How to do Compression Check

[NSXBill2]

Searched around some, here and in manual...maybe not enough .

How do I check the compression while I have all the plugs out? Is it in the manual? Is there a DIY here? I know it is pretty simple, I guess, but I don't see it explained anywhere.

I scew the comp gage in and simply rotate the crank to TDC for that cylinder and read the gage? Is that it? What's it supposed to read? 130 psi? How close to each other?

I have an old Sun compression gage that has two plug thread adapters (for American cars-I used on '70 Camaro). I don't think this will work so I am buying a metric one at AutoZone.

thanks,

Bill

 

[Briank]

Screw the gauge into the spark plug hole, block the throttle wide open and crank the engine over for couple of seconds, read the pressure off the gauge. Move to the next cylinder and repeat. The actual pressure is not that important what you are looking for is similar pressure reading across all the cylinders.

 

[NSXBill2]

Screw the gauge into the spark plug hole, block the throttle wide open and crank the engine over for couple of seconds, read the pressure off the gauge. Move to the next cylinder and repeat. The actual pressure is not that important what you are looking for is similar pressure reading across all the cylinders.

So this is not something that I do with all the plugs out? I just thought I moved the crank with a wrench, like right after I set the valves (head covers off). If this needs to be done with the covers on (obviously, if cranking it), then I will wait.

Thanks

 

[Briank]

Turning the engine by hand will not work the compression leaks by the rings. You can actually hear it.

The main thing is to be consistent, I don't think it matters if the plugs are in or out but don't remove the plug and leave them out as you proceed and conversely install the plug after you finish checking the cylinder.

 

[Sulley]

It sounds like you are thinking of a leak down test. It's a different device than a compression gauge. I see that mostly on airplane engines.

A compression check is done as Brian said. All the plugs out, throttle open and cranking the engine. Some people disable the fuel injection.

Sulley

 

[NSXBill2]

OK guys, thanks for the quick replies. Don't know what I was thinking. Trying to do everything I need to while doing T-belt/WP. Will do comp check later when everything is back together. All six plugs looked same and good...light tan color.

 

[Larry Bastanza]

Looks like you got your answer. Remember just for future reference NSXPrime is not the only place for answers. I did a google on "How do I do a Compression Test" and I got 380,000 hits!!! The second hit gave me a complete detailed description:

http://autos.yahoo.com/maintain/repairqa/engine/ques056_1.html

Regards,
LarryB

 

[NSXclear]

If is recall, when I did it to my car, I read that you have to cut off the fuel. I did this by disconnecting the fuel pump relay behind driver side rear panel. Someone on this forum also said you can disconnect the plug in the engine bay by the Fuel Pressure Regulator. Is this neccessary?

 

[Larry Bastanza]

If is recall, when I did it to my car, I read that you have to cut off the fuel. I did this by disconnecting the fuel pump relay behind driver side rear panel. Someone on this forum also said you can disconnect the plug in the engine bay by the Fuel Pressure Regulator. Is this neccessary?

Yes, you should stop fuel flow, and the connector in the engine bay is the terminating resistor for the fuel injectors, and the one that should be disconnected, or you could pull the fuel pump fuse.

Just for future reference the fuel pump relay you mention will not cut off fuel flow. That relay is used to shunt the fuel pump reisistor for higher fuel flow just prior to VTEC engagement.

Regards,
LarryB

 

[Daedalus]

It's a good idea to remove the fuel supply so the gas doesn't wash away the oil that lubes and seals the rings. Also, all spark plugs should be removed for the test to keep the crankshaft speed at maximum.

 

[Tytus]

I was under the impression that the actual compression number reading was important and not just to be the same or close accross the board. I had a car tested that read 130 to 140 PSI and it was recommended not to buy the car. Various guages confirmed this number at different testing shops as well as Acura. Spec calls for 140 to 190 and many cars report readings above 200 with considerable mileage, say 60,000+. What gives?

The leak down was like 16% so that also contributed to the decision not to buy the car, but what about the compression? BTW the car also had a CTSC for 20k miles, hence the concern. Is a car that is sporting higher compression numbers in better shape? Offering better performance? Or is 130 PSI actually OK?

Tytus