Leakdown test

[Andrew]

Just did a leakdown test with a friend on his 440 GTX. Pulled rockers so all valves were seated. Results were pretty consistant, but up around 40%. The motor has very few miles on it. The leak seemed to be audible thru the crankcase (dipstick).

The reason for the test is the car tends to get hot and the vacuum is low- about 9". We tried to find a vacuum leak all the typical ways- spraying intake etc.. but no luck. so we did this test and don't know what to make of it.

Any ideas???????

 

[scotts74birds]

Read this. http://vmaxoutlaw.com/tech/leakdown_tester.htm Did you do things correctly? My very worn 383 had a leak-down of around 15-18% per cylinder. All through the crankcase, no valve or head leaks. Doing a leak-down test with just the valves closed, and the pistons in "just any" position will be misleading because max cylinder pressure is around, or just after TDC. There may be wear at the bottom of the cylinder, but by then, so much cylinder pressure has dissipated, that cylinder wear, or sealing, wont matter as much because most of the "work" has been done. Clear as mud?

 

[Andrew]

First- thanks for your reply.

I have 2 questions regarding your response.

1- the motor was hardly used and not run hard at all.

2- wouldn't at least one of the cyllinders be at or very close to top dead center even with the crank in a random position? I think the best reading we got was about 34%.

 

[Meep-Meep]

Yes, to do an accurate leak down test you must have the pistons at TDC. However 40% leakage is a lot, so in that case it might not matter where the piston is. I'd be wondering if I had a piston!!

When you say the motor was hardly used I assume that means fresh rebuild?? Rings will seat in 20 mins so I wonder if some rings got broken during assembly. It's not out of the question. Did you do a compression test?

 

[Redpolara]

I had to pull down a fresh engine (with issues) years ago, It was fresh alright but it had numerous broken rings and I think it was 6 out of 8 pistons had the ring gaps lined up.
I'm with Meep-Meep, try a compression test.

 

[Andrew]

Tried the compression test. Nothing substantial there. all cyllinders in 160 - 180 range. Maybe a bad guage? I don't think we did the leakdown test wrong, there were 3 of us doing the test and I'd like to think we had about the sense of one decent mechanic between the 3 of us??? maybe???

 

[Redpolara]

What cam is the engine running,is it installed straight up or adv/ret, hydraulic or solid> valve clearances. And what is the timing set at?
Aside from getting hot and having low vac,does the engine perform well?
I'm just looking at other possibilities as a process of elimination.

 

[Andrew]

cam is hyd flat somewhere under .500 lift. Not sure of other specs off hand. IT doesperform well, though he said it not too good off the line from what I remember. I just know we were side by side at about 50 mph one day, both jumped on it, and he was edging my coronet 440 6pack out and my car runs pretty well.

I know the heads and block were milled, compression in the cyllinders is around 175, and we adjusted the rockers to make sure the valves weren't being held open.

I'll try to find more out about the cam and timing.

 

[Meep-Meep]

Well, if the car runs strong and isn't blowing smoke like a 71 Vega then I'd say it's OK. You will always hear some hiss through the breather because nothing is 100% but as long as the leakage percentage is low. Also, 9" hg is not out of the question for a hot cam.

 

[Andrew]

Thanks all. Passed your info on to my friend.