Compression 115 80 100 100 to low to run?

[Scout23]

Hey everyone,

Was wondering what you thought of compression numbers ranging from the front of the motor to the back as 115, 80, 100, 100? This is from an 86 Tercel with a 3AC. Would these numbers be two low to run? The vehicle doesn't smoke or didn't smoke when it was running and it was just a quick test after it had been sitting for a few months. The 80 pds would come up to 100 but took more cranking to get there. Head gasket was recently replaced and the head looked very good.

Thanks for all your help.

[dlb]

sounds to me like the engine should be pulled and re-ringed, likely the valves need attention too, or at least on the 2nd cylinder. i figure anything under 120 psi is on it's last legs. you might be able to keep driving it for a bit longer, but if you know the engine is that worn i think it is best to be proactive about it.

[Petros]

that will run fine, it will just be down on power. I had an all-trac that had lower pressures than that (80 to 90 PSI), it ran and started fine. You might try some Restore, Motor Honey, STP or similar additive to get a better seal as a temp measure. It should get a rering and valve ground. You did not get the valves reground when you had the head off? that might have helped.

Did you put some oil in the cylinders and try the compression test again? that will tell you if it is valves or rings, if it is mostly worn rings, the compression will come up quite a bit. If the valve seats are worn the compression will change only a little.

When you do a compression test btw, you are supposed to keep cranking until the gauge does not go up any more. Be it five revs of the crank, or ten or more, you keep cranking until the gauges stays at the same peak pressure. You might try that again, do it "dry" and than do it again with oil squirted into to the cyl to seal the rings. than get back with us.

Good luck

[keith]

The fact that is has sat for a couple of months would invalidate the compression test. I would expect a dramatic increase if you added a little oil to each cylinder, but I would not automatically condemn the rings for this. You need to start and run the engine for awhile, then remove all the plugs and block the throttle wide open. Then perform the compression test, and its six cranks, no more, no less. If the numbers are low from this test, then add some oil to the cylinders and repeat. If the numbers come up, you need rings, if not, then its valves.

I would check the valve lash before starting the engine, but if you are confident that it is OK, you can skip this step.

[Scout23]

For future reference it turns out the lower timing gear and woodruff key were broken and I believe sometimes turning as the should but not always. This made the timing jump and was the cause of all my problems. Put a new lower timing gear and a 5mm woodruff key sanded down just a touch for a tight fit and the car runs like a top now. Should have looked at it sooner just didn't think it was the timing belt or gear as the individual I bought the vehicle from had said he had recently changed it. Problem was he used a 5/16 woodruff key instead of a 5mm and it eventually broke. Surprised it ran at all. Hope I didn't damage anything tryng to get it running when the timing was off. The rings probably do need changed it blows a little black when first started but still runs fine with the new gear and belt.

Thanks for all the help everyone provided.