Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > 911 Engine Rebuilding Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered
 
Join Date: Jun 2001
Location: Los Gatos, Ca
Posts: 210
Leak Down test explained?

Can someone describe to me how the leak down test works for diagnosing valve guide wear? How exactly does it work.


thanks!!

__________________
Doug '67 911 2.2
Old 01-21-2004, 12:25 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
dtw dtw is offline
GAFB
 
Join Date: Dec 1999
Location: Raleigh, NC, USA
Posts: 7,842
Compressed air is introduced through the spark plug hole; twin gauges allow you to calculate the actual and percentage pressure drop in the cylinder (IIRC the first gauge measures input pressure at the tester, the baseline reading, and the 2nd gauge measures pressure in the cylinder). The pressure drop is the amount of leakdown. By listening carefully, the leakage can actually be heard through the breather or at the head. The location of the leakdown 'hiss' indicates ring wear (breather) or valve guide (head) wear. Leakdown tests are somewhat subjective depending on technique and equipment, but large or outlying leakdown values (when compared to other cylinders) can help pinpoint problems. Leakdown is a static test (engine is not turned over).

Best results are obtained by considering leakdown in conjunction with compression test results. A best practice (generally anyway) is to conduct both tests back to back. Otherwise, results of either test independently may be inconclusive. Compression tests can corroborate the results of a leakdown test. (If a cylinder leaks down at 11% versus 3% on all others, you can develop the expectation that compression test results on the same cylinder may also be poor due to bad sealing).
__________________
Several BMWs

Last edited by dtw; 01-21-2004 at 12:43 PM..
Old 01-21-2004, 12:39 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
Registered
 
Join Date: Jun 2001
Location: Los Gatos, Ca
Posts: 210
dtw, thanks for the reply. I don't get how a bad valve guide whould show up IF the valves themselves seal. Is it just by listening? If so, seems very subjective. I am debating with a shop the source of smoke at start up, contaminatng the plugs and emmissions.
__________________
Doug '67 911 2.2
Old 01-21-2004, 12:57 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
dtw dtw is offline
GAFB
 
Join Date: Dec 1999
Location: Raleigh, NC, USA
Posts: 7,842
Well if the leakdown value is low/within spec, you probably wouldn't take the test any further. But if the results show higher ranges of leakdown, then yes, there is the somewhat subjective test of listening for the source of the leak.

What color is the smoke at startup? Is it only at startup? Sometimes oil seeps past the rings and pools a bit in the combustion chamber. This then get blown out at startup. I've seen this phenomenon on 911s that had no issues with ring wear...but I've also seen much larger startup 'puffs' on cars with worn out rings.
__________________
Several BMWs
Old 01-21-2004, 01:20 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Irrationally exuberant
 
ChrisBennet's Avatar
 
Join Date: Aug 2001
Location: Nashua, NH USA
Posts: 8,164
Garage
Re: Leak Down test explained?

Quote:
Originally posted by dhaines
Can someone describe to me how the leak down test works for diagnosing valve guide wear? How exactly does it work.


thanks!!
I don't see how it could diagnose valve guide wear. When the valve is closed nothing is going to leak through a valve guide.
-Chris
__________________
'80 911 Nogaro blue Phoenix!
'07 BMW 328i 245K miles!
http://members.rennlist.org/messinwith911s/
Old 01-21-2004, 04:20 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
 
dtw dtw is offline
GAFB
 
Join Date: Dec 1999
Location: Raleigh, NC, USA
Posts: 7,842
Troof. I was thinking valves and saying guides.
__________________
Several BMWs
Old 01-21-2004, 05:23 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
Registered
 
Grady Clay's Avatar
 
Join Date: Dec 2003
Location: Arapahoe County, Colorado, USA
Posts: 9,032
Have you gone to:

leakdown results!!??

Best,
Grady
__________________
ANSWER PRICE LIST (as seen in someone's shop)
Answers - - - - - - - - - - - - - - - - $0.75
Answers (requiring thought) - - - - $1.25
Answers (correct) - - - - - - - - - - $12.50
Old 01-22-2004, 04:20 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
Registered
 
Join Date: Jun 2001
Location: Los Gatos, Ca
Posts: 210
thanks for the replys. based on what I see the leak down is not useful in detecting valve guide wear, mostly rings or the valves themselves. I had understood it could also diagnose the guides, but seems the answer is no.
__________________
Doug '67 911 2.2
Old 01-23-2004, 06:47 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #9 (permalink)
Senior Member
 
Superman's Avatar
 
Join Date: Feb 2000
Location: Lacey, WA. USA
Posts: 25,310
When guides and/or valve stems are worn, the valve rocks and sometimes does not seat squarely. So, potentially worn guides could be found this way. When valves rock and fail to seat properly, they retain, instead of transfer, their heat. They get too hot and burn. Then they do not seal. Intake valve hiss is heard in the intake. Exhaust valve hiss is heard in the exhaust. Ring blow-by hiss is heard from the crankcase, which is easy to hear if you remove the oil tank cap. Before making conclusions, fire the car up, drive it a bit and repeat. Carbon can get stuck under the exhaust valves. YOu can have a pretty good idea of what's going on in a combustion chamber before you remove the engine.

__________________
Man of Carbon Fiber (stronger than steel)

Mocha 1978 911SC. "Coco"
Old 01-23-2004, 07:22 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #10 (permalink)
Reply


 


All times are GMT -8. The time now is 02:30 AM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2025, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2025 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.