Leak down test and a compression test

O

old31

Well-known member
I am interested in knowing what you think of this leak down test.

The other day I went for about an hour ride at the end of the ride the car really started running ratty. When I got home, I pulled all the plugs. Number one and two plugs were very fouled and oily. Number three and 4 had a nice tan color.

I am concerned as to what caused #one and two plugs to go bad.

Most of the cylinders I could hear air in the crankcase, not the exhaust, or carb or radiator.

Compression Leak Down
1 90 Lbs 32%
2 115Lbs 32%
3 110 Lbs 8%
4 110 Lbs 32%
 
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Could be quite a few things. Is your oil filler letting blow by gases out freely? Or is it clogged by a defective cap or metal pads jammed in it? Having pressurized crankcase is not good.

The rings could be sticking. The oil could need changing. The auto parts stores have "miracle" additives claiming to free sticky rings.

The distributor cam could be worn so that it is not opening normally for #1 and 2. Check to see if all four lobes are opening the points the same correct amount.

Look inside the distributer cap for carbon tracks running to the contacts for # 1 and 2.

The guides for the intake valves could be worn allowing oil to get sucked into the cylinders.

The two spark plugs could have had a colder heat range than the others.

The intake manifold runner for 1 and 2 could have a vacuum leak causing intermittent firing, not enough heat to burn the oil off.
 
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The compression readings and the leak down percentages don't make sense for me. For example if you have 32% leakage you should not have a higher compression reading than a cylinder with 8% leakage. More leakage = less compression and less leakage = more compression. Something is off here.
 
Flathead said:
The compression readings and the leak down percentages don't make sense for me. For example if you have 32% leakage you should not have a higher compression reading than a cylinder with 8% leakage. More leakage = less compression and less leakage = more compression. Something is off here.
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Same here! Check for good spark at all 4, all cylinders are high enough in lbs. to make the engine run well, #1 is a little weak.
 
in the old days we would put a shot of oil in the week cylinder. if the pressure went up it was suspected the rings were the issue. if it did not go up the valves could be the issue.

i would only focus on the low cyl.

and swap the plugs around and see if it changes how it runs. and then check them again.
 
I just redid the test and the low cylinder, #1 was were I heard the loudest air in going into the crankcase.

1: I will redew the test wet this time.

2: I will follow Philos idea and get some miracle gunk.

Also, what would be the next hottest plug over a Champion w18?
 
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Still too many unknowns for me, and I would hold-off on pushing out the pistons until you have more accurate information.

So allow me to muddy up the waters a bit. Whose brand of leak-down tester did you use? The cheap 'Chinesium' ones tend to give false readings. I have found that the same applies to Compression Gauges also. For example, elevation affects readings anyway, but a stock engine will normally be in the 55#-60# range. Most Snyder 6:1 Hi-comp Heads I have checked have been in the 80#-90# range. Again, those numbers will be less at higher elevations. Therefore, your numbers look off to me.

Plug fouling can be caused by ignition point gap being incorrect also. A worn Distributor's points block, shaft bushings, or Dist. Cam can cause irregular spark and or weak spark which would allow #1 & #2 to be showing excessively rich. Reading old spark plugs with lots of miles are only good at looking for uniformity. Outside of that, not much can be accurately determined.
 
If you can hear air in the crankcase from the compression test, your rings are bad or there’s major scoring of the cylinder walls or both.
 
AKJurnee said:
If you can hear air in the crankcase from the compression test, your rings are bad or there’s major scoring of the cylinder walls or both.
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Ummm, this is not very accurate information when it pertains to a Model-A engine getting a leak-down test, ...which is where I think he was saying it was heard. If you are testing a high-performance engine with either gap-less Rings, or tight end gaps, the leakage is less. On a big bore engine with0.020"+ end gaps, you will have air escaping past the rings. My experience has been that you will always hear air in the Crankcase, -even if it is an engine that has just been run-in and has sealed-up.
 
Perhaps stating the obvious, but when doing a leak down test on a cylinder you need to be sure that the valves are completely closed and the piston is at TDC compression stroke. TDC exhaust stroke can look like valves closed but depending on the cam grind they might not be. I find it easiest to set TDC #1 using the timing pin, then rotate half turn at a time through the firing order. Always visually confirm TDC visually through the spark plug hole.

And I agree with Brent - on a Model A with its ~0.015 (or so) ring gap you will almost always hear noise through the crankcase. I was taught the same as Old Ugly re: using oil during a compression test.
 
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It's easy to make errors when doing a leak down test I always use 100 PSI and then you're leaked down readings are the exact percentage typically.
 
Brent, the tester was a Harbor Freight cheapie.

With all cylinders, I heard a little air going into the cranckase, but #1 was far worse.

Jay, that is the way I did the test. Pin #1 and then rotate through each dist post 1 2 4 3 .
 
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Fullraceflathead said:
It's easy to make errors when doing a leak down test I always use 100 PSI and then you're leaked down readings are the exact percentage typically.
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I’ve found that 100 psi can turn a Model A engine over during the test. If you plan for that and put the car in gear be sure to set the brake and block the wheels. Or just use a lower pressure.
 
WOW! straight from a 90psi cylinder to a complete tear it down. that does not make sense to me.
i have had good running A's with less compression than that.

-put a another set of plugs into it and drive again
-you can test it when idling with propane at the intake manifold to make sure there is no intake gasket issues.
-do the oil test on that cyl. that will tell you if it is a ring problem or a valve problem. I have not seen this happen on a model A but what if a valve keeper came out? (I have seen that on double spring OHV. still ran but not well) broken spring.
-make sure all the external components are working properly first.
-then if it wont run properly. pull off the cyl head.

don't instantly pull it all down.
 
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Cylinders that are gas washed down from not firing right will be a lot more hissy than the nice firing ones.Checking things out thoroughly comes first,ripping and tearing comes last.Biggest one seems to be,pull the head.I wonder how many heads have been pulled for an ignition problem,or a water pump sucking air and putting a little air in the coolant.There are dozens and dozens of old farm tractors that I have worked on in the last 50 or so years that would have been condemned by a leakdown test.Those went right back out and kept on raking hay,mowing grass,plowing snow,grading driveways,hauling sap,twitching firewood,etc.
 
In my opinion, the leak-down test has little value. The compression test is more important. Oil can get into the cylinders via the valves guides, piston rings, damage to the pistons, scored cylinder walls & cracks in the block. So long as the compression is reasonable for the compression ratio of the cylinder head, & the gases exiting the oil breather pipe are not excessive, clean the fouled plugs, keep driving and enjoy the "A".
 
Bob Bidonde said:
In my opinion, the leak-down test has little value. The compression test is more important. Oil can get into the cylinders via the valves guides, piston rings, damage to the pistons, scored cylinder walls & cracks in the block. So long as the compression is reasonable for the compression ratio of the cylinder head, & the gases exiting the oil breather pipe are not excessive, clean the fouled plugs, keep driving and enjoy the "A".
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Actually, I think most engine builders would feel just the opposite of that Bob. Leaking an engine tells us if there is a slightly leaking intake or exhaust valve that would otherwise be masked by a spinning engine. It will also show us developing gasket or crack issues by injecting air into the cooling system or crankcase. Compression testing is often faulty because there is so many ways to trick or fool that method.
 
I have gone with a hotter plug, found an intake leak, and installed some magic oil.

Will keep you posted.
 
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