Leak Down Test for 454's

[67hat34c]

Going to try leak down test this weekend, just bought the tester. any tips?

I was going to wrap masking tape around the crank pully and cut it at the timing mark, pull off and measure it and divide in 1/4's with marks. then put it back on so i can obtain TDS for all cyl. This should work right?

 

[BUSTER]

Exactly what is it you want to do?

 

[67hat34c]

just base line condition of both engines

 

[Avenger]

For baselining I'd just do a compression test. Leakdown is more of a pinpoint diagnostic tool for when you know you've got a bad cylinder and want to know why.

Of course, my experience is from the diesel world where compression is the holy grail.

 

[spartonboat1]

For baselining I'd just do a compression test. Leakdown is more of a pinpoint diagnostic tool for when you know you've got a bad cylinder and want to know why.

Of course, my experience is from the diesel world where compression is the holy grail.

The old school of gas engine compression testing was a) use a screw in tester with a flex extension (not the cheap press in by hand unit) , b) ensure it won't start, c) test both hot and cold, but maybe you can't start it and warm it up, d) run compression with WOT, e) look for significant differences between cyl's, which is a sign of blown gaskets, or f) if you have a burned valve (not too common today, I would think) you'll little to no compression at all. I would think you wouldn't need to roll it over more than 6-10 times per cyl.

My old Mercedes that was fairly high compression would show 160 #'s on a good day. So I would think you would be looking at the range from 140-160 #'s, similar in all cyl's. I most are the same +/-3-5 #'s, your good to go.

My 2cw...

 

[Akyla]

The leak down test will give you very good indicators of engine status and at the same time tell you where the weak spots are. You really dont need to go through the trouble with the pulley marking unless you want to. Just bring it up to tdc for # 1 and start there. Look at the dist from then on as you hand crank it to the next spot and the location of the rotor will tell you what cylinder is coming up to fire. Taking the rocker covers off and watching the valve positions will help you as well if you aren't sure.

Just remember that the air pressure can make the engine rotate if you don't have it exactly tdc, but it will only go one stroke. Also don't get too stressed out if you have indicators of blow by or valve leaks as even a very well built just reworked engine will show some loss.

 

[Maynard Rupp]

Here is how you do it. Cyl. #1 at TDC, firing stroke verified by rotor pointing to #1. Throttle wide open. Air to cylinder. Listen for leak near carburator, take oil fill cap off on one side and listen for hissing. Go to exhaust, I hope you have a swim platform, listen again for hissing. If rings are leaking, hissing is heard at oil fill cap. This is the common source of leaks and you need to decide how bad it is leaking there. The carb and exhaust hissing indicates valve leaking and this is not normal nor good. One last check is to remove the preassure cap from the coolant tank and look for bubbles. This indicates a blown head gasket. Now to proceed to the next cylinder find the next one in the firing order, install your test hose with no air. Rotate the engine in the correct direction with your thumb holding the air back. When the air stops coming and the rotor is pointing to the correct, and next cylinder position you are good to go with the test. Proceed through the rest. On airplanes we use a differential tester which is 2 gages. one shows the incoming preassure from the air source. That is adjusted to 80psi. The other gage reads the psi that your cylinder is holding. 70 to 75 is great 60 is marginal. Again you want to see that all cylinders produce similar results. I use that differential box on our HAt's Crusaders. I also do a cranking gage test.

 

[Beckytek]

Maynard, I think your test belongs in the FAQ section. Thanks for all the great information you provide. What would we do without you and Mike P, Buster, etc, etc ,etc. Ron ( Is this a great site or what?)

 

[MikeP]

I'm a big fan of leakdown tests and Maynard's info is spot-on. The one thing I would add is just a caution that was previously mentioned:

Typically folks rotate the engine by hand using a socket/breaker bar on the harmonic balancer bolt. BE VERY CAREFUL because IF the engine rotates when you apply the air AND the socket/handle is still attached to the bolt, it can cause a serious injury.

If you want to determine actual TDC, a degree wheel is the best way to do that and also ensure that TDC on the pulley mark/timing tab is actually TDC. You can now print out a degree wheel from the internet and paste it onto a stiff card rather than buying a nice wheel for $20-30!

http://www.tavia.com/free_degree_wheel.html

 

[Beckytek]

I printed out the degree wheel. Now how do you use it? Ron

 

[Maynard Rupp]

Put the zero mark,(the same as 360), on the notch on the harmonic balancer. The next cylinder in your firing order will have its piston @ TDC when you turn the harmonic balancer,(and the engine), 90%. So each 1/4 turn will bring the next one in the firing order up to TDC. So at 0%, 90%, 180% and 270% you will have the next appropriate cylinder at TDC. Now when you get back to zero, #6 cylinder will be at TDC and on its firing stroke. Of Course, your #1 piston is also at TDC, but it is not on its firing stroke. It takes 2 full revolutions for each cylinder to arrive at TDC of its firing stroke. Don't forget that to start this whole check you must be sure that #1 piston is at TDC, and on its firing stroke. The rotor must point to the position where the #1 plug wire is on the distributor cap. An easy way to be sure that #1 is coming up on its firing stroke is to put your finger in the plug hole and feel air rushing out as you turn the crankshaft. Air will only come out if that piston is coming up on its firing stroke. Sorry to make this kinda windy

 

[Beckytek]

Thanks Maynard for clearing that up for me. I'm putting that in my maintenance manual for future reference along with the compressed air test. I was at the boat today but it's still too cold to do anything. Ron

 

[Avenger]

Okay, I probably should just let this go, but we've been kicking the question around the shop this morning and have come to the same conclusion.

Per his post, 67hat34c is looking to do a leakdown test in order "baseline" his engines. A leakdown test is a lot of work, and does not necessarily tell all about the condition of the cylinder. It only tests ring/wall integrity at TDC. Any fault in the lower part of the cylinder will not be detected unless you can lock the crankshaft in a position further down the stroke. Again, a lot of work.

On the other hand, a compression test will tell you about the overall condition of the cylinder and is a lot easier to do. If you're only looking to establish a "baseline" we think the compression test will provide a more valid result with a lot less difficulty.

What am I missing here?

 

[Westfield 11]

Good point about the piston @ TDC covering the lower cylinder surface. But does not the leakdown test also tell you about the valve condition too? I always thought it was more of a diagnostic test than a baseline test ,i.e. there is already something wrong, but what is it, rings, intake valve, exhaust valve?

 

[GaryNW]

Sort of thinking along the lines of Scrod. My stbd 454 has about 1800 hrs. The port unit was long-blocked 800 hrs ago due to water ingestion (the PO!). The higher hr engine has a little lower oil pressue at idle, needs a quart of oil after 30 hrs run time, and turns the oil a lot blacker. But the "older" engine runs just as strong, starts just as easily and consumes the same amount of fuel per the flowscans. What more do I need to know? Oh, 32.7 kts @ 4000 rpm, full fuel and water, 600 lbs of fishermen.
Gary

 

[Avenger]

I always thought it was more of a diagnostic test than a baseline test ,i.e. there is already something wrong, but what is it, rings, intake valve, exhaust valve?

That's what I meant when I wrote:

For baselining I'd just do a compression test. Leakdown is more of a pinpoint diagnostic tool for when you know you've got a bad cylinder and want to know why.

From my perspective if you just want to put the numbers in your logbook for future reference, then good compression is all you need to know about.

But does not the leakdown test also tell you about the valve condition too?

Well, it does but, unless you've got a calibrated ear how are you going to tell where that extra 5% of leakage is coming from by sticking your head in the exhaust pipe, dipstick tube and intake?

I think our hi-performance friends are partial to leakdown tests on good engines because they're trying to squeeze the last pony out of every cylinder and need to know if they're balanced. In the production engine world if #3 has low compression the head is going to come off and a visual inspection will tell you what you took the head off to fix.

Gary, I've got the same situation. The PO replaced the stb engine and the port has an unknown number of hours on it since new in '82. They both make enough power to move the boat. One starts a little easier than the other. What do I do? Pull the port engine out and reman it so it matches the stbd? I don't think so.

 

[captddis]

I agree with Scrod a compression test is simple and a good indicator of engine health. If you find low compression, then no matter what the heads have to come off and then you can tell pretty quickly what the problem is. On most gas engines you can have the heads off in the time it takes for a leakdown test.

 

[yachtsmanbill]

I was not going to add my 2 cents to this however... I am a little confused on timing for the leakdown test.
Several years ago we changed the guide seals on the 454 mercs. We did this in place with air in the cylinders. Unless you can gaurantee that the cylinder is absolutely at TDC and the air will push down on the piston, OR lock the crank from turning, the air will run that slug down to BDC. We chose the latter with a full cylinder of air and changed all 16 seals in about 2 hours.
At BDC, both valves are still closed, PLUS you get the benefit of testing lower cylinder condition. At any rate, I agree that the leakdown test is worthless or a lot of extra work when a compression test will verify if all is within specs. If one or more is that low, you will need to do at least a valve job anyway.
Another nickel; on gas engines, MOST compression issues are from valves, and smoking is from guides/seals. Even on the Mercs when pulled for bottom end bearings and oil pump the rings were almost re-usable. ws

 

[Maynard Rupp]

You should use differential compression tester. 2 gages, a regulator, piping. Air goes to gage1, then through the preassure regulator, 2nd. gage, then the cylinder. We set the regulator to 80 psi. then read #2 gage, the difference between 80 psi and what the cylinder is holding is leakdown. The listening is to figure out why. Now you have a value, like 74/80. You do that for each cylinder and record it for future checking. 60/80 is for pretty tired engines and 76/80 is great. It is just another way of testing. We are required to test airplane engines that way. Also good because you would have to avoid the prop if you used an atomotive type gage and cranked the engine, OUCH.

 

[67hat34c]

could not get the tester to work properly at home, tried it on lawn mower that is only 1 season old. it would not measure the leak down. cheep meter from Harbor Freight tools. Taking it back.

I did compression check a few times in the last year and it is very good, see prior posts.

Have run sea foam several times through the carb and it doesnt seem to be bringing compression down. Still way higher than factory.

I have been chasing a loss of power for a while now. starboard engine runs about 200 less than port. Starboard got carburator rebuild, valve stem seals and general clean up. new plugs etc. 12 Degrees BTDC. Just checked plugs on 1357 since easy to get to . they are a nice brown color and even. I can find nothing wrong with this engine. nothing we have done has made any significant difference.

I find that it looses 50 RPM's more than starboard motor when you put it in gear.

Fuel burn seems very uneven. not sure of exact difference since gen pulls from port tank and we have to run it to keep Refer cold when we are off shore. However last time out port took 70 gallons and starboard took 77 gallons. 12 hr trip and gen most likely ran for 10 hrs at very light load so I am guessing it used 5 gallons or so. This would mean port engine used 65 gallons and starboard 77 gallons. To me this is a big difference. Flow scan also shows more gph for starboard side.

She is comming out in March for bottom and prop change, will have yard check allignment and cutlass wear.

Transmissions sound the same, Fluid is proper color, no visible wobble in shaft when you put it in gear.

only thing i can think of is there may be difference in prop pitch for some odd reason.

Here is another odd thing. plugs in port engine are one season old, pulled the 2468 since easy to get to and 2 was dark brown, others were dry black. smokes at idle, carb needs rebuild. Any how this engine is stronger than the one with new carb and all other work done. Again both engines have nearly identical compression and cylinders are within a pound or 2 even.

Wish I knew more about these things. Learning every time I work on it.

Planning on giving port engine (the good one) same job as the starboard one. carburator, pull intake, clean up any sludge, pull valve covers and clean up and sludge. change the valve stem seals, clean up lifters, push rods etc. bead blast intake and valve covers and paint them up. Etc.