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Leak down test, compression test & Reading spark plugs

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1968SWBBigblock

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Discussion starter · #1 ·
Lots of questions that might all be intermingled to the same issue.
When I started my 454 for the 1st time this year a couple of weeks ago I had an intermitant back fire through the carb.
I could not pin point a rhytham to the back fires.
Started trouble shoot based upon guidence from others better suited than I.
1 - I drained the 6 month old gas - not pretty looking fuel - dark in color strong on odor. I had 1.5 gallons left in the fuel cell
2 - OHM checked all of the plug wires and coil wire _ I have one that a question - looks like it is high in resistivity depending upon how the plug connector is bent
3 - pulled the plugs - all plugs are close to the same color except for the back two on the passenger side - they look nearly new. The plugs are New NGK's but the same style I have used for years
4 - pulled the valve covers - it looks like all the springs are in good shape

So I want to run a compression check and a leak down test - I have run the compression checks in the past but have never done a leak down so I have a few questions
1 - can I run a compression check with the carb off?
2 - can I run a leak down with the carb off?
3 - does the motor really need to be warm to run a leak down test?
4 - any advice or guidence on doing the leak down test?

454 - .040 - 10.3-1 compression, sst valves, holley 770 SA carb, pertronix triple strike distributor, MSD plug wires, NGK UR5 plugs gapped at .045
Truck ran just fine when I put it away last fall.
I changed the plugs and fueld filter prior to firing it up for the 1st time this year.
Looking for any thoughts on any of the above items.
 
#2 ·
1968SWBBigblock said:
3 - pulled the plugs - all plugs are close to the same color except for the back two on the passenger side - they look nearly new. The plugs are New NGK's but the same style I have used for years
Click to expand...
Vacuum leak from the brake booster or other vacuum hose into those runners?

1968SWBBigblock said:
1 - can I run a compression check with the carb off?
Click to expand...
Yes. Won't hurt a bit. If you had the carb in place, you'd want to block the throttles open.
1968SWBBigblock said:
2 - can I run a leak down with the carb off?
Click to expand...
Yes. Listen at the plenum opening for air leakage noise, and don't drop anything into the plenum as you work.
1968SWBBigblock said:
3 - does the motor really need to be warm to run a leak down test?
Click to expand...
No. As soon as you start pumping compressed air into the cylinders, the piston and rings are going to start cooling off anyway.
1968SWBBigblock said:
4 - any advice or guidence on doing the leak down test?
Click to expand...
Do the test on a couple of KNOWN GOOD ENGINES before you test the engine that may have problems. Leakdown testers vary in construction, and the TESTER ITSELF has as much affect on the indicated leakage as the actual leakage in the cylinder. You need to know how much leakage is acceptable for the tester you're using.

When testing EACH CYLINDER,
Listen at the intake for intake valve leakage.
Listen at the exhaust for exhaust valve leakage
Listen at the oil fill cap for ring leakage--you'll always have some.
Listen/look at the radiator cap for air leakage into the cooling system.

ANY leakage into the exhaust, intake, or radiator is cause for concern.
 
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#3 ·
Using a compression tester to check the basic condition of an engine is better than nothing, but it is NOT the best choice. Compression test results can be inaccurate and inconsistent. This is because the condition of the battery, variations in ambient temperature, and the effects of the presence or lack of oil (if the engine has been sitting) around the rings which helps seal them, can all affect the results. This makes a compression test only a mediocre test at best.

Leakdown testing is by far the best method for checking an engine’s basic condition. It is done by checking each cylinder at TDC of the compression stroke. 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 leakage is what % of the 70 psi dynamic input pressure? And you punch into the calculator 65/.70 (don’t forget that its “point” 70 here) and the answer comes up 92.8 (you could round up to 92.9 if you wanted), 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

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

http://www.goodson.com/store/templa...store/template/product_detail.php?IID=4467&SID=8d21b88540d51ac02dd17bf9c8dfc641
 
#4 ·
i agree with everything posted above---- perfect data and well written to boot as usual..

BUT--- he can never leak down my nitrous motor---lol.. he would make me tear it down after its first 20 minutes of run time.... well no, im sure he wouldnt-- he just let out the part where nitrous motors run HUGE ring end gaps for big hits so they leak like the titanic....
 
#6 ·
Just look for equality more than anything, whether its compression or leakdown, amd not get too concerned about the actual #'s. As mentioned, too many variables to rely on the actual #.
 
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#7 ·
The important part of the leak-down tester is the size of the orifice between the supply side and the cylinder side. The larger the orifice the lower will be the "percentage" of leak-down.
Since there is no standard on the orifice size, the manufacturer decides which size to place in his particular device. I have seen various sizes from .040 to .060 recommended when constructing your own tester.
The safest way is to use the tester for comparison between cylinders and it is also nice to get to know your own leak-down tester by using it on various engines. You will get a feel for what the numbers are by experience.
Two different testers will more than likely come up with two completely sets of numbers from comparable tests on the same engine.
Squido
 
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#8 ·
1968SWBBigblock said:
Should I hear any noise at all during the test or should the cylinder take the air and hold it?
Click to expand...
Schurkey said:
When testing EACH CYLINDER,
Listen at the intake for intake valve leakage.
Listen at the exhaust for exhaust valve leakage
Listen at the oil fill cap for ring leakage--you'll always have some.
Listen/look at the radiator cap for air leakage into the cooling system.

ANY leakage into the exhaust, intake, or radiator is cause for concern.
Click to expand...


swcash said:
The important part of the leak-down tester is the size of the orifice between the supply side and the cylinder side. The larger the orifice the lower will be the "percentage" of leak-down.
Since there is no standard on the orifice size, the manufacturer decides which size to place in his particular device. I have seen various sizes from .040 to .060 recommended when constructing your own tester.
The safest way is to use the tester for comparison between cylinders and it is also nice to get to know your own leak-down tester by using it on various engines. You will get a feel for what the numbers are by experience.
Two different testers will more than likely come up with two completely sets of numbers from comparable tests on the same engine.
Squido
Click to expand...
True for most leakdown testers, and exactly what I was trying to suggest in my earlier post. As I learned from 540 Rat quite recently, the FAA has a specification for the orifice size and requires a specific tapered entry to the orifice; since NO standards exist for automotive leakdown testers, I'd suggest buying one that's compliant with FAA standards.

I'm making plans to disassemble MY leakdown tester to see what it has for an orifice, and to see if I can make it FAA compliant.
 
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Discussion starter · #9 ·
Guys thanks a ton for your help.
I finally finished up the leak down test tonight
All of the numbers look good all in the mid 90's.

FWIW - I pulled the Pertronix distrbutor cap off and was suprised at the wear and the plastic "bleeding" pattern on the inside of the cap.
I have about 2000 miles on this cap and I am a little suprised at the difference between this one and a new one - rotor looks good.
I will replace both of the sunday and start putting things back together.
A new set of plugs wire will also be put on.
 
#11 ·
1968SWBBigblock said:
Should I hear any noise at all during the test or should the cylinder take the air and hold it?
Click to expand...
Yes, anytime you do a leakdown test, at least with conventional rings, you will hear some air leakage. My 540 BBC with conventional rings, shows only an average of 3% COLD leakdown, yet I can still hear some air leakage hissing out of the breathers, from the ring end gaps, even for this small amount of leakdown.
 
#12 ·
swcash said:
Two different testers will more than likely come up with two completely (different) sets of numbers from comparable tests on the same engine.
Squido
Click to expand...
I tested 2 different high pressure leakdown testers, each with matching dual psi gauges, on my 540 BBC. The results were:

The Goodson high pressure leakdown tester set to 80 psi static input pressure = 3.5 % average leakdown. This is a very nice tester that I recommend.

The Summit high pressure leakdown tester set to 80 psi static input pressure = 3.0 % average leakdown. On this tester, the cylinder head fitting/hose combo would NOT seal air, so I had to use the Goodson cylinder head fitting/hose combo to do the test.

Obviously these 2 high pressure testers showed very consistent results.
 
#14 ·
540 RAT said:
Using a compression tester to check the basic condition of an engine is better than nothing, but it is NOT the best choice. Compression test results can be inaccurate and inconsistent. This is because the condition of the battery, variations in ambient temperature, and the effects of the presence or lack of oil (if the engine has been sitting) around the rings which helps seal them, can all affect the results. This makes a compression test only a mediocre test at best.

Leakdown testing is by far the best method for checking an engine’s basic condition. It is done by checking each cylinder at TDC of the compression stroke. 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 leakage is what % of the 70 psi dynamic input pressure? And you punch into the calculator 65/.70 (don’t forget that its “point” 70 here) and the answer comes up 92.8 (you could round up to 92.9 if you wanted), 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

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

http://www.goodson.com/store/templa...store/template/product_detail.php?IID=4467&SID=8d21b88540d51ac02dd17bf9c8dfc641
Click to expand...
===============

GREAT INFORMATIVE POST!

But link doesnt show testor,also looked for it on site and found many tools but not 1 leakdown testor.

So with that said please check link to a testor i found on ebay with dual gauges to let me knw if thats the dual type matching type you are reffering too.

Checked summit but thier 2 gauge leakdown testors either show % on 2nd guage or 3 color good /srv'cable/bad but no dual guage units that have both giages reading PSI only.

It has what looks like 2 same exact guages that both read-show PSI like you said to look for.

Heres the item # & link to a couple of them.

Item number in case links NG: 350538726058

http://www.ebay.com/itm/Engine-Cyli...726058?hash=item519dbc7aaa&item=350538726058&pt=Motors_Automotive_Tools&vxp=mtr

Item number to search in ebay in case links no good: 130660625357

http://www.ebay.com/itm/New-Speedwa...625357?hash=item1e6bfaebcd&item=130660625357&pt=Motors_Automotive_Tools&vxp=mtr

Thanks.....Scott
 
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#15 ·
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#16 ·
SWHEATON said:
Image
http://www.ebay.com/itm/Engine-Cylinder-Leakdown-Tester-Dual-Gauge-Kit-OTC-5609-/350538726058?hash=item519dbc7aaa&item=350538726058&pt=Motors_Automotive_Tools&vxp=mtr

Item number to search in ebay in case links no good: 130660625357

Image
http://www.ebay.com/itm/New-Speedway-Dual-Gauge-Leakdown-Tester-Use-w-100-PSI-/130660625357?hash=item1e6bfaebcd&item=130660625357&pt=Motors_Automotive_Tools&vxp=mtr

Thanks.....Scott
Click to expand...
I bought a batch of OTC stuff a couple of years ago. Genuine OTC labeling, not the imported "Stinger" line. Stuff I bought was trans or engine oil pressure tester, vacuum gauge, compression tester accessory, bigass prybar, remote starter, etc. Every bit of it was imported crap. All came from China except for the battery terminal brush which was from Taiwan or South Korea.

First Guess: Same with the OTC leakdown tester.

Second Guess: Same with the Speedway leakdown tester.
 
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Schurkey
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