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