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

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)

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

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)

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

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

Re: Leakdown Testing
 

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.

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

Re: Leakdown Testing
 

Have you been able to verify your TDC is correct. If it would have slipped that would throw off your timing which could possibly be your miss. Not sure just taking a WAG.

Re: Leakdown Testing
 

I have a related question that has been keeping me up at night for years, LOL. When using a twin gauge setup, do you set the incomming gauge to 100psi before connecting the cylinder hose, or do you adjust after connecting? The reason I ask is, if you set it before connecting, the reading on the incoming gauge drops slighty when connected. Chuck

Re: Leakdown Testing
 

Mine is supposed to be connected and then adjusted to 100psi then backed down to 60 psi before disconnected so then when connected to next cylinder it starts @ 60 psi
Mike Taylor 3601

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.

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

Re: Leakdown Testing
 

Those Sun Leak Down testers are like Gold I have two one in the shop and the one in the trailer. When there is a auction in the area I look for them. As I remember no mater what the input pressure is the only put 30 or 60 psi into cylinder.

Re: Leakdown Testing
 

Thanks to everyone for all the info. Chuck

Re: Leakdown Testing
 

That's what I use. Have had mine for forty years. (got it in 1972) Still works like new. Just needed a new hose once.

Re: Leakdown Testing
 

Yep had mine bout that long too-NO its not for sale---one more thing remember to RESET TO ZERO the % gauge before you go do the next cylinder( part of the attach/ unattach/attach tap the lense LIGHTLY routine---REPEATABILITY is what counts here---this is not rocket science merely comparing one cylinder to another looking/hearing for wear or malfunctions/damage---we always leak a new engine before start up/after initial start up to get a baseline for the future to see how much wear we really are getting as we put runs on the car--but we do leak it at the end of EVERY day racing just to see whats going on-FED387

Re: Leakdown Testing
 

It's also good to check for leaks with the piston all the way down (with rockers backed off). Just a little more information.

Re: Leakdown Testing
 

Good day, can a leak down test be done on a cold engine?

Re: Leakdown Testing
 

Sure.

Re: Leakdown Testing
 

Engine temp really does not come into play here!!!!!