302 compression test

[PBR]

i just bought a '77 early bronco for my wife and it's smokin' a ton so i am going to do a compression test on it. what should i be seeing per cylinder if it's good to go, and what am i going to see if it's rebuild time???

[richard cretsinger]

id be looking for anything above 115-120. i know my 302 is about 150ish but its fairly new. anything less with it smokin like it is id be tearing into it.

[Josh_K]

Also, use the average of the cylinders to tell you if you have one of two bad ones. If all the cylinders are in the 120 to 130 range and one of two are alot less like 50 or less, it will tell the tail.

Hint; Take the air cleaner off and open the carb and listen. I you hear air hissing you got a bad intake valve on that hole. Listen at the exhaust too for bad exhaust valves and at the case vent for cracked/worn rings.

It the standard thing to do with aircraft each year.

[scottm]

An engine with a weak cylinder on a compression or leakdown test will usually run rough too, especially on acceleration. Its also possible to run good and check ok on compression test, but still smoke. If it blows a cloud just when it first starts up, that could mean just valve seals. I recently did a valve job on a 4.3 l chevy v6 that ran good but smoked only at startup, and most of the problem was just petrified stem seals.

[PBR]

so i did the compression test and here are the results:
4 @ 75psi
1 @ 70psi
1 @ 85psi
1 @ 80psi
1 @ 65psi

now what??? run it and add oil??? rebuild it stock??? as usual no budget or i would buy a stroker crate motor and call it a day.

[scottm]

Wow, that is weak. Was the carb butterfly wide open?

[Ramsey_ElWardani]

With compression readings that low, I would question the compression gauge or the method used to test the compression. Readings that low would mean all cylinders have at least one burnt valve (highly unlikely) or that all cylinders have really bad rings or cylinder walls, in which case there would be a tremendous amount of blow-by and crankcase pressure with oil being blown all over the place. If the thing runs well otherwise, more than likely you smoking problem is valve stem seals and worn guides.

[Josh_K]

Hopfully Ramsey is right about the stem seals. But if the problem is the rings, I think your looking at a rebuild. You could just run it and add oil but if the engine has enough miles on it to be worn that bad, your risking something stupid like a timing chain to break or the oil pump to fail and cause a total junking of the engine instead of a simple rebuild with new pistons and a hand honing of the cylinders.

[Ramsey_ElWardani]

This might help

http://www.race-dezert.com/forum/showthread.php?t=17860&highlight=compression


Tony,

You are always going to have cylinder pressure leakage, even on a “new” cylinder. A compression test if properly done will identify a cylinder compression problem. A Leak Down test will help you isolate the problem’s cause.

Compression test should be done with all sparkplugs removed and throttle plate held open (for consistent vacuum) and the same number of compression strokes. I count them and make sure to use enough to let a normal cylinder build to full pressure. You can also perform a “Wet Test” where motor oil is placed in the cylinder to help seal the rings in a “bad hole” and if the pressure doesn't increases substantially you can assume that the leak is past a valve.

You can get more specific results with a Leak Down Test. In a Leak Down test a set regulated air pressure (say 100 psi) is introduced into the cylinder and the retained pressure in the cylinder is measured. If the retained pressure is 80 psi, then you have 20% Leak Down, it’s that simple. A good race motor with Total Seal type rings may have anywhere from 2% to 8% Leak Down, a production vehicle can have considerably more with standard rings and be in perfect condition.

When performing a Leak Down test it is important to test all the cylinders at the same point in their stroke (usually tested at TDC, but testing at various points in the stroke can identify cylinder wall damage at different the points) and with the valves totally closed (this is usually done by removing or loosening the rocker arms) and the engine recently run to clear any obstructions (corrosion) from the valves and seats.

The cool thing about a Leak Down test is that you can listen to where the air is escaping from to identify where the problem may be.

Bad rings/broken piston – listen to the oil filler cap, valve cover breather, oil dipstick tube.
Bad exhaust valve – listen to the exhaust pipe.
Bad intake valve – listen to the carburetor/throttle body.
Bad head gasket (cylinder to water jacket) – look for leakage into the cooling system.

In your case Tony, the leak that you are looking for is probably not related to cylinder pressure and none of these tests will find the problem. I’m guessing that your leak is between the water passage in the head and the oil system including the top of the head where the oil ends up before it runs back down into the pan. The test you need to perform is a cooling system pressure test, where the cooling system is pressurized (low pressure ~ 15psi) and loss of pressure indicates a problem. Look for the coolant leak while under pressure, and if one is not found externally it is probably in the head. Pull the valve cover and check the cylinders for coolant while performing the test.

Have fun!

[PBR]

thanks ramsey and others!!! i did not pull all the plugs out and the butterfly on the carb was closed during the test (i am not a motor guy). the truck has been sitting for about 10 years and besides the smoke it seems to run okay for a totally stock old 302 with ALL the smog equipment.

do you think i should just do a simple tune up on the motor (plugs, wires, cap, rotor, coil) and run it??? or don't spend the $$$ on it and just get it rebuilt???

[richard cretsinger]

i dont thing a tune up is going to solve those low compression readings. i know having the butterflys open all the way are not going to improve those readings alot. im definatly not an expert but i would pull the heads and check the cylinder walls for wear and piston side to side play. id also take a good look at your heads, the valves may not be seating correctly from carbon build up and just so many miles on them. i dont believe in any fix it in a bottle formulas or any thing like that, it either has to be right or its just a waste of time. just my opinion. good luck