Leak down test and a compression test
 

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%

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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?

Re: Leak down test and a compression test
 

Air in the crankcase indicates worn or broken rings.

Re: Leak down test and a compression test
 

I will stand with Jim Brierley on this one. You need to pull the pistons and check the rings.

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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 .

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

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".

Re: Leak down test and a compression test
 

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.

Re: Leak down test and a compression test
 

I have gone with a hotter plug, found an intake leak, and installed some magic oil.

Will keep you posted.

Re: Leak down test and a compression test
 

FYI

I ran a wet compression test on #1 and it only went up by 1 lb. I also checked the valve clearances on #1 and they were ok.

I should have it running again next week.

Re: Leak down test and a compression test
 

Having worked in aviation for over 40 years, I don't quite get the leak down numbers listed so I'll relate the method used in aviation. I use a differential pressure test rig which is a control manifold with two gauges, a pressure regulator and a balance orifice in the manifold. The cylinder to be tested is first set to top dead center on compression stroke with the input air fitting installed in the test cylinder's spark plug boss. I use a low enough input air pressure so that the crank can be turned while watching the pressure gauge for slight rises in pressure while bringing it up to TDC. At TDC the pressure reading stops increasing with movement. If it decreases with movement then the technician went too far. It takes a while to get used to getting a cylinder exactly at TDC so that the piston will not move as pressure is increased. Once at TDC, the technician brings the regulated input air slowly up to 80 PSI. Once at 80 PSI, the tech reads the differential gauge PSI. On a good cylinder, the differential pressure should give a reading of 65 psi or better. A good cylinder will read from 70 to 79 psi. If it reads 80 or more then the gauges should be calibrated since it can not read accurately over the input pressure reading of 80 PSI. If the readings are less than acceptable, the tech then listens at the intake, exhaust, and vent locations to hear where the leakage going. Hissing at a valve port indicates that the respective intake or exhaust valve is leaking. Hissing or building pressure at the vent tube is indicative of piston ring leakage.

All rings leak a little and that's normal. The rings move some in the piston grooves and can move enough to align ring gaps so one with ring leakage can be rechecked after a run with power being pulled to get them to move out of alignment. Squirting oil in the cylinder will generally increase the differential reading but usually not by more than 5 PSI or so but it does help to confirm ring leakage.

The intake or exhaust valves can have carbon on the valve seat which can be blown off by running the engine for a while and then rechecking the differential pressure. A person has to figure that an engine with a lot of carbon in the cylinders is likely starting to burn more oil whether its coming around the rings or valve guides. Flathead motors generally don't leak as much around valve stems but they can leak some when the clearance is worn a lot in there.

Re: Leak down test and a compression test
 

That's interesting, thanks.

Re: Leak down test and a compression test
 

Get or borrow a good boroscope and look into the cylinder. Scratches and grooves will be obvious.

BTW how much air is blowing out the oil fill when engine is running?

Re: Leak down test and a compression test
 

BK, I never noticed the blowby from the oil fill tube. I have a draft tube attached.

I actually have a good boroscope, had not thought of it.