chevy engines so I'd like to try doing it myself. Of course what I really want to do is to drive this thing!

I definitely want to keep it original. The rest of the car is in reasonably good condition.

sballard · 12-19-2015, 08:25 AM

Any other opinions out there?

dad911 · 12-19-2015, 08:55 AM

Quote:

Originally Posted by sballard

Oil consumption has been hard to judge in years past because the engine leaked so much oil.
.....
Of course what I really want to do is to drive this thing!

I definitely want to keep it original. The rest of the car is in reasonably good condition.

Well, since no one else chimed in..... I assume you have the compression & leakdown gauges.... So I would drive it, and check it again in a few months/few thousand miles, and you will have a better handle on oil consumption.

wdfifteen · wdfifteen's Garage

i would go for a rebuild at this point. 21% is a lot, and all cylinders are high. This tells you the engine has substantial wear, but doesn't reveal wear on bearings. I would be concerned about them. If a rod bearing lets go your wallet will be in a world of hurt.

Jase77 · 12-19-2015, 11:37 PM

Hey Steve,

I am slowly starting to consider a rebuild too. Some of the symptoms you mention are familiar to my car. Over the festive period I will change as many gaskets as I can get to with the engine in situ. I will replace all of the suspension bushes too so this will take up much of my cash. My local mechanic thinks my motor is pretty good and he recommends that I just drive it and not open it up until it really needs it (if it goes BANG).
I will also do a compression and leak down test but I feel that I already know what the results will be...
Ideally I will get all of 2016 driving it and do (attempt) the engine and trans rebuild this time next year.
Good luck with yours and keep us posted.