I finnally got my leak down tester and want to check the bottom end of my engine (heads are fairly new).

What is the consensus of pressure loss that would be considered normal and what would be unacceptable?

My compressor probably delivers around 95 psi. The guidlines with the unit says that 5% is normal, 20% is too low. Reasonable? Also, how long should I wait?

Next question. If I find a low cylinder, would adding oil through the plug hole reduce the leak down rate much like a compression test? The reason that I ask is my third question.

When you get new heads, do you lap the valves or do you assume that it's done? I did the latter and now I'm second guessing myself.

 

Five percent is going to be very hard to achieve unless this is a brand new engine with precision machine work and file-fit rings. Closer to 10 percent would be a pretty good engine. Twenty percent is on the high side for what would still be a useable driver engine.

How long should you wait for what? If you mean how long do you keep pressure on the cylinder, it doesn't matter. Once pressure hits it, it's going to show a differential. The only reason to leave it hooked up longer is to determine where you have leaks.

Adding oil to a cylinder will only slow leakage through the rings. This practice is counter to the diagnostics the tester is trying to show you. If the rings are leaking, you want to know that, not mask it. That is a compression tester method. You test the compression and if it's low, you add a little oil in the cylinder. If the pressure comes up, then you have a good idea that it's going past the rings.

Lapping already lapped valve does nothing harmful. New heads or rebuilt heads would or should already have the valves lapped as a part of the valve job.

Edit: I wanted to add that be prepared to spend a good part of your day leaking down all eight. You'll probably do this only once for an engine in the car. It ain't a fun job and precision counts when you bring that piston to TDC.

 

This comment makes me wonder if I have the procedure correct.

I thought that you apply full pressure from the compressor, then turn the input off. You then read the differential between the input and the pressure remaining in the cylinder. My assumption was that unless you have a near perfect seal, the longer you wait, the lower the pressure will drop.

Also, this is not a new engine. It is an unmolested 90K L81 motor.

 

Depends on what kind of leakdown tester you have. A dual gauge unit allows you to compare the pressure differential as they exist. A single gauge unit (which I assume you have) is a hit it,release, and read. Yes, it will bleed down if you wait too long.

I would expect to see around 10psi on a well-maintained 90k engine. And that's not bad.

 

Doing a leak down test, particularly in conjunction with a compression test, will give you a great deal of additional information and allows problems to be pinpointed. The condition of the engine is determined by measuring the degree to which a cylinder with valves closed leaks air.

In simplified terms, if air is pumped into the cylinder at 100 PSI and the gauge reads 97 PSI, then the leak down percentage is 3%. That’s why it’s important to use a big enough CFM compressor to maintain 100 psi input no matter how bad the cylinder is. Doing a leak down test is a fairly simple matter. The tester is connected to an air source and is attached to the engine through the spark plug hole. There are two types of instruments, those with single and double gauges. I have the double gauge unit. The double gauge constantly measures the input pressure and the cylinder pressure. The single gauge instrument relies on checking the input pressure and then switching to measure the cylinder pressure. Each requires some adjustment to be accurate. The engine should be rotated so that the valves in the cylinder to be tested are closed and the piston is at top dead center. Tests should be conducted when the engine is warmed up.

Problems are pinpointed simply by determining where the air is leaking out of the cylinder. Air leaking out of the exhaust system (you can hear it in the exhaust pipe) indicates a problem with the exhaust valve. Air coming out of the carb indicates a bad intake valve or seat. Air going into the crankcase is leaking past the rings and does not indicate a problem if the percentage is low enough. A leak where the air is going into an adjacent cylinder or into the coolant indicates a blown head gasket or cracked head.

A brand new street engine might measure from 5% to 8% depending on the engine, manufacturer, and degree of break in. I checked my Ford 4X4 pickup with 120k miles and it tested recently measured 10% to 11% per cylinder. Although indicating some wear, the consistency between cylinders and the fact that all of the air was leaking past the rings into the crankcase indicated a reasonable street engine for daily driving that did not need any immediate work.

A new race engine should measure under 2%. My current 427 engine measured under 2% after my 600 mile easy breakin period. My 383 measured between 3% and 4%. After two years of hard running.

Using the leak down test, should allow you to quickly determine the basic condition of any engine. If you are not sure what the levels should be, differences in readings between cylinders is a key indication of a problem. After testing, you will know whether the top or bottom end really needs a rebuild.

 

Could you guys make some suggestions on where to find a leak down tester? I've sort of looked around with little success...the 2 guage sounds like it would work very nicely.

tia,
trw

 

I don't know where I got mine years ago. But this might work. I got to looking at it and it doesn't look like it has threaded ends to screw into the spark plug holes. which is good in one sense. Your not going to have to fiddle around trying to thread something with the pipes in the way. but my threaded attachment is also good when you want your hands free.

I used it to supply air when I was removing my valve springs and seals a couple of months ago.

http://www.harborfreight.com
4317-1VGA

US General
CYLINDER LEAKAGE TESTER $49.99

 

For some reason your link doesn't work on my computer, but I did find it:

http://www.harborfreight.com/cpi/cta...temnumber=4317

I might pick one up, $50 beats having to pay someone to do it.
thanks gkull, appreciate it.

trw

 

got mine at NAPA yrs ago

 

A leakdown tester has a very small orifice between the compressor guage and cylinder guage. It will only allow a small amount of air through. On a good engine, that wont matter because the cylinder side is sealed, so [si eqilibrates. On an old engine the flow through the orifice cant keep up with the loss by the rings.
FYI, if you apply 100psi to a cylinder, it has to be EXACTLY at TDC, otherwise the piston wil be pushed back down. You can cheat, if you want to finish quickly by lowering the pressure to 50-60psi, and it shouldn't force the piston down if it's close. Just be sure to be close enough to TDC so that both valves are closed.

 

Try this.

http://www.harborfreight.com

Then do a search.

I've also found that instead of finding TDC on every cylinder I've just loosen every rocker nut so ever valve is closed and gone straight down the line without ever rotating the motor.

The bores on an older motor will be more worn at @ 1/2 of the stroke because of years of ring friction.

My motors are never that old and I have to set the solid roller lifters anyway so i inspect the pushrod tips for chipping or ruffness.

 

[QUOTE=Garys 68]
A leakdown tester has a very small orifice between the compressor guage and cylinder guage. It will only allow a small amount of air through. On a good engine, that wont matter because the cylinder side is sealed, so [si eqilibrates.
QUOTE]

Ahhh. Now I get it.

Thanks all; now I understand.