2015 Prius V compression leakdown numbers 75000 miles

Install an oil catch can (OCC) in the PCV circuit. The amount of liquid in the OCC is a good indicator of piston ring condition. It is easily checked, so it is easier to monitor than a leakdown test.

 

Seems those numbers are fairly good? What do others think?

 

Hate to ask a question to answer another question, but do you like reading? If yes, make some coffee because there’s a thread that’s over 200 pages long that covers all of it very well.

 

Need for an Oil Catch Can (OCC) in this instance is a reference to early model Gen3 Prius which has a design flaw in the Exhaust gas recirculation being taken too far upstream so it's really dirty oily exhaust, which mucks up the valve and can lead to a blown head gasket. These days in Gen4 Prius the exhaust gas is acquired further downstream where it's cleaner. I'm still not clear if this only applies to regular Gen3 Prius or if Prius V also needs it? I've recommended it to a Prius V owner before, but they didn't understand it well enough to want it.

 

Oil Catch Can, Eliminate that knock! | PriusChat

 

Those numbers are within the margin of error of a leak down test. All cylinders are essentially identical, healthy engine. Put another 200k miles on it, and test it again.

 

That's what I was thinking too... But didn't have enough experience to say for certain...

 

I expect your engine is fine since the numbers are all similar to one another, but my understanding is that these engines have a pretty high compression ratio. Like 13:1 for compression and 15:1 for expansion. So it's good that your numbers are all close to one another, but it might mean the test was not done properly. ??? Or maybe it is not possible on these engines for some reason. ??? Was the gas pedal floored while cranking? Engine warmed up?

Leakdown numbers also look good since they are similar to eachother. It is not likely that a true problem exists and affects all cylinders equally. The actual percentage may not be meaningful for these tests, unless you know what pressure it was run at and what the orifice size used was. I.e., 96% does not mean anything other than that if the guy always runs tests the same way, and uses 100 psi, and the same orifice (or does not know what that is), and knows that 96% is pretty tight, that's what it really means.

 

FWIW, it's 9.5:1 effective compression and 13:1 expansion due to valve timing. There's been some debate whether or not the variable valve timing (VVTI) can increase the effective compression ratio closer to 13:1 at WOT.

Edit: On further thought, assuming atmospheric pressure (related to barometric pressure) at your elevation is the standard 14.7 psi, the pressure at TDC in absolute pressure should be (14.7 x 9.5) = 139.65. Gauge pressure (the reading on your gauge) is this number minus atmospheric pressure or: (14.7 x 9.5) - 14.7 = 124.95 which is very close to your leakdown numbers.

 

The P*V analysis does not account for the rise in temperature as the compression occurs, so it is a little more complicated than that.

There is an adiabatic compression going on, so it is not P*V = constant, it is P*V^gamma = constant, where for a diatomic gas (such as a mixture of O2 and N2), gamma = 7/5. And BTW, very cool stuff, that 7/5 seems crazy, but it can be derived based on statistical physics - and is due to the degrees of freedom of the gas molecules. And for monatomic gas, it is 5/3, due to the reduced degrees of freedom.

It is surprisingly easy to do a compression test wrong. Perhaps obviously, if you're taking in air, compressing it, and measuring peak pressure, the pressure of the air intake is important. You expect it should be atmospheric, so in a regular old school test, you keep the gas pedal floored to keep the throttle open during the cranking. Some mistakenly think that if you have a drive by wire throttle, there is never a need for this. My approach is to not think I know the details of how the engine is controlled (including throttle opening, variable valve timing, etc.), and get the manual, follow the instructions, etc.

On the compression ratio, it is complicated due to Atkinson Cycle and variable valve timing (or whatever Toyota calls it here). I had seen the 13 and 15 numbers somewhere before and never looked further into it (foolish me). I still don't know the answer, but I can believe the 9.5 and 13 - and that makes a lot more sense to me.

 

Sounds like you are in the process of creating a problem where there is no problem. Just enjoy the car and ignore whatever readings you did as long as it drives fine.

 

The V Wagon and the hatchback have the same powertrain. I have an OCC on my '13 V Wagon.

 

I don't know what the spec is for compression on one of these engines -- and frankly, I don't fully understand how it would be accurately measured without somehow disabling the VVT. (I'm not saying it can't be done, just that doing it is way above my pay grade.)

But the fact that all 4 cylinders are nearly identical tells me everythings in excellent condition. If there was a problem, you'd see one or maybe two cylinders that are off.

 

Easy, VVT is not activated at idle.

 

Well there you go... I learned something today. I think I'll take rest of the day off.