While trying to diagnose why my engines running rough, I compression tested it and got numbers I wasn’t expecting. The spec for this car is 128psi, 99psi minimum, and 14psi max difference, but my measurements were very high, 150, 180, 180,and 180. 180 is incredibly high. In fact, it’s right about the expected compression for a 13:1 engine, but it’s to my knowledge the 128 number represents the apparent 8.5:1 ratio from the Atkinson cycle engine. Obviously, the 150 number is too low in comparison, this is probably a bad piston ring, given my oil consumption and blowby. Am I missing something about the compression for this car? I did the math and would need over 8ccs of carbon buildup for the 180psi number to be right, which sounds improbable.
Do you think there could be an intake timing issue? The timing adjuster isn't working, or the chain is mistimed, or somebody worked on the engine and put in an intake cam from a different 1NZ? If I remember right, the Prius engine sort of mechanically is a 13:1 engine, if the intake valves close when an Otto engine would close them....
Yeah, it is 13:1 physically (and the toyota numbers note this). At bottom dead center theres 13x the volume than at top dead center. Very well could be. I've only had the car for a few months. I can't say for sure anyones been inside the engine before though, when I replaced the valve cover gasket it was completely petrified, and Id hope anyone that was doing a timing job would replace that while it was in there. Maybe not. Im not sure how I could verify this without taking the timing cover off which I... really don't want to do.
With the valve cover off you should be able to roll around and make marks come up in certain order to get ideas if this has happened . There are also codes for crank n cam timing being fubar . I have an engine with this issue right now driving daily
From a quick google search, the 2001 Toyota Prius has a mechanical (expansion) compression ratio of 13.0:1, but an effective Atkinson-cycle compression ratio of 9.5:1.
Yep! which is why I found it odd when the compression measured was basically 13.0:1, rather than 9.5:1
Thats what I did to test it lol. I'll borrow one from a different store when I get a chance and double check.
Those tools aren't treated very carefully. Once I have to get two replacements. First one was leaking - missing a seal, second one was off calibration. I always check my tools before using them; so I don't chase my tail with bad information. Good Luck.....
