Since I have my valve covers off, I figure this will be a good time to do a leakdown test.

I was wondering if anyone has a procedure. If I do an internet search I see all kinds of complicated variances in the procedure.

For example, some people say use a hand crank to crank the engine so piston is at BDC. Others say it doesnt matter where the piston is.

My leakdown tool says all you need to do is loosen the rocker nuts (on whatever cylinder you are testing) to ensure the valves close completely?

Does my piston need to be at BDC (or TDC for that matter?) ?
*(Its a major hassle if so because I dont know how to 'hand crank' my engine...)

Also, what kind of air compressor do I need? Like specs, what kind of PSI, tank size, HP motor?

Can I just get a cheap electric one?

Any help would be appreciated!

 

anybody? I want to get some parts after work today..

 

With the piston on TDC of the compression stroke you won’t have to mess with the rocker arms since both valves will be closed. Make sure you use a two gauge leakdown tester and adjust the input to 100psi for each cylinder, then you can read the leakdown directly from the second gauge. I have a 20 gal compressor and it works ok. The only issue is my compressor does not turn the pump back on until the pressure drops to 95psi. The engine has to be hot . As I remember on my engine the leakdown was 2-5 percent worse when it was cold.

You can turn over the engine with a socket on the crank pulley. (This is a PITA).
Finding TDC on # 1 and #6 is easy. For the other cylinders I made a paper donut with
lines 90 degrees apart and mark the firing order on each line. So one line has 1&6, 8&5,
4&7, and 3&2. Tape the donut to the inside of the crank pulley when the crank is on TDC and line up the 1&6 line to the O pointer. This assumes your balancer has not shifted. I wish I had a picture of this it is much easer than it sounds. If you follow the firing order you never have to turn over the engine more than 90 degrees at a time.

Get the engine hot.
Remove all the spark plugs.
Turn over the engine by hand to # 1 TDC.
Install the leakdown tester in #1.
Adjust the input air to 100psi.
Read the leakdown off of the other gauge.
Write down results.
Turn the engine over to the next cylinder in the firing order and line up the next line on the donut to find TDC and repeat above steps.

I don’t know how long the engine will be considered hot for this test. Removing the plugs and screwing in the tester it can take some time. I found screwing in the tester to be a major PITA. Good luck.

 

Dave's got some good tips. In the airplane game, 80 psi is used as the input and usually a screwdriver is placed in the upper spark plug hole and TDC is found in each cylinder by hand turning the prop. Mid-70's are good numbers for planes. If you find a weak cylinder, you can determine where the loss is by listening to the intake, exhaust or crankcase while the compressed air is charging the cylinder. That will tell you whether you have an intake valve, exhaust valve or ring problem. BTW, I don't know about SBC's (I tried to leakdown test my 91, but gave up - hey: it runs ok), but the air will move the piston (and the prop!!) on an aircraft if the piston's not right at TDC. Good luck.

 

Its going to be easier for me to test it without having to move the piston to TDC...although maybe if I put the transmission in neutral the air will cause it to move to BDC?

So the loosening of the rocker arms method (vs. hand turning the nut on the crank shaft) is better for me


I should also mention if I HAVE to have the engine warm, I might as welll not do it since my computer chip is out.

I can imagine, as metal expands when its hot, that readings would be lower(less good) directionally on a cold engine.... so maybe if you knew it read ok on a cold engine, that leakdown is minimal on a hot engine....

Whats got me wanting to do this is I sometimes spray a few drops of oil (nothing much) out my valve cover breathers at the end of a quarter mile run (I can tell this bc theres a slight bit of oilbeneath the breathers)....

Since I run open breathers (and the stock engine was closed) I dont know if this little bit of oil -under extreme conditions is bad.

44,000 miles on the car. 93 hot cam'd LT1 ZF6 100 shot nitrous

Sprayed a little bit of oil both before nitrous installed and after

 

You will get a more realistic number with the pistons at TDC than at BDC. There is much more wear at the tops of the bores than the bottom.

RACE ON!!!

 

In performing a leakdown test, unless you have the piston EXACTLY at top dead center, it's gonna spin the motor over when you pressurize the cylinder because it's gonna slam the piston to the bottom of it's bore.

I would prefer to do it this way just because you know you're consistent on all cylinders.

 

Do I have to have the engine hot?

Seems to me if it was hot Id just be burning my hands and stuff on my headers when removing the plugs to put the leakdown tester bung in the spark plug hole...

 

I did mine cold. What is important is that all the cylinders are tested under the same conditions. Don't test some hot today and then the rest cold tomorrow. Don't put oil in one and not another, etc, etc.

 

FWIW, You can't avoid having to turn the engine by hand either way. (Not sure if you trying to avoid that altogether or just trying to make sure the valves are closed) You will have to turn the motor to reset lash/preload.

 

Nathan,

did you test yours at BDC?



Rain Delay...Good point about having to turn the engine by hand when resetting valve lash.

Im trying to avoid that altogether.

I might just go with a simple compression test. If the results are ok then..Im happy.