Leak down test. School me.

[bocco]

So I tried a leak down test yesterday to see if I could gauge the condition of my motor. Watched a bunch of videos and read a few online articles. I ended up with a lot of questions.

The motor is a 1997 500HP carbed gen 6. It has about 330 hrs on it. Still has the wire seals. It runs great and pulls strong right up to the rev limiter. My only reasons for doing the leak down test id that I read lot of stories about valve spring and valve issues with these motors. Also I just wanted to learn how so I bought a tester from summit racing. Both gauges on this tester read PSI so you get to do some quick math but it's easy.

The process I attempted was to warm up the motor first. Then pull all plugs and test each cylinder at TDC starting with number one and then rotating 90 degrees and working through the firing order. I got numbers all over the place and I'm not sure if it's how I'm using the gauge or if the engine cooled off to much which seems hard to avoid. On the slightly low reading I could hear air at the intake. On the bad readings I could hear air out the valve cover. But these came later in the process and I'm not sure if it's temp related.

I've read different opinions on TDC vs BDC. Also whether to dial the left side gauge to the correct pressure then plug the gauge into the hose from the spark plug hole or hook all of the hoses and the adjust the pressure and take a reading.

Sorry for the long post.

 

[Runs2rch]

Simple definition.

An engine leakdown test is a compression test in reverse. Instead of measuring the engine's ability to create pressure, compressed air is introduced into the cylinder through the spark plug hole. ... The loss percentage will indicate the condition of the cylinder and overall condition of the engine.

What are your numbers 1-8?

 

[ONE-A-DAY]

Ive never done it on an engine with valves, only two strokes, those are always tested at TDC, it has to be in order to close off the ports.

 

[obnoxious001]

The fact that you can hear air is exactly why you do a leak down test instead of compression test. The sound of the air indicates your problem area. If you hear air in he valve covers (crankcase), means the rings are leaking. The engine does not need to be hot, but running prior helps to knock debris off valves. The intake leaking indicates you need a valve job, but if the other numbers are bad, the engine is likely tired.

What sort of percentages are you seeing on the "bad" ones?

 

[bocco]

The fact that you can hear air is exactly why you do a leak down test instead of compression test. The sound of the air indicates your problem area. If you hear air in he valve covers (crankcase), means the rings are leaking. The engine does not need to be hot, but running prior helps to knock debris off valves. The intake leaking indicates you need a valve job, but if the other numbers are bad, the engine is likely tired.

What sort of percentages are you seeing on the "bad" ones?

Unfortunately around 50%. My best one was close to 90%. Seems like a big spread so that's why I was thinking (OK hoping) operator error may have been involved.

 

[bocco]

Ive never done it on an engine with valves, only two strokes, those are always tested at TDC, it has to be in order to close off the ports.

Some texts recommend bottom dead center since it won't rotate the engine. TDC is easier to find and also makes sense as that is where the cylinder sees the most pressure.

 

[oldschool]

We've always done it at TDC and cold. Cold is cold.

 

[Instigator]

FYI
At BDC depending on the cam profile the exhaust valve may already be open.
I have always found it best to do a Leak down at TDC .
Assuming that your leak down tool has a Quick Coupler on the Cylinder test hose and it has a check valve on the manifold, I always adjust inlet pressure regulator to read 100 psi on the 1st gauge without the test hose to the cylinder connected to the tester manifold. The second gauge should be the same 100 psi as the test hose is not connected yet. This would represent 100 PSI of compression with zero % leakage shown on the 2nd gauge as the test hose is not connected yet. Then when you connect the test hose to the cylinder you can take a reading and what ever the 2nd gauge reads can be considered the % of leakage. For example, 100 psi inlet pressure and the second gauge reads 90 psi then you can assume 10% leakage. Simple system but make sure the engine doesn't rotate once you apply air to the cylinder during the test. If it does, you weren't on TDC.
You can listen to Exhaust and Intake for Valve leakage, valve cover breather for rings. You can also squirt some engine oil in the spark plug hole and retest if the rings are suspected to be leaking and a significant decrease in initial leak down will be noted upon retest until the compressed air blows the engine oil past the rings.
Hope that helps.
Here is the one that I use but truly you can actually make your own too.
https://www.matcotools.com/catalog/product/CLT200/cylinder-leakage-teste

 

[LOBOATOMY]

What pressure does your tester use? Some of the cheap ones use 10-20 lb as reference, and throw cheap gauges and they will show numbers all over the map. Remember with a 10 lb gauge and 2-3 lb diff it shows 30%. 3# at 50 as a base is 6%. Also a good regulator is a must for accuracy.
I modified mine to a good gauge on both ports and like to test at 50#. After testing a few you will know if you have a problem and what the nature is.
Good Luck

 

[bocco]

What pressure does your tester use? Some of the cheap ones use 10-20 lb as reference, and throw cheap gauges and they will show numbers all over the map. Remember with a 10 lb gauge and 2-3 lb diff it shows 30%. 3# at 50 as a base is 6%. Also a good regulator is a must for accuracy.
I modified mine to a good gauge on both ports and like to test at 50#. After testing a few you will know if you have a problem and what the nature is.
Good Luck

My gauges go to 100#. I was using 80 as a reference since my compressor doesn't quite get to 100. What I was noticing is that when I connect the hose from the cylinder my gauge drops to about 75#. Not sure if I should use 75 or 80 for my calculation.

 

[LOBOATOMY]

OK sounds like you have a good tool. If you are doing the exact same on all cylinders and you get a lot at 90%-80% and you have a hole at 50% then you need to, 1) recheck all and in particular that hole, and 2) If you are getting the same results (a cylinder way out of norm) pay attention to where the air is escaping. All holes will have loss, as long as they are reasonable and close to the same all ok. If you have one or two way out of the norm, recheck a couple of times and plan your remedy. Hopefully they will be "hissing" from either the intake (carb) or the exhaust. This would indicate a valve job. The valve covers hissing would mean the rings/pistons/cylinder walls. Hope this helps, I am just a poor backyard mechanic I am sure there are more knowledgeable peeps on here.

 

[farmo83]

This is really depressing as I have an HP 500 carb.

 

[obnoxious001]

Been sort of a long day here, forgot when I was posting earlier that if I have a bad cylinder, I use a deal blow hammer to tap the two rockers for that cylinder while the air is going. Sometimes you might dislodge a bit of rust or carbon that is on a valve seat. Makes a neat sound too, Blue Man Group might be able to use it.

 

[Racer56]

When you do the leak down test pull the valve covers. That allows you to easily verify that you don't have any spring pressure on the valves and you are on TDC of the cylinder you are testing.

 

[LOBOATOMY]

If the air is coming from the valve covers it is not the valves so "tapping" in the valve steam and/or pulling the valve cover to verify tdc will yield no effect. Air from the valve covers indicates ring seal problem.

 

[bocco]

When you do the leak down test pull the valve covers. That allows you to easily verify that you don't have any spring pressure on the valves and you are on TDC of the cylinder you are testing.

Unfortunately the exhaust manifolds have to come off in order to remove the valve covers. I know that at least 2 cylinders are leaking past the rings so rebuild is in my future. For this summer at least, I'm just gonna run it even though it's getting tired.

 

[farmo83]

Unfortunately the exhaust manifolds have to come off in order to remove the valve covers. I know that at least 2 cylinders are leaking past the rings so rebuild is in my future. For this summer at least, I'm just gonna run it even though it's getting tired.

This isn't true you can pull them without taking the manifolds of, I changed the lifters in my HP 500 this way 3 years ago. Although in hindsight it would have been easier just to pull them after them.

I guess I will leak mine down now.

 

[bocco]

This isn't true you can pull them without taking the manifolds of, I changed the lifters in my HP 500 this way 3 years ago. Although in hindsight it would have been easier just to pull them after them.

I guess I will leak mine down now.

What year is yours? How many hours.

 

[farmo83]

I have a 1996 Gen V.

As far as hours go I would guess around 350 +- 50, I don’t have an hour meter. The previous owner said it had around 200-250 when I bought it and I would guess I’ve put 100-150 on it. The previous owner had the springs changed right before I bought out so I lucked out there. I had to change lifters a few years ago, other then that it’s been a great engine(knock on wood). Pulls hard right to the rev limiter and doesn't use a drop of oil all season. It’s not getting any newer though. I’m hoping to get it rebuilt this upcoming winter and it won’t go back stock.

98% of it's time is spent between 2000-2500, there's too many surf boats to wind it up anymore then that.

 

[racerden]

So I tried a leak down test yesterday to see if I could gauge the condition of my motor. Watched a bunch of videos and read a few online articles. I ended up with a lot of questions.

The motor is a 1997 500HP carbed gen 6. It has about 330 hrs on it. Still has the wire seals. It runs great and pulls strong right up to the rev limiter. My only reasons for doing the leak down test id that I read lot of stories about valve spring and valve issues with these motors. Also I just wanted to learn how so I bought a tester from summit racing. Both gauges on this tester read PSI so you get to do some quick math but it's easy.

The process I attempted was to warm up the motor first. Then pull all plugs and test each cylinder at TDC starting with number one and then rotating 90 degrees and working through the firing order. I got numbers all over the place and I'm not sure if it's how I'm using the gauge or if the engine cooled off to much which seems hard to avoid. On the slightly low reading I could hear air at the intake. On the bad readings I could hear air out the valve cover. But these came later in the process and I'm not sure if it's temp related.

I've read different opinions on TDC vs BDC. Also whether to dial the left side gauge to the correct pressure then plug the gauge into the hose from the spark plug hole or hook all of the hoses and the adjust the pressure and take a reading.

Sorry for the long post.

Sure fire way to count out operator error is to relieve the valve spring pressures on all cylinders, hand crank it using a breaker bar until the the leak tester produces the least amount of leakage at or around TDC. It's a PITA but accuracy is highly enhanced not to mention the cylinder pressure kick-back(s) occurrences. Depending upon ring end gaps & cylinder tapers, percentages will vary but a round good number on a COLD engine (60F or so) will be around 10-12% or less. Be sure before relieving the valves to crank it over 30-40 revolutions to 'free' things up before you begin the process.

 

[Lavey 29]

Do vacuum gauge testing from a intake manifold source. Also blow by meters can tell how good ring seal is. Or do running compression testing.

Will ad that leak down and compression testing are good tests to do but they are NOT 100 percent to pin point internal engine issues. Neither one of them are.

 

[22hotkat]

Been there done that. It really needs a warm motor which is hard to do for a boat motor.

After years of tinking and going through the process the intelligent answer is

IF IT AINT BROKE DON'T FIX IT

Unless your looking for a project

 

[Lavey 29]

for leak downs I tape a very lite weight tissue to the exhaust side and then to the intake side / carb or throttle body and then another one to an unscrewed oil cap and or or a pulled dipstick. do the leak down and watch which tissue is moving as the air is blowing out. Oil cap / dip stick tissue is rings / pistons / cylinder walls ----------- carb or throttle body tissue is the intake side of the heads ------------------- exhaust tissue is the exhaust side of the heads.

 

[bocco]

Been there done that. It really needs a warm motor which is hard to do for a boat motor.

After years of tinking and going through the process the intelligent answer is

IF IT AINT BROKE DON'T FIX IT

Unless your looking for a project

I'm leaning this way. It just runs to good. I have used no oil all season and the oil still looks clean.

 

[2FORCEFULL]

Unfortunately around 50%. My best one was close to 90%. Seems like a big spread so that's why I was thinking (OK hoping) operator error may have been involved.

what did

I'm leaning this way. It just runs to good. I have used no oil all season and the oil still looks clean.

you'll be leaning over the side with a paddle to get back to the dock....these motor eat valve seats...and valves... pull the heads,,, put in the trash... get a good set of aluminum

 

[2FORCEFULL]

 

[2FORCEFULL]

those 500hp have blind hole head bolt holes in the block... the fix was they offer head bolt shims to get the correct torque....torqueing helps but the bolts bottom out...

 

[ibelio]

you were correct at what you described butt it also had water in cyl [look at rust ] possible from detination head gasket?

 

[bocco]

So why do these motors tulip valves more than other big blocks? Does the particular cam in these motors just beat the valves to death? My plugs don't indicate that it's running lean in any cylinders. Are these heads any different than the original square port heads that have been around since the early seventies?

What changes would you make during a rebuild to make one of these motors run 400 hours instead of 2 hundred.

Another question I've had. Is there any difference better or worse between the gen 5 and gen 6. Mine is a gen 6 for sure. It has the cast timing case cover.