Prpoer way to do compression test.

[Norton47]

What is the proper way to do a compression test.
I understand cold and warm, dry and wet.
But should all plugs be removed or only the one under test?
Thanks
I am also going to build a leak down tester.

 

[Geo Hahn]

I remove all the plugs... lets the engine spin faster for more meaningful results plus having the adjacent cylinders' plugs in place could disguise a head gasket leak into an adjacent cylinder.

You're going to pull all the plugs anyway so it's no more effort to do 'em all at the get-go.

P.S. I also remove the air cleaners and prop the carb pistons full up and prop the throttle linkage open just to be sure any intake valve leakage is recognized.

 

[jdubois]

All spark plugs out, throttle wide open, and fuel line disconnected/carb floats empty so you don't wash the cylinders down with gas.

 

[tdskip]

Throttle wide open is key, your reading will send you into cardiac arrest otherwise...

Ask me know I know that!

 

[MichaelG]

If you have a compressor this alternative should be considered. Really the only way to go in my opinion, and a better way to assess engine condition. If for example you have a bad valve, that cylinder would not hold the same pressure a "good" cylinder and you would be able to hear the hiss of air leaking past the valve. FYI, This compression test that is used on aircraft engines.


https://en.wikipedia.org/wiki/Leak-down_tester

https://www.xs11.com/tips/misc/misc3.shtml

 

[Norton47]

I had already found the second link and scrounge everything for it in the garage. I am making the orifice and was having a had time with .040 or #60 drill on hand. I have a #72 and a 1/16". The 1/16" most likely be the one I use based on the Wiki link.
I will post a picture when I am done.

 

[RonMacPherson]

This thread is starting to evolve.

In diagnosing internal engine conditions there is a "tree" type of growth of diagnostic procedures.

FIrst start with cranking rhythm.;
Ground the coil wire, or remove primary ignition wire.
Then crank the engine over for 10-15 seconds. Just as if you were trying to start it. Listen for any abnormalities in engine speed, does it get faster, then slower, or just a steady rhythm.(Lot easier(easier being that the Lucas was slower and so the variance would be more detectable) test with the older Lucas starters).

Then Open the throttle and repeat the test.

Start the car and let it run to operating temperatures. Shut it off, remove the plugs, jam the throttle open, ground the coil wire or remove the primary ignition. Battery fully charged, insert the compression tester(strongly recommend a screw in type) then crank the engine over for 8 seconds each hole. Watch the needle on the compression gauge and see how it rises. Does it jump up to pretty much full pressure first couple of revolutions, or does it go up only part way each revolution, building to highest pressure?

Then you perform a leakdown test. If needed. Compression gauge and vacuum gauge are invaluable diagnostic tools. Might take a few tries at using them before you learn the intricacies of what "clues" are being given you.

 

[KVH]

Why would it be helpful to have the throttle wide open?

 

[Dave Russell]

It's sort of "the air that can't get in can't be compressed".

On some engines, testing with the throttles closed can result in slightly lower compression readings. On engines with a fairly fast normal idle setting it may make no difference in the compression readings.

Since no two compression gages read the same & you are more trying to determine variations between cylinders than actual compression numbers, it doesn't really make much difference whether open or closed.

BTW, a little bit of trivia - the type of valve core in the compression gage can have a huge effect on how a particular gage reads. The refrigeration type valve cores will give far higher gage readings than will a tire valve core. Most compression gages are calibrated to work with the low pressure drop refrigeration type of cores.
D

 

[piman]

Hello all,

for those of you who are about to use a leak down tester for the first time, I can prepare you for what I learned the hard way.

Set the cylinder to be tested to TDC, insert the gauge then turn the air on. Unless you are very lucky and have absolute TDC the engine spins due to the air pressure and you need to start again. (Incidentally some large Diesel engines use compressed air in that manner to start them.)

So what I did was to put the car in gear, (Handbrake hard on) once I was ready to do the test.

Anyone else got a better idea to leep the engine in position?

Alec

 

[Brosky]

Let me add this to the leak test side.

If you use the sequential method of moving the engine to open and close valves for valve adjustment you can save yourself a lot of hassle. This requires removing the valve cover and following the manual, but will assure that the valves are completely closed in the cylinder that you want to do the leak test on.

I have attached the page from the TR6 manual with the correct sequence of which valves will be open to allow others to be completely closed.

 

[davidk]

Do the pistons have to be at TDC for a leak down test? If not, wouldn't it work to remove the valve cover, unload all the valves so that all of the valves are closed, and then test all cylinders?

 

[Bugeye58]

Yes, the piston being tested must be at TDC. If it's not, the air pressure introduced into the cylinder will rotate the engine.
And, you want the piston at TDC, as that's where it sees it's highest pressure during the combustion cycle.
Jeff

 

[TR3driver]

Maybe you guys are better at finding TDC for cylinders other than #1 than I am, then. When I was trying to use air pressure to hold the valves closed (installing valve seals with the head on the engine), I found it very difficult to hit TDC precisely enough that the engine would not turn; even with the gearbox in 4th and the handbrake set. After the car ran over my foot, I gave up and did it at BDC.