Compression / Leak Down Testing

[Tom Bond]

I've seen a number of posts on here recently asking about compression tests and leak down tests. Came along this web page with a pretty good explanation of it. I think this was for a Lotus or something but the idea and procedures are pretty much the same.

http://www.wcengineering.com/articles/leakdown.html

 

[Texas!]

The problem with a leakdown test is that you have to maintain the piston at TDC and the compressed air will be trying to push the piston down. We do leakdown tests on aircraft engines all the time but it's easy to keep the piston at TDC- just hold onto the propeller. What are you going to hold onto in a tractor engine?

 

[Royse]

(quoted from post at 22:28:58 08/31/13) The problem with a leakdown test is that you have to maintain the piston at TDC and the compressed air will be trying to push the piston down. We do leakdown tests on aircraft engines all the time but it's easy to keep the piston at TDC- just hold onto the propeller. What are you going to hold onto in a tractor engine?

Stick a crank through the grill?
Sorry Alan, couldn't resist!

 

[Texas!]

That will only work if you have a tractor that has a hand crank AND the air is trying to push the piston down so that the crankshaft is turning backwards.
You may be able to hold a crankshaft that has a hex head bolt in the snout with a breaker bar and socket.

 

[Royse]

(quoted from post at 23:44:33 08/31/13) That will only work if you have a tractor that has a hand crank AND the air is trying to push the piston down so that the crankshaft is turning backwards.
You may be able to hold a crankshaft that has a hex head bolt in the snout with a breaker bar and socket.

You are, of course, correct. I was trying to be funny.
I am funny, but looks aren't everything!

 

[john in la]

This is why I always rely on a compression test to see if something is wrong. Once you find out there is a problem and the engine needs to be torn down a leak down test can be done with the rocker covers off and all the rockers loose. This will then pin point if you can stop at taking the head off or if you need to go into the bottom end and you can do the test at BDC cause the valves opening is out of the equation.

But then again I do not work in a shop setting where a estimate has to be given before the work starts.

In real life leak down test are for motor rebuilders to check out fresh rebuilds before installation or shipping.

 

[Geo-TH,In]

I only did one leak down on a 20 hp kohler. Got it at tdc, didn't move om me. Must have been lucky.

It told me I didn't have a leaky head gasket, like everyone told me I had.

 

[36 Coupe]

Park it near a tree and put it in gear.Whats happened to common sense?

 

[Texas!]

Yes, I saw the humor in your response (unlike someone saw in my response, which has now been deleted, to a post referencing a gay) and I had to think about why the hand crank won't work.

 

[Hobo,NC]

leave quickly and politely and never return. They do not have the proper diagnostics knowledge to be working on your engine.

Looks like you are weeding out the ones that you should not take your engine to... The value of knowing were the leakage is at is priceless its well worth the time to check for what you can not see....

As for the excuse that its hard to keep a engine from turning while performing a leakage test I will admit it can be difficult on some engines... I regulate my air to 100 this guy said 80 I can live with that... If you do get a piston at TDC dead nutz its impossible to move that piston with compressed air,,,,Y'all think about that while Y'all are making excuses for what you don't know....

Its only needed when a compression test confirms poor numbers, its the next step in the process to confirm what you can not see... It takes the guess work out of the equation 8)

Its a simple test that can be done at the spark plug hole no disassemble required on most engines. I guess a Lotus is a bear and you have a customer with deep pockets 8) You can take out Difficult to perform on most every thing else...

Y'all tell me, had you rather pay me to go after what I have confirmed are pay me for my best guess... The best guess normally wins from a DIY guy he's lazy and looking for a silver bullet so will take the advice from other lazy DIY'ers as his silver bullet..

Most of the Time you are going after one cylinder your focus is on one cylinder is wasting 10/20/30 min. to confirm what you can not see a waist of time. Is it a waist of time to have knowledge you need to know if you have to tear it down so you can check the mechanical parts of that system to confirm its not a mechanical fraught... It can be like looking for a needle in a hay stack I,,, If you have 8 stacks I narrator it down to one stack theirs no need to focus on all of them if the needle is in only one of them,,, a leak down test will guide you to your next step...

 

[Bern]

Put the tractor in gear and apply the parking brake. Works just fine unless you have a SOS.

 

[Bern]

Oops, I meant put the trans in HIGH gear. I wish this forum had an edit feature.

 

[Hobo,NC]

(quoted from post at 00:04:24 09/02/13) Oops, I meant put the trans in HIGH gear. I wish this forum had an edit feature.

It does on the modern side...

 

[NoNewParts]

I sorta leakdown test every tractor I buy
I don't have the gauge kit, just plug my air line into the compression tester hose with the shrader? valve removed with the cylinder at TDC.
If it's in the shop, it already has the wheels blocked, and I just put it in gear for added peace of mind.
Never had a gas tractor 4 cyl engine turn though.
fill the radiator full up temporarily and look in there, stick my ear on the exhaust, intake, oil fill, etc

I check em all because the normal maintenance of adjusting the valves on a solid lifter engine seems to be neglected on most tractors. So first they get an adjustment, again, for my peace of mind, then a leak test. Especially those exhaust valves, where a faint pfft, pfft, when running can turn into a $ problem quickly.
and as I've posted, at the price level of the tractors I buy, None of them are worth machine shop work, so I try to head problems off early

 

[Hobo,NC]

I highly recommended you regulate the air pressure to no more than original compression readings are as I do no more than 100 PSI...


I run 180PSI in the shop I have blow a head gasket while replacing a valve spring. I had rebuilt the engine about 1 year prior... We could debate the gasket was ready to go anyways,,, it did turn a simple profitable job into a hard days work that I had to eat....

I am not looking for trouble passed 100 PSI all I need to know will show up at 100 psi are less :wink:

 

[Bern]

If shop air pressure blew a head gasket, then it was already hanging by a thread anyway. Pressure in the cylinder during the power stroke is a whole lot more than 180 PSI.

 

[Hobo,NC]

One's all I plan to eat in my life time... I don't plan to go looking for issues that can be pined on me...

 

[36 Coupe]

My old air system has an outlet with a regulator on it for painting.Aint hard to figure that some jobs dont need high pressurs.

 

[36 Coupe]

Until hydraulic lifters came out I checked valve clearance on every new Ford they sold where I worked.Found some tight and some loose.I always favored extra clearance on my valve jobs.

 

[Bern]

30 PSI won't find a pinhole in a cylinder wall, I can tell you that.

 

[Hobo,NC]

(quoted from post at 23:05:28 09/02/13) 30 PSI won't find a pinhole in a cylinder wall, I can tell you that.

I recon that would be a conversation for a different topic...