Compression Test

The procedure for checking the compression requires the Toyota Scan Tool so that you can put the Prius into "Inspection Mode" and then choose the menu item: DIAGNOSIS / ENHANCED OBD II / HV ECU / ACTIVE TEST / CRANKING RQST.

I believe it is possible to put the Prius in Inspection Mode without the scan tool, but I don't think it is possible to command the Cranking Rqst without the scan tool.

If you have access to a Leakdown tester, you might be able to run a Leakdown test instead of a Compression test. I believe you could do the leakdown test without any proprietary scan tool, but I haven't tried (and haven't had a reason to).

 

you're right. maintenance mode is easy.. anything else requires a tool.

 

Because of the Atkinson cycle that is used (after induction about 1/3 of the inlet charge is pushed back out again) the compression ratio is effectively about 9:1.

kevin

 

The pressure you saw is about right for the above effective compression ratio. Note that the engine is spun faster than in a "normal" car.

No, you wouldn't do any damage to the car doing the test the way you did. Nice bypass of all the computer systems! You -could- get the computer all upset doing what you did as the expected engine start didn't occur, but I guess if you shut it off after a few seconds (the time it takes for a compression reading) it doesn't enter a code.

 

I'm sure you didn't do any harm. However I don't know what to make of your pressure readings, since a normal engine will be spinning at ~300 rpm via the starter motor while the Prius engine was probably spinning at 1K+ rpm via MG1.

You are right that the main concern is that the readings should be relatively constant.

 

You said, "turned the engine over...", but didn't say how. It seems everyone else understood what you meant but me - guess I'm kinda' slow. Did you just go to READY mode and wait for engine to try starting then shut it down as soon as it started to crank?

 

I did the same thing, I removed all the spark plugs, unplugged all the ignition coils/igniters so there would be no spark and attached the vacuum gage to the first spark plug hole. I then turned the car on to ready (brake on, power button) and waited until I heard the ICE turn over (the MFD also indicated ICE operation) and quickly hit the power button to shut it down. I repeated the process with the other 3 cylinders. With only a few seconds of operation all 4 cylinders read 135 lbs. Just to make sure that was the maximum reading (I agree with the other poster who said the absolute reading is not as critical as any differential in the readings) I let it crank for 4-6 second, at that point the engine shut down on its own, turned on the check engine light, the red triangle and the ! on the MFD. I looked at it with my iPhone Engine Link app and it showed 4 seemingly random diagnostic codes:
P0403 - Exhaust Gas Recirculation Circuit Malfunction
C1103 - ABS Hydraulic Brake Switch Circuit Failure
C1203 - Engine Control System Communication Circuit Malfunction
C1303 - Right Front Isolation Clamping Circuit Failure
I then checked it with PriusDash on my laptop and saw the codes I would have expected:
P0351 - Ignition Coil 'A' Primary/secondary Circuit
P0352 - Ignition Coil 'B' Primary/secondary Circuit
P0353 - Ignition Coil 'C' Primary/secondary Circuit
P0354 - Ignition Coil 'D' Primary/secondary Circuit
I cleared the codes using Engine Link (not sure how to do it from PriusDash) and all is well.

I was checking the compression because I had the spark plugs out anyway to replace them. I have a 2005 with 114,000 miles that I just purchased and I recently put new NGK IFR5T11 Laser Iridium plugs (gapped to .044), cleaned the MAS (Mass Airflow Sensor) and the throttle body thinking it was all nearly due. Shortly thereafter my mileage which was in the high 40's started to sink and after a few weeks was down in the low 30's. Since I had a spare set of plugs that I was going to put in the 2004 Prius I decided to swap them out to try to isolate the problem. The old ones looked fine but sure enough the new ones brought the mileage up into the high 40's, a little better than before I started.

 

I've got 293,000+ miles on my '05 Prius. Just had it checked 125-129, all four, as I was replacing the plugs (after 160k). The only "repairs" in nearly 300k miles: replaced both water pumps, one twice, and an idler pulley (with that mileage these could be deemed "maintenance," not repair). Other than regular maintenance that's it. Most unbelievably reliable and least cost to operate car on the planet; and it's comfortable and safe! Lifetime average MPG 47.9.

 

does anybody know what the compression should be? if you are using a conventional over the counter Napa discount bin compression tester you compression test will read lower than it actually is. it would not cost very much to take it to Toyota and get them to use a high quality compression tester to do the test properly.

 

I did a compression test & got 90-97 between all 4 cylinders - I have 272K miles, but I'm assuming that since the numbers were basically the same between all 4 cylinders that the engine is fine - this is especially since I felt like i couldn't tighten the tester in the cylinder - it would just keep turning forever...

 

I would not say your engine is "fine". At best you could say that the cylinders show even wear. However that wear may exceed spec.

 

Patrick,

What are the consequences of exceeding spec?

 

The engine operates less efficiently and produces less power than when new.

 

I've never known a compression tester to do that, unless perhaps it came with multiple ends and you were using one with a different (but similar) thread to what the Prius plugs use? Anyway, if it couldn't be tightened down, I don't think I'd consider the readings to be worth much.

-Chap

 

So I just double checked - it did come with an adapter which I assumed was for bigger holes, but now I see its about the same size but the threading may be a tiny bit tighter. Either way, it wasn't like I was able to just pull the hose out of the cylinder - i did have to turn it counterclockwise to get it back out and when I disconnected the guage from the hose it did release a whoosh. I suspect that the reason it didn't tighten is because the hose would turn around the metal bottom part that would get threaded into cylinder - once it reached a certain tightness it wouldn't turn the hose & metal together but only the hose.

 

Hmm, still seems weird to me, but if you think it was tight, then the question is back to why 90 to 97. What was the throttle position while you cranked? Most manufacturer instructions I've seen for compression testing instruct to have the throttle clamped full open. I'm sure if you were using Techstream to command the test, the throttle motor would just handle that for you.

Good point; OTC 5609 is one example of a leakdown tester. To use it, you need an air compressor, but it's another way to get the same info, without needing a VCI to make the engine crank. I like it, it seems more civilized, you're not trying to watch the compression needle pump up and decide how long to keep cranking. You just block the crankshaft from turning, open the air valve and read the numbers. It's quiet, and if there's leakage, you can actually hear whether it's going into the head or the crankcase or one of the manifolds or the water jacket. Kind of neat.

You do need to put a socket and handle on the crank bolt and secure it somehow, or the compressed air will flip it around.

-Chap

 

Chap,

I was actually thinking about this this morning and I realized that maybe the reason is because I did the test on the car first thing in the morning before warming the engine to operating temperature. It was a pretty warm day (already about 75 degrees when I did it) but the reason I didn't warm it up was because I wasn't sure how warm I should really get it, and I was primarily concerned with getting consistent number between cylinders - I was under the impression that without the techstream it wouldn't be accurate anyway...

Again, my primary concern was to eliminate the possibility that one of the cylinders was bad and causing a misfire so I could focus on other troubleshooting steps (like buying expensive coils). Since sparkplugs are so cheap it might make sense to just swap out all 4 and do the compression test again after reaching operating temperature before I put in the new plugs. As per my post here: Misfiring or something else? | PriusChat no one has yet commented on the condition of my plugs. What are your thoughts?

Thanks!