Coolant System Pressurized When Cold

Please describe how to do a "combustion chamber leak test".

 

Given a choice, consider one made to Federal Aviation Authority (FAA) specs.

There is no legal definition or "industry standard" for automotive leakdown testers. However, the FAA specifies the construction of leakdown testers for aircraft use. Going further, certain aircraft engine manufacturers also require a "Master Orifice" that can be used to verify the allowable leakdown rate of their engines. This is VERY handy especially for those who only use a leakdown tester occasionally, or are new to leakdown testing. The only caveat with the Master Orifice is that it's on the "generous" side, allowing more leakage than a performance automotive engine should have. I allow 1/2 to 2/3 the leakage of the Master Orifice on the vehicles I test.

Example:
https://www.aircraft-tool.com/Detail?id=2EM

You are specifically looking for a tester having a .040 testing orifice (this is not the Master Orifice, and the .060 testing orifice on some leakdown testers is for engines with a larger-than-five-inch bore, which excludes most automotive engines) and a cylinder adapter with 14mm threads. The tool above normally ships with an 18mm adapter, but they'll give you a 14mm instead if you request it. There are 10mm, 12mm, 14mm, 18mm, and long-reach adapters available.

 

The description in that link does not match the picture of the test equipment on that page.

They say:

But then they show a photo of a combustion-gas analyzer that apparently can detect ten different chemical compounds in the combustion gas.

In short...a really confusing and poorly-laid-out web page.

 

Good grief, back off the rockers to close the valves, put shop air into each cylinder one at a time and watch for a rising tide in the radiator. It's not rocket science.

 

So it's essentially a leakdown test using generally-higher air pressure and without the need for the instrumentation. You'd use additional time and effort to back off the rockers (and then re-setting them afterwards) instead of quickly and easily putting each cylinder at TDC-Compression before testing.

Depending on the leakdown tester and the air compressor in use, you'd go from 60--100 psi to 100--175 psi. A typical single-stage air compressor has a maximum pressure of about 125 psi, regulated downward to 60--100 psi by the leakdown tester. You're just using "tank pressure" without regulation.

The additional air pressure--IF the compressor can achieve higher pressure--would be useful. Everything else is pretty-much a wash.

The two tests are effectively the same other than the air pressure involved, and only then if the compressor is a high-pressure single-stage, or a typical two-stage. NOT a "big difference" after all.

 

Incorrect. A leak-down test as you've stated is done at lower pressures. Ripe for missing a head gasket issue. If you don't have the proper equipment, what good is performing a test using other equipment that may miss the very thing you're trying to discover?? Sounds kind of foolish to me. And you can also put each cylinder at TDC for a combustion chamber leak testing. If you can't remove valve covers and back off and then tighten (adjusting lash or preload possibly if the valve train is adjustable), then how do you expect someone to be able to remove a head and check a head gasket and then reassemble? Most of these folks are DIY. Most of these Buicks have non-adjustable shaft-mounted rockers.

Many newer engines w/ OHC's are all but impossible to easily perform a traditional combustion chamber leak test. What I've done for years and have passed on to my techs is to apply shop air to a cylinder, then rotate the engine with a big 1/2" drive ratchet, extension and socket on the crank bolt until the valves close and the air leakage stops. You will be on the compression stroke but not at TDC. You are essentially physically holding the engine against shop air for 15-20 seconds, while looking for the rise in coolant in the radiator or expansion tank. You could even do this with old OHC engines like V8 Buicks.

I got into with a machine shop guy over a 5.3L engine in his GM truck. It had a misfire at idle on one cylinder and he claimed he did a leak-down test on it and it was fine. It was less than 30% by far IIRC. The cylinder next to it leaked more but didn't misfire. I told him his leak down test missed a bent valve. He disagreed, and I said I could here the air leaking from the throttle body while I had it aired up with the valves backed-off. I asked if he wanted to talk about it more or did he want me to fix it? I pulled the head and it had a bent valve. I confirmed it first it by removing the intake and spraying soapy water on the back of the intake valve. I did some consulting with folks smarter with me and they said I would have found it by cutting the leak down input pressure to 50psi IIRC.

 

What is the peak pressure in the combustion chamber of a performance-oriented Buick Big Block, WFO/torque peak RPM?

The head gasket (along with the engine block and head castings and fasteners) has to withstand that (intermittent) pressure for as long as the throttle is open, every time it's open, for the life of the engine.

By comparison, the difference between regulated air pressure via a leakdown tester, (60--100 psi) or "tank pressure" of a two-stage air compressor (175 psi) is essentially meaningless. It would be possible, but so extremely unlikely that a leak would show at 175, but not show at 60--100 psi. (And even that assumes the "DIY" dude has a two-stage compressor. A small single-stage probably can't even maintain 125 psi once it's connected to the cylinder.)

My overall point is that while I recommended a leakdown test of the engine, you're saying that using "tank pressure"--in your words:

But the truth is there's hardly any difference at all.

 

You certainly are entitled to your opinion. There is not a lot of volume requirement in a cylinder at TDC compression stroke. There is not a lot of leakage even if ahead gasket is bad. I'd bet a pancake compressor would be OK but no matter the compressor, the last thing in the world I would do is choke-off pressure or volume that a traditional leak down tester does. There IS a big difference if accurate results are what you're after. You want to stack the deck in your favor as much as you can. My leak down tester is in the bottom drawer, near the back (I think). There is nothing worse than pulling a head thinking you have a head gasket issue w/o a specific cylinder to scrutinize or as a "guess" and seeing not anything. Heck, many modern engines use coated MLS gaskets and even if you know what hole you're after it's hard to see definitive prove sometimes.

Speaking from 38 years in the trenches and counting, the best way IMHO to find a bad head gasket (internal) is to put shop air into the cylinders.

 

Well, now from the files of could I have really been that stupid! The problem was the upper radiator hoses was on backwards! A couple people had suggested an airlock of some kind in the engine. That’s why the hose was pressurizing. So I put the new thermostat back in. I had disconnected the passenger side spark plug wires because they get in the way, so I’m hooking the wires back up after I hooked up the radiator hose and I’m like hold it. This hose is never in the way of the wires in the past??? Earlier in the week I had put the wires on before installing the new radiator so I failed to notice this! I started looking and realized I had the upper hose on backwards. The hose is apparently molded so that air gets out of the engine into the radiator. I took it for a ride and it was fine. Never went over 180°. Got back to the shop open the hood, radiator hose didn’t have a bit of pressure in it. Guess I’m going to Bowling Green after all.

 

Glad you found it, but I don’t understand how the upper hose on backwards would produce the symptoms you were having.

 

Me too - I need a picture!