Normal PSI for cylinders

[4AU4LIFE]

What should the normal PSI be for a 3.0L ?
If it matters it is OEM compression, JE pistons with ~28k miles on engine.
I looked in the Helm service manual in the specifications pages and found a section that mentioned compression but wasn't sure if that was the answer I was looking for.
TIA

 

[Daedalus]

199 psi nominal, 142 psi minimum, and 28 psi variation maximum betwen cylinders. Personally I'll be a bit surprised the first time I measure a car that's more than 10 or 15 psi below nominal when dry.

Service manuals are here.

 

[menfou]

On my engine with 9.2:1 pistons I have 167psi on each cylinder

 

[EdinKali]

Just make sure you have the throttle open when you crank it over. Lots of folks try to do compression tests with the throttle closed and wonder why they get really low readings.

 

[NCC-1701D]

Call me weak but I did a compression test last year with my engine cold because I didnt want to work with a hot engine.

Anyways my compression numbers on a cold engine with about 65,000km on it is

1-210
2-200
3-210
4-200
5-210
6-200

 

[4AU4LIFE]

Just make sure you have the throttle open when you crank it over. Lots of folks try to do compression tests with the throttle closed and wonder why they get really low readings.

Rats, I don't remember if I had the throttle open or not.
My #'s were very close to the ones listed above.
When I check it I pull each coil, then plug and turn the engine over for ~3-5 secs to get the reading. I then reinstall the plug and coil and repeat the process for each cylinder, never having more than one plug removed at a time. Is that correct procedure?
And yes, it is on a cold engine.

 

[EdinKali]

If the numbers were close to the above, you should be fine. The butterfly was probably open enough to get air into the cylinders. On some cars/motorcycles if you don't open the throttle you can get low readings due to a lack of air getting past the throttle plate.

As long as the car hasn't been sitting for days/weeks you should be ok with a cold engine. The reasoning is that you want to have a little oil on the cylinder walls to help with ring seal. If the engine hasn't been run in a long time and the cyl. walls are dry you may get some low readings.

Oh and if you get really high compression readings(and your compression ratio is stock) you likely have high carbon buildup on the piston/combustion chamber(highly unlikely unless you are burning a lot of oil or running super rich).

 

[Daedalus]

never having more than one plug removed at a time. Is that correct procedure?

No, all plugs should be out, throttle wide open. I also disconnect the fuel pump. Your numbers would only get better.

 

[Big_D]

Just make sure you have the throttle open when you crank it over. Lots of folks try to do compression tests with the throttle closed and wonder why they get really low readings.

+1

No matter what method you used, the main point is that all cylinders are consistant. If the throttle was closed or not doesn't really matter in terms of finding a weak cylinder.

Also remember that altitude has an effect on the compression ratio. I had my PPI done in Denver and the Acura dealer tested with a closed throttle and it came in at 145 psi across the board.

 

[4AU4LIFE]

No, all plugs should be out, throttle wide open. I also disconnect the fuel pump. Your numbers would only get better.

I pulled the fuse for the fuel pump so I know that isn't working.
All 6 plugs should be pulled at same time when doing?

 

[Daedalus]

I pulled the fuse for the fuel pump so I know that isn't working.
All 6 plugs should be pulled at same time when doing?

Having all the plugs out means the starter motor doesn't have to work (waste energy) to build compression in the other 5 cylinders. If you look at the manual, the 199 psi spec value is @ 200 RPM. The slower the crank turns, the more time air has to leak past the rings during the test.

I should have clarified that if you pull the pump fuse, do so while the engine is running so there's no residual pressure in the rail. Probably a bit on the nit-picky side. On my other cars I'll leave the fuel pump fuse alone and pull the injector fuse.

 

[4AU4LIFE]

Having all the plugs out means the starter motor doesn't have to work (waste energy) to build compression in the other 5 cylinders. If you look at the manual, the 199 psi spec value is @ 200 RPM. The slower the crank turns, the more time air has to leak past the rings during the test.

I should have clarified that if you pull the pump fuse, do so while the engine is running so there's no residual pressure in the rail. Probably a bit on the nit-picky side. On my other cars I'll leave the fuel pump fuse alone and pull the injector fuse.

I never looked for just an injector fuse. There is one?

 

[Daedalus]

Not on the NSX IIRC, which is why I only do that on my other cars . I think someone (Larry B?) mentioned you can disconnect the injector resistor on the NSX to shut the injectors off, and this is confirmed by the wiring diagram. Probably a good way to go unless the connector is old and looks/feels brittle.

 

[4AU4LIFE]

I just did it again with dramatically different, but better and more consistent results.
Last time I did it I didn't let the engine warm up and I pulled one plug, test it and then reinstalled it before moving on to next plug. My readings were as follows, from LF, FC, RF and the RL, RC, RR
190-195-190
230-210-230
I think that by the time I had gotten to the rear bank that the engine had been turned over enough times that maybe those cylinders were more ready. Doing it this way took over an hour probably ~15 mins between tests.

This time around, I let the engine run for ~5 mins to get it warm but not so hot that I couldn't work on or above it. I removed all plugs and it took ~10mins to test all cylinders. These are the numbers I got this time.
240-240-240
240-240-240.
Yep, exactly the same number on all cylinders.
Needless to say, I was quite pleased to see those numbers this time around.
Thanks to those responding and providing me with the proper procedure for doing this test to get more accurate results.