compression test results: what do they mean?

[smbiorn]

Pulled the plugs. Here's what they look like:

compression results as follows:
1: 117.5
2: 92.5
3: 115
4: 94
The compression seems low in all four, but especially in 2 and 4. does anyone know what the manufacturer's specification is for engine compression? After that, I added a tablespoon or so of motor oil to each cylinder and got:
1: 247
2: 224
3: 214
4: 237.5

I also have terrible oil leaks, seemingly from everywhere.

What are your thoughts about next steps? rings? gaskets? what else?

[Curly]

The way cylinders 2 and 4 came up after adding oil, coupled with your statement about oil leaks everywhere, I'd suspect worn or broken rings contributing to the low compression and causing blow by with the associated oil leaks.
Next step is Leak Down test where you fill each cylinder in turn with compresssed air and by using a pressure gauge watch how quickly the pressure drops and listen via a stethoscope to trace whether the air is leaking past the valves, the head gasket or the rings.

[wikkid]

Could you outline how you performed the compression testing? Did you have it at WOT (wide open throttle)?

a) those are "out of spec" numbers: all low, and too much variation between cylinders.
b)the plugs look pretty healthy, form the photo.

Leakdown test for sure. Fortunately for me, I have never had to do one of those. Unfortunately for you, I can't therefore offer any tips.

[baltobernie]

WOT, yes. Also make sure the engine is warm, and all of the spark plugs have been removed, and battery is charged.

The psi numbers "with oil" are very high. Is it a stock engine? My high compression motor is in the 190's.

Those spark plugs look awfully dry to be suffering from bad rings. Does the car smoke under trailing throttle? (High engine vacuum condition with car in gear, throttle closed and engine providing braking).

[majicwrench]

Does car run well? What inspired you to run compressin test in first place?? I am not a fan of oil in cylinders, and it looks like you must have added a bunch.
Keith

[smbiorn]

So I have a stock (best I can tell) 1978 with 1756 cc weber carbureted motor. It runs pretty well and does not smoke at all, whether at startup or after warm or under trailing throttle. There is probably a timing adjustment that could be made to improve some very mild backfiring or sputtering that happens occasionally while driving but not during idle.

I did the compression test because of the copious oil leaks and because I one time saw bubbles in the coolant overflow reservoir; it overheated (redlined) at the same time and has neither overheated (redlined) nor had bubbles since then. These conditions led me to believe that a head gasket could be at issue as well as other gasket leaks. I read on here that a compression test could help determine whether a blown head gasket had occurred.

I have owned the car about a month and I'm no mechanic.

I did the compression test with a warm engine and fully charged battery, all plugs pulled, and WOT. Obviously, by the time I added oil to the cylinders for the second half of the compression test, the engine was significantly cooler than when I started.

Due to the dry plugs (and they seem about the same to my untrained eye), I too doubted that the rings are bad enough to allow oil into the cylinder. It is likely relevant that the air filter is wet and seems oily (like black drops of liquid on it). Also, it runs hotter than 190 on the gauge, say in the 220 range.

I probably did put in too much oil into the cylinders for the test. It smoked for several miles when I drove it to burn it off.

I guess I'll try a leak down test and see if I can tell why compression is so low. Thanks, and if any information I've added here helps in the diagnosis, please let me know your additional thoughts. Thanks!!

[majicwrench]

If car runs OK your compression test results are way off. A car with that low of compression--as per your results-- would NOT run well. WOuld not bother with leakdown test, not sure what you are chasing, but again, if car runs OK compression is probably fine. A compression test WILL show a dramaticly failed head gasket, but not a slight leak. Your temp gauge doing the 220 thing is what needs work, not the compression. Engine should run in the 190deg range give or take. Oil leaks, find em and fi em. Pretty much all these cars leak a little bit...helps with the rust underneath.
Keith

[burlybryan]

Concurring for the sake of concurring with Keith. Before worrying too much about compression (esp given the appearance of your plugs) I'd go over the coolant system. Bubbles (and the overheating) could be from a leaky hose or poorly bled system. Start there, clean the engine if possible and start tracking oil leaks.