: How to air pressure test cylinder?



larryntripp
06-24-05, 08:58 AM
I've read in a number of posts, that you can use a spark plug body to serve as a an air pressure connection to test cylinders for a head gasket leak. But exactly how do you convert a spark plug to do this and how do you then get the air connection secure enough to apply enough pressure into the cylinder?

turbojimmy
06-24-05, 09:10 AM
You want to do a compression test. You can get a compression tester for about $20 at an auto parts store. One end has a fitting that goes into the spark plug hole. The other end has a gauge on it. It will tell you how much PSI is being generated on the compression stroke in the cylinder. If it's 0 then there's a hole somewhere - like in the head gasket or a piston. You want the cylinders to be relatively close to each other in compression. One that's significantly lower than the rest could point to a problem.

There is also a leakdown test where you actually do pressurize the cylinder with compressed air from a compressor. The tool is a lot more expensive but the test helps you better pinpoint a low-compression problem.

Here's a link that explains the difference between the tests:
http://www.lcengineering.com/TechNotes/TechNote47.htm

That page shows pretty high prices for the tools. Again, you can get a cheap compression tester for around $20-30. My leak-down tester like $80 or so I think.

Jim

lry99eldo
06-24-05, 09:13 AM
Go to a good NAPA store and tell them what you need to do, they will give you several options to help with performing your tests.
lry99eldo

Geno Castellano
06-24-05, 11:57 AM
A compression test wont tell you anything at all about a potential head gasket problem nor will it help diagnose or pinpoint which cylinder(s) might be affected by a head gasket. Actually, I'm not sure what a compression test does for you other than give a general indication that the valves are sealing up to some extent.

If you want to do an air pressure test of the cylinders it is actually pretty simple and is a VERY conclusive means of telling if there is a cylinder head gasket problem or not.

Find a spark plug that fits the engine in question. It can be anything as long as the threads are the correct size and length and the seat is a tapered seat or whatever seat the engine you have uses. A Northstar has a tapered seat. Look at the plug. See the rolled area at the top of the spark plug shell where the shell was rolled over to seal against the porcelean? Take the plug to a grinder and carefully grind away the rolled area at the top of the spark plug shell. Just remove it all the way around by grinding with the edge of the grinding wheel while you rotate the plug. Grind the ground electrode off the spark plug shell while you are there. Now, put the plug in a vise and drive the porcelean out from the chamber end of the plug. Sometimes you can wiggle the porcelean and just pull it out with your fingers. It is pretty easy to get out this way. Took be about 45 seconds to grind and remove the last one I did.

With the porcelean out, find a 1/8 PIPE tap. Run the tap thru the ID of the spark plug shell. Most of the time the hole left by the porcelean is very close to the correct diameter for the 1/8 pipe tap. If not, then run the correct drill thru it first. Screw a 6 inch 1/8 pipe nipple into the threads tightly. Put an air quick connect or appropriate fitting on the other end of the pipe nipple.

Remove the spark plugs in the engine. Turn the engine over using the accessory belt until the piston is at TDC with the valves closed. You can rotate the engine until you sense compression in the hole to find the compression stroke and then drop a small wooden dowel down the spark plug hole to feel the piston approach TDC. Screw your adapter into the spark plug port (it doesn't need to be that tight usually) and hook it up to shop air at 100 to 120 PSI. Take the cooling system pressure cap off and watch the coolant for bubbling. If the system does not show any signs of bubbling for 5 minutes then do the next cylinder... The head gasket is fine in that cylinder. If you see bubbling from the cooling system when the cylinder is pressurized then the head gasket is failing and leaking. Plain and simple. It is helpful to do this when the engine is cold and temporarily overfill the cooling system with coolant so that the coolant level is up at the cap fitting on the pressurized surge tank so that any bubbles are easy to see.

This test is simple, cheap, relatively easy and dead reliable in determining the status of the head gaskets. If you pressurize the cylinder when it's off TDC, the air pressure will cause the piston to move and rotate the crank. That is why it's helpful to have the cylinder exactly at TDC unless you have some other way of locking the engine so it will not turn.

turbojimmy
06-24-05, 01:20 PM
A compression test wont tell you anything at all about a potential head gasket problem nor will it help diagnose or pinpoint which cylinder(s) might be affected by a head gasket. Actually, I'm not sure what a compression test does for you other than give a general indication that the valves are sealing up to some extent.

If you want to do an air pressure test of the cylinders it is actually pretty simple and is a VERY conclusive means of telling if there is a cylinder head gasket problem or not.

Find a spark plug that fits the engine in question. It can be anything as long as the threads are the correct size and length and the seat is a tapered seat or whatever seat the engine you have uses. A Northstar has a tapered seat. Look at the plug. See the rolled area at the top of the spark plug shell where the shell was rolled over to seal against the porcelean? Take the plug to a grinder and carefully grind away the rolled area at the top of the spark plug shell. Just remove it all the way around by grinding with the edge of the grinding wheel while you rotate the plug. Grind the ground electrode off the spark plug shell while you are there. Now, put the plug in a vise and drive the porcelean out from the chamber end of the plug. Sometimes you can wiggle the porcelean and just pull it out with your fingers. It is pretty easy to get out this way. Took be about 45 seconds to grind and remove the last one I did.

With the porcelean out, find a 1/8 PIPE tap. Run the tap thru the ID of the spark plug shell. Most of the time the hole left by the porcelean is very close to the correct diameter for the 1/8 pipe tap. If not, then run the correct drill thru it first. Screw a 6 inch 1/8 pipe nipple into the threads tightly. Put an air quick connect or appropriate fitting on the other end of the pipe nipple.

Remove the spark plugs in the engine. Turn the engine over using the accessory belt until the piston is at TDC with the valves closed. You can rotate the engine until you sense compression in the hole to find the compression stroke and then drop a small wooden dowel down the spark plug hole to feel the piston approach TDC. Screw your adapter into the spark plug port (it doesn't need to be that tight usually) and hook it up to shop air at 100 to 120 PSI. Take the cooling system pressure cap off and watch the coolant for bubbling. If the system does not show any signs of bubbling for 5 minutes then do the next cylinder... The head gasket is fine in that cylinder. If you see bubbling from the cooling system when the cylinder is pressurized then the head gasket is failing and leaking. Plain and simple. It is helpful to do this when the engine is cold and temporarily overfill the cooling system with coolant so that the coolant level is up at the cap fitting on the pressurized surge tank so that any bubbles are easy to see.

This test is simple, cheap, relatively easy and dead reliable in determining the status of the head gaskets. If you pressurize the cylinder when it's off TDC, the air pressure will cause the piston to move and rotate the crank. That is why it's helpful to have the cylinder exactly at TDC unless you have some other way of locking the engine so it will not turn.

What you've described is a leakdown test without using a gauge to measure the leakdown. Rather than torture yourself by making a tool out of a sparkplug you can just buy one at NAPA or Autozone. One end has the sparkplug, the other has a fitting for the air compressor. I bought one for like $10.

EDIT: Here's a pic to help you find one:
http://www.turbojimmy.com/air_hose.jpg

I don't understand why a compression test won't help you identify a cylinder with a compression problem - like a leaky headgasket? If you've blown the gasket into the cooling system, why wouldn't it show up with a compression test? You need the leakdown test (or the method you described) to listen to where the air is going, but the compression test should narrow it down to a cylinder for you....wouldn't it? It does on every other car I've owned - what's different about the N*?

Jim

Ranger
06-24-05, 02:36 PM
The difference is, with a compression test you are only pressurizing the cylinder for a split second and even with a bad head gasket, it will still pressurize and probably about the same as the rest of the cylinders unless the gasket is completely gone. The leak down test on the other hand gives the air time to slowly leak through a likely very small breach in the head gasket and the bubbles time to find their way to the surge tank or radiator cap.

turbojimmy
06-24-05, 02:40 PM
The difference is, with a compression test you are only pressurizing the cylinder for a split second and even with a bad head gasket, it will still pressurize and probably about the same as the rest of the cylinders unless the gasket is completely gone. The leak down test on the other hand gives the air time to slowly leak through a likely very small breach in the head gasket and the bubbles time to find their way to the surge tank or radiator cap.

Gotchya. We turbo people are used to blowing the headgaskets in a big way. Big holes. A compression test finds them easily most of the time. If a compression test is inconclusive, or highlights a suspect cylinder, I follow it up with a leakdown test. A leakdown test is a pain in the butt compared to a compression test, which is why I usually start with a compression test.

Jim

Geno Castellano
06-24-05, 11:37 PM
Yes, if you have completely pushed the head gasket out or have a hole in the piston a compression test will pin point the problem. Otherwise, for small head gasket seepage (like most people that are talking about on this forum) a compression test is useless. The brief time interval of the pressure will just not show anything with a small leak.

We use the pressure test of the cylinders on all of our development engines all the time. Typically we will use nitrogen at much higher pressures (MUCH higher) but 120 PSI shop air will certainly do the trick.

Nice thing about the pressure test approach is that makes exhaust valve or intake valve leakage easy to spot also as you can hear the air hissing in the throttle body or exhaust pipe.

The air pressure and air flow rate is so low in a typical leak down test that it will not really work here. Certainly is a similar approach and similar mechanics but different philosophy.

lry99eldo
06-25-05, 11:01 AM
The NAPA suggestion was to address the "hardware" needed. NAPA has a wide selection of sparkplug adapters for pumping air into a cylinder, and usually easier to just purchase. I have used dual gage and single gage leakdown testers for years but mostly for cylinder seal % after a new build and during the racing season to determine efficency loss in order to predict a freshen up.
Either one of the two types of leakdown tester will provide the needed "ratio" data of loss but the single gauge unit is just easier to read, no calculation. If it's a sensory or tactile answer your looking for then all you need is the air adapter to sparkplug thread fixture. Last I saw the adapters, (3), were $12 or $15, with the 1/8 NPT already tapped in. The 1/8 NPT tap will cost you that much if not more, they're expensive for a one time use.
I'm not one that has any stored nitrogen around and most leak down testers are efficent at 80 to 120 psig, so a home garage shop usually has an air compressor that will provide steady air supply.
I'm sort of surprised at the amount of question I read in these forums about headgasket failure. A headgasket is not a simple device in design, it's quite complicated and it's asked to do a job that is really important. Not only in all the forces that it must contain but also in all the circumstances that suround it to keep it efficient. Yet there are just a few things that are pretty evident that will indicate a failure. Just remember that the "repair" is not simple though and that all those things that need to be in place to assist in helping a gasket perform are all in place when the repair is attempted. The easiest recommendation I have to offer is just thinking "sealing surfaces" and keep it CLEAN.
lry99eldo

Geno Castellano
06-25-05, 11:49 AM
Sorry. I didn't mean to imply that NAPA didn't have the hardware available to adapt for the test. Just that most people have never heard of that style of test nor what it can tell you. If you want to use a compression tester adapter or something that will work fine. And I would certainly not recommend at all a home or shop using nitrogen to run the test. It is possible to easily damage a perfectly good head gasket with the 1000 PSI available in a nitrogen bottle. The head gasket lives at those pressures and above because the time interval is so short during combustion but if that pressure is held for minutes steady at a time it can permanently damage the head gasket. That type of testing is just for engine development work and I only mentioned it to add credence to the concept of pressure testing the cylinder in general because basically that is the only test we use. Leak down and compression testing is not that effective except for specific circumstances.

The nice thing about the spark plug shell approach is that it is basically free. If you can find an old plug around and have a 1/8 pipe tap and some scrap pipe nipples and such you can make it for nothing and have that satisfaction of making a very useful tool.

The other problem with some of the compression tester adapters and such is that they are often short and/or on the end of a flex hose. Since the spark plug wells are so deep on DOHC engines and the Northstar it is more usefull to have an adapter with a 6 inch extension as mentioned to make it easy to screw the adapter into the spark plug port.

As discussed in the past the head gasket is very complicated as you mention. There is no single "failure mode" or "failure characteristic" of a head gasket so it is often hard to describe the failures or how to detect them. People think that if it is a head gasket then coolant gets in the oil or a compression test will find it, etcetera... Not true at all. A head gasket can fail and leak compression into the cooling system and yet the engine might never loose coolant. In other situations it can leak coolant into a chamber during shutdown, coolant "disappears" yet it seals combustion fine. In the case of catostrophic failures like in a turbo motor where the head gasket is completly blown out or torched both will occur quite noticeably. So a "head gasket failure" covers a myriad of possible failure modes and failure characteristics. As far as I know the only real good test that will catch ANY head gasket problems is the cylinder pressure test at 120 PSI. Holding it for 5 or 10 minutes and watching the coolant system for bubbles streaming through. Much like the final and sure way to find a leak in a tire is to submerge it in water and find the bubbles. :)

lry99eldo
06-25-05, 05:37 PM
Yes, indeed, there is a myriad of conditions that can and will cause headgasket failure. Yet in the spirit of DIY'ers, the road to discovering that a headgasket has failed has a fairly small list of home type diagnostics with the results easily seen or discovered. I dislike the word obvious so I refrain it's use.
At home headgasket repair is complicated all the same and cudos to all that attack it and complete it. For some like me that have taken years to equip themselves with tools and the skills it's still a tough job. These N* engines are beauties and I personally never want to see the inside of mine really, LOL.
So keep on keepin on all you Caddy DIY'ers, it's fun to see what those crazy engineers have laid out in front of us, just not on my car, please!!!!
lry99eldo

JbellELDO
11-21-05, 06:06 PM
Geno Castellano- I need to make the spark plug adapter. Can you give me some detailed instructions on how to do this, I have no idea what a pipe nipple or a 1/8 pipe tap are...Anything you have will help! Thanks!!


-Jon

chevelle
11-23-05, 01:25 AM
Find an old spark plug that fits your engine. Take it to a bench grinder and grind the rolled land off the top of the spark plug shell. That is the steel part of the spark plug where it is rolled to seal to the porcelean part of the plug insulator. Just use the edge of the grinding wheel and slowly turn the plug to grind away the entire rolled land all the way around. When it is gone you can put the spark plug in a vise and drive the porcelean out of the plug shell leaving just the shell.

A 1/8 pipe tap is a tap used to cut threads in a hole. It will leave 1/8 inch pipe threads. Use the tap to thread the ID of the spark plug shell. If you go to most any decent hardware store or autoparts store or tool store they will have a "1/8 inch pipe tap."

A pipe nipple is a piece of pipe that is threaded on both ends. You want a piece of 1/8 inch pipe about 6 inches long that is threaded on both ends. Any hardware store should have this.

Screw the pipe nipple into the threads you cut in the spark plug shell using loctite to secure it. Put a quick connect fitting on the other end of the pipe nipple or fashion something to hook the shop air hose to . Screw the plug shell into the cylinder head spark plug port . The pipe nipple is long enough that it will be sticking up above the cam cover. Hook the air hose to it to pressurize the cylinder.

The piston in the cylinder must be at the very top when you do this. To find TDC use a 12 inch piece of wooden dowel. Take the plug out, slide the dowel down into the cylinder until it touches the piston. Slowly turn the engine over using the bolt on the end of the crank. The dowel sticking in the cylinder will rise as the piston rises in the cylinder. When it stops moving the piston is at TDC. If you apply air and it leaks out of a valve then the engine is at TDC exhaust and you need to turn the crank 360 degrees and find TDC again. Then all the valves will be closed so you can pressure test the cylinder.