Leakdown Testing

Ben Kallies · 06-04-2012, 05:17 AM

Question: When performing a leakdown test on an engine, how does one stop the 100psi of air pressure from pushing the piston back down in the hole, which in turn moves the crank/cam, opening up the valve, and letting all of the air you just pumped into the cylinder? I tried reinstalling all of the other spark plugs, along with putting the trans in gear, but it still pushes the piston down enough to open the valve back up.

Answer:?

Thanks,

-Ben

Rsmith38 · 06-04-2012, 07:52 AM

1/2" bar with socket on crank pulley bolt. (probably held by a helper- lined up at TDC it won't move)

buzzinhalfdozen · 06-04-2012, 07:55 AM

Make sure the piston is on TDC for each cylinder tested. You can use the timing marks on the balancer to confirm piston position. Most leakdown testers don't actually put the 100PSI into the cylinder, though the gauge shows 100 the actual cylinder pressure is much less, believe they use an orifice of some type. Maybe someone else has a more accurate explanation.Joe

Jim B · 06-04-2012, 08:15 AM

From the sound of your post I would doublecheck the accuracy of your 0 - 90 -180 - 270 markings on your damper and insure the 0 mark on the damper is really #1 piston TDC. I've seen a partially spun damper off by less than 5 degrees cause what you are describing. I am assuming you don't have some outrageous piston wrist pin offset that might contibute to this scenario. (I've never seen up to a 0.60" offset have any effect)

junior barns · 06-04-2012, 10:38 AM

Back off all rockers and do one at a time. Little time comsuming but at least you will know the valves are closed

Ben Kallies · 06-04-2012, 11:07 AM

Thanks to all for the responses. I think that I may not have been at TDC. I'll give it another try tonight.

-Ben

69Cobra · 06-04-2012, 11:29 AM

Quote:

Originally Posted by junior barns

Back off all rockers and do one at a time. Little time comsuming but at least you will know the valves are closed

X2 We always back the rockers off as well. Also you set the incoming pressure to 100 and then what the cylinder is holding out of 100 is the percentage. For example if you have 100psi coming into your gauge but the cylinder side is only showing 92psi. Then you have 8% leakage on that cylinder. At least thats how we've done it.

buzzinhalfdozen · 06-04-2012, 11:47 AM

As stated previously, start with number 1 cyl. put pointer dead on TDC of balancer, if engine turns over, you may have an issue with the pointer not being set correctly. Typically an engine will not turn if the piston is on TDC. Of course you have the option of removing the valve train also.I would caution against using a bar and socket to try and hold it unless you have someone who wouldn't mind getting their knuckles busted. Joe

Marine One · 10-04-2012, 08:36 AM

There are a lot of ways to check "in cylinder performance". I have experimented with hot engines, cold engines, dry cylinders, wet cylinders, compressed air, compressed nitrogen, various grades of oil, etc. I have even gone as far as recording how much vaccum a cylinder pulls to understand how the rings seal to the tops of the ring grooves on the way down during the intake stroke. Much harder to get rings to seal while pulling vacuum.

Regardless of your method, you need to check frequently and you need to check consistently. Since you are trying to predict wear, you need to have a good benchmark of what is good before you can say what is bad. Get a baseline reading at rebuild and make it a point to check all cylinders again, the same way you established your baseline. Then measure and record everything the exact same way after you have burnt so many gallons of fuel. Example - at every 50 gallons of fuel burnt, measure, record and look for a trend.

FED 387 · 10-04-2012, 12:05 PM

usually somewhere either on the gauge or in the instructions it will give you a range of air pressure needed so that the gauge will function properly----my particular gauge is a Sun it ranges from 70-200 pounds needed----turn the engine to a position where BOTH rocker arms have NO tension on them so that the neither of the valves are open at all-- then go do your test---we pump air into the cylinder , disconnect the air line to that cylinder then re- attach the air hose and take the reading--works for us just remember to do your routine the EXACT same way every time--- hot ,cold, medium temp engine odd numbers first whatever how ever it works best for you

06-04-2012, 07:52 AM	#2
Rsmith38 Member Join Date: Mar 2011 Posts: 186 Likes: 0 Liked 28 Times in 15 Posts	Re: Leakdown Testing 1/2" bar with socket on crank pulley bolt. (probably held by a helper- lined up at TDC it won't move)

06-04-2012, 07:55 AM	#3
buzzinhalfdozen Senior Member Join Date: Sep 2006 Location: Bellevue Ohio Posts: 984 Likes: 0 Liked 11 Times in 8 Posts	Re: Leakdown Testing Make sure the piston is on TDC for each cylinder tested. You can use the timing marks on the balancer to confirm piston position. Most leakdown testers don't actually put the 100PSI into the cylinder, though the gauge shows 100 the actual cylinder pressure is much less, believe they use an orifice of some type. Maybe someone else has a more accurate explanation.Joe __________________ Joe Buchanan SS/BX 3117

06-04-2012, 08:15 AM	#4
Jim B Member Join Date: Dec 2008 Posts: 100 Likes: 0 Liked 1 Time in 1 Post	Re: Leakdown Testing From the sound of your post I would doublecheck the accuracy of your 0 - 90 -180 - 270 markings on your damper and insure the 0 mark on the damper is really #1 piston TDC. I've seen a partially spun damper off by less than 5 degrees cause what you are describing. I am assuming you don't have some outrageous piston wrist pin offset that might contibute to this scenario. (I've never seen up to a 0.60" offset have any effect) Last edited by Jim B; 06-04-2012 at 08:20 AM.

06-04-2012, 10:38 AM	#5
junior barns Senior Member Join Date: Jun 2006 Posts: 541 Likes: 11 Liked 20 Times in 14 Posts	Re: Leakdown Testing Back off all rockers and do one at a time. Little time comsuming but at least you will know the valves are closed

06-04-2012, 11:07 AM	#6
Ben Kallies Member Join Date: Jan 2005 Location: Milwaukee, WI Posts: 107 Likes: 2 Liked 0 Times in 0 Posts	Re: Leakdown Testing Thanks to all for the responses. I think that I may not have been at TDC. I'll give it another try tonight. -Ben

06-04-2012, 05:17 AM	#1
Ben Kallies Member Join Date: Jan 2005 Location: Milwaukee, WI Posts: 107 Likes: 2 Liked 0 Times in 0 Posts	Leakdown Testing Question: When performing a leakdown test on an engine, how does one stop the 100psi of air pressure from pushing the piston back down in the hole, which in turn moves the crank/cam, opening up the valve, and letting all of the air you just pumped into the cylinder? I tried reinstalling all of the other spark plugs, along with putting the trans in gear, but it still pushes the piston down enough to open the valve back up. Answer:? Thanks, -Ben

06-04-2012, 11:47 AM	#8
buzzinhalfdozen Senior Member Join Date: Sep 2006 Location: Bellevue Ohio Posts: 984 Likes: 0 Liked 11 Times in 8 Posts	Re: Leakdown Testing As stated previously, start with number 1 cyl. put pointer dead on TDC of balancer, if engine turns over, you may have an issue with the pointer not being set correctly. Typically an engine will not turn if the piston is on TDC. Of course you have the option of removing the valve train also.I would caution against using a bar and socket to try and hold it unless you have someone who wouldn't mind getting their knuckles busted. Joe __________________ Joe Buchanan SS/BX 3117

10-04-2012, 08:36 AM	#9
Marine One Junior Member Join Date: Mar 2010 Location: Indiana Posts: 69 Likes: 2 Liked 0 Times in 0 Posts	Re: Leakdown Testing There are a lot of ways to check "in cylinder performance". I have experimented with hot engines, cold engines, dry cylinders, wet cylinders, compressed air, compressed nitrogen, various grades of oil, etc. I have even gone as far as recording how much vaccum a cylinder pulls to understand how the rings seal to the tops of the ring grooves on the way down during the intake stroke. Much harder to get rings to seal while pulling vacuum. Regardless of your method, you need to check frequently and you need to check consistently. Since you are trying to predict wear, you need to have a good benchmark of what is good before you can say what is bad. Get a baseline reading at rebuild and make it a point to check all cylinders again, the same way you established your baseline. Then measure and record everything the exact same way after you have burnt so many gallons of fuel. Example - at every 50 gallons of fuel burnt, measure, record and look for a trend. __________________ Chad Sheets \| STK 319 \| 10.69 \| 1.39 Last edited by Marine One; 10-04-2012 at 08:38 AM.

10-04-2012, 12:05 PM	#10
FED 387 VIP Member Join Date: Jun 2006 Posts: 3,420 Likes: 529 Liked 333 Times in 180 Posts	Re: Leakdown Testing usually somewhere either on the gauge or in the instructions it will give you a range of air pressure needed so that the gauge will function properly----my particular gauge is a Sun it ranges from 70-200 pounds needed----turn the engine to a position where BOTH rocker arms have NO tension on them so that the neither of the valves are open at all-- then go do your test---we pump air into the cylinder , disconnect the air line to that cylinder then re- attach the air hose and take the reading--works for us just remember to do your routine the EXACT same way every time--- hot ,cold, medium temp engine odd numbers first whatever how ever it works best for you