How much compression should I be reading on a Chevy 3.1 engine? The three cylinders that I checked are reading 75 lbs.


75 PSI is way too low for any engine!

But here’s good rule of thumb. If any cylinder is more than 10% below the readings from the best cylinder reading, time to think about a new engine.



That’s about as bad as it gets. To determine if the problem is valve or ring related, squirt a small amount of oil in those cylinders one at a time and restest them.
If the readings take a significant jump upwards then it’s time for an engine overhaul.

A good engine should be producing (depending on mileage and other factors) a 150 on up.

Does this thing even run or barely run?


Two things will (other than a worn out engine) effect compression. The throttle must be held wide open while cranking and the elevation of the test site plays a considerable part. Also, the engine should be warmed up before doing the test. 90 psi is about the bare minimum for a reasonably healthy high mileage engine. Also, remove ALL the spark plugs, not just the one you are testing…


Before you start running out to buy a new engine, make sure your tester is calibrated correctly. I’ve seen them to be WAY off…at least some of the cheap ones. One I used was about 50lbs less then the actual preasure.


Was the throttle held open when you did the compression check?


No, I didn’t open the throttle. The car idels rough but runs like new and has plenty of power. I would think it’s possible that I just didn’t test it right. Also, I only removed the plug from the cylinder that I was testing. Thanks for your imput.

Why not tell more about the symptoms the engines has?


When you’re performing a compression test you should always remove all spark plugs and prop the throttle plate wide open.
The engine should be cranked over at least 5-6 revolutions when testing each cylinder.

Low compression, even on one bank, can cause an engine to idle rough and smooth out when the throttle is opened.

I would advise revisiting this process; performing the dry test and the wet test with oil if the readings are still low.
You could also connect a vacuum gauge to the intake manifold and see what the vacuum reading is. If you legitimately have low compression it will show up instantly on the vacuum gauge. Vacuum gauges are cheap and worth their weight in gold IMHO.

(Note. Sometimes the Schrader valves on the compression tester can act up and leak off a bit. This can give you an erroneous reading. The valve can be worn or even a tiny piece of grit can be blown into the tester and get wedged on the Schrader valve, which will cause it not to seat and give you a low, or no reading at all. If you suspect a faulty Schrader valve then get the valve designed for the tester. One out of a tire valve stem is not suitable.)


Throttle must be held wide open so there is something to compress…Remove all the plugs so the engine can spin freely. Year make model will get you better answers…