Advice wanted on the Proper leak down process - Chevelle Tech
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post #1 of 12 (permalink) Old Oct 5th, 16, 8:27 PM Thread Starter
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Advice wanted on the Proper leak down process

Ive leaked down fresh engines and hurt ones that are being used regularly etc .....but need to leak down a restored mustang that has been sitting for at least 5 yrs with only occasional start ups ........looking for advice .... I assume I should get it running and warmed up fully maybe even fresh plugs to ensured all cylinders running clean with no oil etcetc does that make sense ? can I wait for it to cool down to leak it or do it SAP hot??

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post #2 of 12 (permalink) Old Oct 6th, 16, 7:34 AM
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re: Advice wanted on the Proper leak down process

as long as it's still warm it'll be OK. I wouldn't burn myself or anything.
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post #3 of 12 (permalink) Old Oct 8th, 16, 3:35 AM
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re: Advice wanted on the Proper leak down process

Most leak down's I did in the shop were for stickin' valves on a cold engine so i would think it might depend on what your looking for. Though it would be bit lower cold I'd be more concerned with variance than the actual % but that's just me.
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post #4 of 12 (permalink) Old Oct 8th, 16, 11:21 AM Thread Starter
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re: Advice wanted on the Proper leak down process

Yes not concerned about the %number just looking to see if a ring seal problem or valve leak ..... on my own motor I do it cold as a condition ck ......... in this case was thinking doing it hot ? already know compression test shows 1 cyl on each bank down

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post #5 of 12 (permalink) Old Oct 8th, 16, 6:26 PM
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Re: Advice wanted on the Proper leak down process

I would do both a hot and cold test if you have the choice.
If you see a change in the numbers between the two, that might take the guess work out of wondering if it's valves or gasket or ring seal.

If you do your tests at TDC as I prefer to, make sure you remove the breaker bar that your're turning the engine over with before you pressurize. 100 psi in a 4.25" bore will generate 1500lbs of force on the piston, plus the leverage of a ~4" stroke, all in a split second.
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post #6 of 12 (permalink) Old Oct 11th, 16, 9:00 PM Thread Starter
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Re: Advice wanted on the Proper leak down process

289 Ford
Ended up doing it cold set tester at 90psi (at 95psi it rolled the engine over even with plugs in) no leaks at valves, no sticky valves
4 cylinders leaked pretty bad 40%+ you could hear it coming out the oil drain back holes in head rest of engine 15-20% ( had the valve covers off) saw no need to redo hot leak% might have better hot but IMO not enough to change the obvious problem

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post #7 of 12 (permalink) Old Oct 12th, 16, 5:10 AM
von
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Re: Advice wanted on the Proper leak down process

I don't know if it's entirely accurate but I always used 50 psi air pressure and doubled the % readings. I could never keep an engine from rolling over at 100 psi with piston at TDC. That's with the spark plugs in all remaining cylinders. Sometimes when a reading seems high % wise and a lot of air is going past the rings, bumping the crank just a hair causes the rings to seat in the bottom of the piston grooves and % reading drops a bunch.

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post #8 of 12 (permalink) Old Oct 12th, 16, 1:45 PM Thread Starter
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Re: Advice wanted on the Proper leak down process

Thanks for reply Von ...... that's why I try to leak down at 90 psi for sure any higher will roll most engines over as you say some even lower and I too adjust %

I never rocked the pistons on the bad cylinders as you say good idea !! I'm gonna redo those as the engine did not use oil or bad blowby like the bad cylinders would indicate so maybe my technique TKS

FWIW I'm using a borrowed Proform Tester ........ looking to buy a decent one of ,y own ........ what are you guys suggest a good one for a hobby user

Considering the OTC in the $$110-130.00- range ??

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post #9 of 12 (permalink) Old Oct 13th, 16, 2:38 AM
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Re: Advice wanted on the Proper leak down process

Quote:
Originally Posted by von View Post
I don't know if it's entirely accurate but I always used 50 psi air pressure and doubled the % readings. I could never keep an engine from rolling over at 100 psi with piston at TDC. That's with the spark plugs in all remaining cylinders. Sometimes when a reading seems high % wise and a lot of air is going past the rings, bumping the crank just a hair causes the rings to seat in the bottom of the piston grooves and % reading drops a bunch.
Did you use a piston stop to double check your TDC? Maybe your mark was off a touch.
I've never had a problem with the engine rotating and have always used 100psi.
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post #10 of 12 (permalink) Old Oct 13th, 16, 3:36 AM
von
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Re: Advice wanted on the Proper leak down process

Quote:
Originally Posted by BlackoutSteve View Post
Did you use a piston stop to double check your TDC? Maybe your mark was off a touch.
I've never had a problem with the engine rotating and have always used 100psi.
Yes TDC was established with a piston stop and degree wheel.

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post #11 of 12 (permalink) Old Oct 13th, 16, 11:45 PM
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Re: Advice wanted on the Proper leak down process

How would you hurt and engine when you are doing a leak down test?

Are you blowing out the valve seal?
Are you using too high a Pressure in the cylinder?
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post #12 of 12 (permalink) Old Oct 14th, 16, 2:56 PM
 
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Re: Advice wanted on the Proper leak down process

Leakdown testing is by far the BEST METHOD for checking an engine’s basic condition. It is done by checking each cylinder at exactly TDC of the compression stroke so it won't roll the engine over. And any leakage heard, helps to pinpoint where any problems are located. Air leaking out of the carb indicates a leaking intake valve. Air leaking out of the exhaust system indicates a leaking exhaust valve. Air leaking out of a breather indicates ring leakage. And air leaking out of the radiator cap opening indicates a leaking head gasket.

I’ve tested the 3 different types of leakdown testers.

One is a single gauge tester that reads leakdown percentage directly. This one is NOT recommended because its accuracy is typically not the best.

Another one is a dual gauge “low input pressure (typically around 35 psi or less, depending on the particular unit)” type that has one psi gauge and one gauge face that shows leakdown percentage directly. These are usually fairly inexpensive, and are also NOT recommended because of their typical inaccuracy.

And the last type is a matching dual psi gauge “high input pressure (usually can go up to 100 psi)” type. This type is convenient to use, and has good accuracy, making it clearly the best of the 3 leakdown tester types. So, if you decide to get a leakdown tester, do yourself a favor and get this type.

Note: Input pressure can be referred to in two ways, static and dynamic. Static means you set the regulator to the desired input pressure, say 80 psi (more on that below) with the tester NOT connected to the engine yet.

Then once you do connect the tester to the engine, the pressure will drop somewhat, becoming dynamic input pressure. You can then readjust the regulator to bring that dynamic input pressure back up to the original 80 psi, if you want. But I’ve found no difference at all in the final leakdown percentage results between doing that, or just letting the pressure drop somewhat and leaving it there. So, the most convenient method is to simply set the static input pressure to 80 psi and simply leave it.

The way to get to the final answer for a given test is:

For example, after you connect the 80 psi static pressurized tester to the engine, the left side regulator controlled gauge may say something like 70 psi after it drops, while the right side engine leakage gauge may say something like 65 psi.

You just plug a few numbers into your calculator, in the following manner:

You ask yourself, 65 psi on the leakage gauge is what % of the 70 psi on the dynamic input pressure gauge? And you punch into the calculator 65/.70 (don’t forget that its “point” 70 here) and the answer comes up 92.8, which means that the right side leakage gauge is showing or holding 92.8% as much as the left side input gauge. And because the original 70 psi dynamic input pressure was 100% of the dynamic input pressure, you simply punch into your calculator 100 – 92.8 = 7.2% leakage in that cylinder, which is your final accurate answer for that cylinder. That’s all there is to it.

For those who don’t use much math, that may seem like too much trouble. But if you read through what was done a couple of times, and then actually do it a couple of times, you’ll see that it’s no big deal at all. And you’ll be crunching the numbers freely after the first couple of cylinders.

There is no universally accepted input pressure for automotive leakdown testing. But the FAA (Federal Aviation Administration) has established 80 psi input pressure as their standard for leakdown testing on piston aircraft engines. And they allow up to 25% leakdown in those aircraft engines.

That 80 psi input pressure works perfectly fine for car engines too, so I use that as my input pressure as well.

And the reference chart I use for COLD leakdown testing on High Performance Engines is:

0-10 % = good condition

10-15% = though not ideal, still acceptable

over 15% = tear down and repair recommended for optimum performance

(for non-performance daily driver/grocery getter type vehicles, over 30% = tear down and repair recommended)

As a point of reference, my 540ci BBC Street/Strip engine shows a COLD leakdown of about 3%, using conventional Speed Pro rings, with a top ring end gap of .021” and a second ring end gap of .027”. And keep in mind that anytime you do a leakdown test, at least with conventional rings, you will hear some air leakage. Even for the small amount of leakdown that my engine shows, I can still hear some air leakage hissing out of the breathers, from the ring end gaps.

Here’s the excellent leakdown tester that I use and like real well. It’s from “Goodson Tools and Supplies for Engine Builders”, and costs about $100.00.

http://www.goodson.com/Dual-Gauge_Leak-Down_Tester/

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