Leak Down Test Results

[Pilotdave]

I'm now poking at the coolant loss in my 400-A and did a leak down test to see if the cause might be internal to the engine or a radiator problem. Got these results:

1. All 4 cylinders results are in the high moderate % loss (60-70%).
2. All 4 cylinders are leaking past the rings (air coming out oil fill tube) into the crankcase indicating leakage past the rings.
3. #3 cylinder is leaking air to #4; the reverse is not happening, suggesting a valve issue in #3?
4. No sign of leakage into the cooling system.

I think these results suggest these conclusions:

A. My coolant leak is a radiator problem, not an internal engine issue.
B. Might make sense to tear down the engine to investigate the high leakage rates and the valves for #3.

But being a novice at this sort of thing, I've got a couple of questions:

1. Given results 1.-4., do A and B make sense?

2. I've seen postings on other sites suggesting that anything worse than 15% is a cause for concern for a modern engine....but I don't know whether this applies to our engines. What % loss would be reasonable for a Model A engine in good repair?

Thanks for your help!
Dave

[100IH]

Dave, #1 says rings are poor. #2 indicates a blown head gasket. Forget the percent readings or not, its the relationship to the other readings that count. You'll find the worst and the least worst. Sometimes something like a blown head gasket in a cyl. will cover up a slightly leaking valve. Attend to the head gasket and while it's off, check valve lash and then consider doing the valves, or risk having to redo the head gasket to do the valves. With high ring leakage, maybe the rings or the bores need attention. A. you said it's the radiator, I'd look for cracks anyway while the head is off. B. yes on the valve and ring part.Consider this, If you test with higher pressure, all of the readings will show more leakage, I always test at 35# so its not a variable. Best to check all of the valve lashes before the leakage test so as not to condem a valve that is being held open. If air is passing from 3 to 4, then air must also pass from 4 to 3. That is a gasket problem.

[RawhideKid]

I would do another leak down test and this time, squirt some engine oil in each cylinder and crank the motor over a couple revs with the hand crank. I have noted way too many folk will raise the engine rpm slightly before turning the key off, and by doing so wash the cylinders down with gas erasing the oil that would normally be left in the cylinders, drying them out.

No accusations, just a bit of common sense.

Then post the results.

[100IH]

Rings are always going to leak some. What is the compression on 1 and 2 ? also when wet with a dab of oil. If the compression is fair:55-65 then that hiss at the crankcase on the test is a norm and not major problem.

[Pilotdave]

Thanks to you both.

My thought re: air leaking from #3 to #4 is that is could be a head gasket failure BUT why doesn't air leak from #4 to #3 when testing #4? On the other hand, perhaps one of the #3 valves is leaking air into either intake or exhaust manifold, thence to #4 whose valves will be open when #3 is at TDC. Is this nonsense or could it happen?

Thanks much.
Dave

[Patrick L.]

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I'm wondering how much pressure you're sticking in the cylinders. It doesn't need much.

You didn't mention any leakage from the carburetor or exhaust, only the filler spout. That pretty much tells me the valves are OK.

I'd still recommend a compression test and if cylinders are fairly even I wouldn't worry about it especially if it runs good. These don't really need much compression, mine is 45# in all 4 holes [ done in 1961]. Does this monster burn much oil ?

Its concerning that it leaks from 3 into 4 but not the other way. I suppose a gasket or a crack could do that, but,,, I don't think I'd pull the head just for that. Not until the end of the season anyway.

How much coolant is it losing ? It appears as though you can't find where its disappearing. It could just be going out the overflow. You aren't overfilling it are you ? Just cover the fins, no more. I think I'd recommend pressure testing the cooling system with very little pressure and have a good look.

[BRENT in 10-uh-C]

Originally Posted by Pilotdave

I'm now poking at the coolant loss in my 400-A and did a leak down test to see if the cause might be internal to the engine or a radiator problem. Got these results:

1. All 4 cylinders results are in the high moderate % loss (60-70%).
2. All 4 cylinders are leaking past the rings (air coming out oil fill tube) into the crankcase indicating leakage past the rings.
3. #3 cylinder is leaking air to #4; the reverse is not happening, suggesting a valve issue in #3?
4. No sign of leakage into the cooling system.

I think these results suggest these conclusions:

A. My coolant leak is a radiator problem, not an internal engine issue.
B. Might make sense to tear down the engine to investigate the high leakage rates and the valves for #3.

But being a novice at this sort of thing, I've got a couple of questions:

1. Given results 1.-4., do A and B make sense?

2. I've seen postings on other sites suggesting that anything worse than 15% is a cause for concern for a modern engine....but I don't know whether this applies to our engines. What % loss would be reasonable for a Model A engine in good repair?

Thanks for your help!
Dave

Dave, were all these readings taken with each piston at TDC?

[Pilotdave]

Thanks for the questions and suggestions. Some answers:

1. All readings were taken with pistons at TDC as indicated by the distributor rotor's alignment with the spark plug contacts inside the distributor cap, with the engine warm, throttle and choke wide open.

2. The only place air leaked out was the oil fill tube and at #4 when testing #3.

3. Calibrating the leak down tester required 100+ psi delivered to the tester, so that's what was applied to each cylinder.

4. The engine doesn't burn any oil.

5. I suspect that I'm losing coolant out the overflow when running at sustained high engine rpm. I've rigged an overflow tank to capture/measure the overflow loss, but I haven't had an opportunity to test it at high rpm.

6. I run the coolant just over the top of the tubes.

I'll do a compression test tomorrow and report those results.

[SeaSlugs]

its normal for an engine to make a air leaking noise past the rings eventually - rings arent 100% air tight - hence hand cranking you can hear each piston "squish" the air. Kinda like pulling a lawnmower rope very slowly you can feel the compression pressure eventually bleed out.

When you say 3 leaks to 4 - in what way? do you feel the air escaping out #4 spark plug hole? is it leaking out the intake or exhaust? since it doesnt leak from 4 to 3 id lean towards a valve issue in #3. Could be burnt, slightly bent from someone prying the head off, improperly gapped at the stem if thier adjustable(or adjuster wandered), messed up seat, etc.

If the car runs smooth and idles nicely with no loping or goofyness i wouldnt worry about #3 right yet.

Now when you say coolant loss - is it overheating (steaming out) or is it leaking, do you notice white vapor leaving your tailpipe?

do you have the rubber hose slipped over the end of the overflow to raise it up? ive seen a few radiators spring a pinhole leak on the bottom of the upper neck - can only leak when hot or when engine is running or frame is bouncing.

[Pilotdave]

SeaSlugs, thanks for your response. Some answers:

The engine eventually boils coolant out around the radiator cap and I have to add ~ a gallon of coolant. No white smoke out the tailpipe.

The leak from #3 into #4 comment is based on feeling air flow out of #4 spark plug hole and by inserting a listening tube into through the hole. I was thinking as you mention that there might be a valve issue in #3, though I didn't hear air leaking at the carb or tail pipe. That said, checking for an air leak at the tail pipe isn't the easiest thing and I may have missed it.

The engine runs/idles nicely.

I'm not clear on your question about "...rubber hose slipped over the end of the overflow tube...." The overflow tank I installed is connected to the lower end of the overflow tube using a plastic tube and a hose clamp.

I've seen no indication of a leak at the base of the upper neck, but that may be hard to spot.

Dave

[SeaSlugs]

is this radiator new or unknown? how rusty is the block? if its boiling out and not a leak (you did have the radiator cap off while doing the leak test correct and looked for bubbles?) id say your cooling system isnt up to snuff - granted your timing, carb, and use of advance settings are correct as they can make an engine run hot as well.

[Pilotdave]

It appears to be an original radiator.....condition unknown. I don't know how rusty the block might be. Yes, radiator cap was off while doing the test. Timing has been set several times, and I adjust the advance lever for driving conditions.

[Jacksonlll]

Your radiator is plugged enough to restrict flow, like most of our old radiators. Very common problem. Ok at lower speeds, but not at 45+ MPH.

[V4F]

radiator partially plugged

[SeaSlugs]

My thoughts too. Theres several posts about at home cleaning. Fyi generally its old grease .

[100IH]

Air will travel through either of the manifolds to reach adjacent cylinders. Exact location of the crankshaft is an imperative. Having the valve cover off and observing the point of overlap on the companion cylinder, is a way to find true TDC. Also, does your leakage tester have a diaphragm regularor after the 100psi. supply hose? If so. then it is most likely ok, BUT the crank must not turn when air is applied. If the manifold assembly was off of the engine there would be no question about what was leaking. Air leaking out of the exhaust or intake port on #3 could not travel to #4.

[Pilotdave]

Thanks for further replies!

I might be able to install a regulator on output side of the tester. Not sure if that would affect the readings but it would reduce the pressure applied to the cylinders. But in the short term, I will tackle the radiator cleaning project.

[RawhideKid]

Dave, the easiest way to get the cleanest radiator is to take it off, lay it face down with the cap on and fill with vinegar, let soak for 2 days, then dump the vinegar through a strainer and flush from bottom. The vinegar can be used again, and again. You can let this soak as long as you like, it will not harm the radiator.

[1crosscut]

Have you tested the actual temperature of the water to see what it is? Using an inexpensive meat thermometer to check the coolant temperature is a sure fire way to get a better information as to what is going on with your engine. I think that this is a tool every Model A owner should keep in their car/truck.

[Jacksonlll]

When you get the radiator out, you might try to measure the flow rate. about 36 GPM is good. Here is a video of the set up we use.
https://www.youtube.com/watch?v=93CJ...cz4R0&index=19