332-428 Ford FE Engine Forum-Cylinder leakdown test (2024)

I can sorta understand the principle, but I've never seen a leakdown tester. Compression testing is about as far as I've ever gone to check out ring sealing.

How much did the leakdown tester cost and where did you get it. It sounds like a potentially worthwhile tool, but I'm just not sure.

Shoe.

I think it's a cross between a compression tester (the portion that screws in to the spark plug hole) with something like an A/C recharge gauge cluster attached. Basically tells how much blowby there is in the cylinders (eg; with leaking or sticking valves or the rings)
That may not be correct but I think I'm close. Please set me straight if I am wrong. I could use the knowledge.

<dd>Neppy</dd>

1966 Galaxie 500, two door hardtop, 352/Cruise-O-Matic, Mooneyes chrome smoothies and baby moons.

I have just bought a leakdown tester and need a little advice on my results.
First of all, can anyone tell me what the leakdown of a 390 should be?
My results were cylinder #1-23%,#5-23%,#4-29%, #2-39%,#6-25%,#3-23%,#7-33%,#8-35%. It looks like #2,7, and #8 are a little high.
To achieve these results I rocked the crank back and forth slightly to help seal the rings. Is this acceptable or should the crank be left static? Rocking the crank reduced leakage around the rings by as much as 12%.
Also, Is there a rule of thumb for the amount of variance that can be allowed between the highest and lowest cylinders? Thanks

Anything over about 15% indicates problems happening. Can you hear where the air is coming out, if just the valve cover breathers it is the rings but listen at the exhaust & carb to check for potential valve problems.

I have just bought a leakdown tester and need a little advice on my results.
First of all, can anyone tell me what the leakdown of a 390 should be?
My results were cylinder #1-23%,#5-23%,#4-29%, #2-39%,#6-25%,#3-23%,#7-33%,#8-35%. It looks like #2,7, and #8 are a little high.
To achieve these results I rocked the crank back and forth slightly to help seal the rings. Is this acceptable or should the crank be left static? Rocking the crank reduced leakage around the rings by as much as 12%.
Also, Is there a rule of thumb for the amount of variance that can be allowed between the highest and lowest cylinders? Thanks

being an aircraft mechanic i know a little about leakdown tests. they are req every 100 hrs or so on piston powered airplanes. the tool cost about 50 bucks i think and is available through a company called ATS (aircraft tool supply). i'll see if i can get more info on them. the tool is just 2 press gages in line with a venturi inbetween the two. a certain amount of press is applied which reads on the first gage and the amount of press the cylinder can hold is shown on the second one. for aircraft engines we use a standard 80psi input press and can accept a cyl that holds 60 or more psi. 25% leakdown. if a cyl is marginal some people squirt oil into the cyl to bring it up ( i think thats cheating though) or rock the piston back and forth which sometimes will bring it up also. i cant remember if we had a spec for the allowable diff in press between cyl's but on a car engine i would say if they are within 10 or 15 psi of each other they are ok. its definitly worth having this tool as a comp tester doesnt tell you much. you can troubleshoot the engine with this. and dont forget to take the rachet/brkr bar off the crank before applying press because if the cyl is not right at tdc and it kicks that thing is gonna fly.

I have just bought a leakdown tester and need a little advice on my results.
First of all, can anyone tell me what the leakdown of a 390 should be?
My results were cylinder #1-23%,#5-23%,#4-29%, #2-39%,#6-25%,#3-23%,#7-33%,#8-35%. It looks like #2,7, and #8 are a little high.
To achieve these results I rocked the crank back and forth slightly to help seal the rings. Is this acceptable or should the crank be left static? Rocking the crank reduced leakage around the rings by as much as 12%.
Also, Is there a rule of thumb for the amount of variance that can be allowed between the highest and lowest cylinders? Thanks

The leakdown test will tell you more about an engines condition than any other tool.An engine is basically a air pump,so the better it seals the more air it will pump or efficiant power it will produce.On my racecar after 80 passes it started slowing down more and more.when fresh all cycl were 3-5% leakage at the rings.when the car went from 10.50s to high 80s and low 90s i leaked it it showed 29% and 32% in two holes.I could hear the air (i use a 1/2" hose down through the carb towards the port and one up the open headers.If you have cast ex manifolds a stethescope works on some so you dont have to remove the manifold.Anyway i had 2 exahaust valves last one on each head leakin.Freshened them up a mild surfacing and was back in the 50s again.The #s you gave are beyond what i call good.Did you have youre motor at running temp when you checked it? This is important for a proper % drops.I agree no more than 15% for a averaged mile street motor.10% or below ofcoarse would be great.Leakdowns can pinpoint a problem like no other test and show you exactly where the leakage lies. Be it rings-intake-ex valves or head gasket.Everyone should have one and leak youre motor every 6 months.I have a snap on model that i paid to much for but summit has them for 65.99 that work as good.

I have just bought a leakdown tester and need a little advice on my results.
First of all, can anyone tell me what the leakdown of a 390 should be?
My results were cylinder #1-23%,#5-23%,#4-29%, #2-39%,#6-25%,#3-23%,#7-33%,#8-35%. It looks like #2,7, and #8 are a little high.
To achieve these results I rocked the crank back and forth slightly to help seal the rings. Is this acceptable or should the crank be left static? Rocking the crank reduced leakage around the rings by as much as 12%.
Also, Is there a rule of thumb for the amount of variance that can be allowed between the highest and lowest cylinders? Thanks

N/M

I can sorta understand the principle, but I've never seen a leakdown tester. Compression testing is about as far as I've ever gone to check out ring sealing.

How much did the leakdown tester cost and where did you get it. It sounds like a potentially worthwhile tool, but I'm just not sure.

Shoe.

It is pretty much as Jeff Curry says. I got mine through the Eastwood Co. for $44.99(part #46049).

I have just bought a leakdown tester and need a little advice on my results.
First of all, can anyone tell me what the leakdown of a 390 should be?
My results were cylinder #1-23%,#5-23%,#4-29%, #2-39%,#6-25%,#3-23%,#7-33%,#8-35%. It looks like #2,7, and #8 are a little high.
To achieve these results I rocked the crank back and forth slightly to help seal the rings. Is this acceptable or should the crank be left static? Rocking the crank reduced leakage around the rings by as much as 12%.
Also, Is there a rule of thumb for the amount of variance that can be allowed between the highest and lowest cylinders? Thanks

The numbers you gave show to much leakdown. How much time does the engine have on it? If it is fairly fresh you have a problem. If it has some time on it its probably worn out and in need of a freshen or rebuild.
I use a Moroso tester that I bought 25 years ago and the price seems to have gone down since then. It is a must have one if you do any racing and nice to have if you don't.
I check my race engine every 20 or so passes or when the ETs start to drop. We also leakdown all the engines we run on the Dyno after break-in and prior to any pulls.
I like to see less than 10% or less +/- 2% variation but 15% isn't that uncommon in a good engine. Engine should be hot, don't rock cylinders, and don't oil.
I also know several guys that tune fuel-injection using a leakdown tester but I don't know the procedure as I haven't done it myself. I may learn as I have an Enderle set-up for an FE that I've haven't run yet.

I have just bought a leakdown tester and need a little advice on my results.
First of all, can anyone tell me what the leakdown of a 390 should be?
My results were cylinder #1-23%,#5-23%,#4-29%, #2-39%,#6-25%,#3-23%,#7-33%,#8-35%. It looks like #2,7, and #8 are a little high.
To achieve these results I rocked the crank back and forth slightly to help seal the rings. Is this acceptable or should the crank be left static? Rocking the crank reduced leakage around the rings by as much as 12%.
Also, Is there a rule of thumb for the amount of variance that can be allowed between the highest and lowest cylinders? Thanks

I see what the results should be but how much pressure do you put in the cylinder and how long to you wait before taking your results? I want to try this on a 428CJ and a 289 4V.

Roger

I have just bought a leakdown tester and need a little advice on my results.
First of all, can anyone tell me what the leakdown of a 390 should be?
My results were cylinder #1-23%,#5-23%,#4-29%, #2-39%,#6-25%,#3-23%,#7-33%,#8-35%. It looks like #2,7, and #8 are a little high.
To achieve these results I rocked the crank back and forth slightly to help seal the rings. Is this acceptable or should the crank be left static? Rocking the crank reduced leakage around the rings by as much as 12%.
Also, Is there a rule of thumb for the amount of variance that can be allowed between the highest and lowest cylinders? Thanks

http://www.briggs-racing.com/LEAKD.HTM
If you have a compression tester with a removable hose you could use the hose part of it.