Is there a Wiki on How to Properly perform Compression Test on our NSXs?

[AcuraNSX]

Hello Prime Members,

As some of you know, I recently purchased a 91 NSX, and I had no service records on the car. Ugh....Needless to say I have been attacking every maintenance item, since I am flying blind. I used Larry's and Gary's fantastic write up on the Timing Belt replacement. I have slowly and arduously replaced every coolant hose in the car, had Brian at NSX e repair fix my cooling fan relay because my fans were running randomly, changed the main relay and ignition switch (preemptively).

Since I have the sparkplugs pulled and coilpacks pulled, I thought I would do a compression test to check the overall health of the engine/cylinders/pistons. I know the compression values to look for in our cars, and the deviation between any of the cylinders, but I could not find a WIKI article on how to PROPERLY perform it on our cars. I have done it on a 4 cylinder 1989 Mazda before. I wanted to know if there was any ignition plug or fuse to pull or fuel pump/relay/fuse or plug to pull. I don't want to harm any other components by doing it improperly. Any advice or and DIY articles would be greatly appreciated. Thanks again!

 

[Lude_2_NSX]

A compression test on the NSX isn't any different than any other car. If you have done it in your Mazda, you should be able to do it on your NSX.

You have already pulled the coils and plugs. Disconnect the fuel pump, make sure the battery has a good charge (or connect another power source to the battery).

Thread the compression tester into the first cylinder. Hold throttle wide open and crank over the engine 6 or so rotations. Generally you will see the gauge sort of "peg out" at a pressure around 5 or so rotations. This is your PSI. Write this number down. Repeat for the rest of the cylinders.

You want to see PSI values greater than 200 PSI. Anything less than that may be cause for concern. More important than the PSI value is the difference between cylinders. You generally want all cylinders to be within 10% (or 20 PSI) of each other. These are all "general" specifications, as all engines are different. Usually compression related problems are very obvious.

Perform your test and post your results, and folks can provide feedback. Welcome to NSX ownership, it sounds like you are putting in the wrench time. I did the exact some thing when I bought mine, and it's been a great journey and learning experience.

 

[AcuraNSX]

Thanks Lude! btw, how do you disconnect the fuel pump? Pull a particular fuse or unplug the main relay behind the passenger seat? I appreciate the feedback.

 

[Honcho]

Here is my method:

1. Warm up the car until the radiator fan kicks on.
2. Remove the fuel injector resistor plug next to the engine bay fan (cuts fuel)
3. Remove the coils and spark plugs (cuts spark)
4. Starting with Cylinder #1 , crank for a few seconds and take the measurement. Repeat for 2,3,4,5 and 6.

Honda says that any compression value above 142 psi is within spec and the "new" standard value is 199 psi. Honda also says that the maximum variation between cylinders should be 28 psi or less. Any more than that and you may have a problem. Historically, most NSXs with good maintenance records and no history of forced induction test above 200 psi. A perfect NSX engine (e.g., Kaz's car for example) routinely tests above 230 psi at sea level. Remember to take into account the altitude, as it will yield lower results. My garage is at exactly 6,000 feet, for example. On my last NSX, I got a result of about 185 psi between cylinders and had a heart attack. After realizing my error, I applied the 0.8 correction factor and got 231 psi, which made me feel a lot better. You can find a standard atmosphere chart on the web.

 

[RYU]

Not sure if mentioned but a leakdown test would be my prefer choice.

 

[Lude_2_NSX]

Honcho, great point with elevation. It does need to be taken into consideration.

AcuraNSX - Honcho's method sounds like the easiest way to go to cut off fuel.

 

[Honcho]

Another point for the OP, if you plan to keep your NSX for a long time and have the tools/ability, you should do a compression test every year. This is the method Kaz follows for his personal NSX and, over time, it will provide useful data about the overall health of the engine. For example, it can show you how good your valve adjustment was. It will also potentially highlight engine problems before they become more serious. With my last NSX, I did it with my annual oil change service. Remember to wait until the engine is cold to put the spark plugs back in!

 

[Old Guy]

Pulling the #18 20 amp fuse in the engine compartment box cuts power to the ECU and the +12v supply to the injectors. Its effectively the same as disconnecting the main FI relay. A de energized ECU will prevent the fuel pump, injectors and ignition from operating during cranking. Its a lot easier to pull that one fuse than it is to pull the injector resistor (or the shunt plug on the later non resistor equipped cars) which is in a slightly inconvenient place and can be uncooperative as multi pin connectors sometimes can be. Just doing the injector resistor plug will cause the fuel pump to continue to run during cranking unless you also remove the fuel pump fuse.

The #18 fuse is easy - you don't even have to scrounge around under the dash like you have to do for the #7 'clock' fuse.

Edit: Does not work - see post #13

 

[AcuraNSX]

Thanks Honcho!

Thanks for your help Honcho. As for the heart attack, LOL.....I could imagine the nervousness you felt.....all the thoughts that must have been running through your head.....I could only imagine. Again Thanks Lude, and Honcho...as for the leakdown, I am sure that is more comprehensive, but I shall start off with a simple compression test as I have a compression gauge to perform the test with.....just picked up a fuel pressure gauge from Acura, hoping to test the health of the fuel pump....planning on changing the fuel filter but would be kind of cool to see how the pressure changes if at all. But thank you for the suggestion, much appreciated.

Here is my method:

1. Warm up the car until the radiator fan kicks on.
2. Remove the fuel injector resistor plug next to the engine bay fan (cuts fuel)
3. Remove the coils and spark plugs (cuts spark)
4. Starting with Cylinder #1 , crank for a few seconds and take the measurement. Repeat for 2,3,4,5 and 6.

Honda says that any compression value above 142 psi is within spec and the "new" standard value is 199 psi. Honda also says that the maximum variation between cylinders should be 28 psi or less. Any more than that and you may have a problem. Historically, most NSXs with good maintenance records and no history of forced induction test above 200 psi. A perfect NSX engine (e.g., Kaz's car for example) routinely tests above 230 psi at sea level. Remember to take into account the altitude, as it will yield lower results. My garage is at exactly 6,000 feet, for example. On my last NSX, I got a result of about 185 psi between cylinders and had a heart attack. After realizing my error, I applied the 0.8 correction factor and got 231 psi, which made me feel a lot better. You can find a standard atmosphere chart on the web.

 

[AcuraNSX]

Pulling the #18 20 amp fuse in the engine compartment box cuts power to the ECU and the +12v supply to the injectors. Its effectively the same as disconnecting the main FI relay. A de energized ECU will prevent the fuel pump, injectors and ignition from operating during cranking. Its a lot easier to pull that one fuse than it is to pull the injector resistor (or the shunt plug on the later non resistor equipped cars) which is in a slightly inconvenient place and can be uncooperative as multi pin connectors sometimes can be. Just doing the injector resistor plug will cause the fuel pump to continue to run during cranking unless you also remove the fuel pump fuse.

The #18 fuse is easy - you don't even have to scrounge around under the dash like you have to do for the #7 'clock' fuse.

My man! I mean My Old Guy! Thanks for the suggestion! Seems like a quick no fuss option. I am sure others who do a search for this will find these suggestions beneficial.

 

[Lude_2_NSX]

Good luck, post your results!

 

[AcuraNSX]

Compression Test Results.....

Good luck, post your results!

Hey Lude et al.,

Here are the results of the compression test. Btw, I had to follow Honcho's technique of disconnecting the fuel injector resistor after letting the car warm up until the radiator fan kicked on. I tried Old Guy's Technique of pulling the #18 fuse and my car wouldn't even turn over....I freaked out a bit, so I plugged the fuse back in and did Honcho's technique. Old Guy did write back to me to inform me that this might be the case in the 91 NSXs as it cuts power to the starter, hence why my car wasn't turning over......I did a timing belt change prior to this test and this is what I got:

Cylinder 1 226 psi
Cylinder 2 233 psi
Cylinder 3 237 psi
Cylinder 4 226 psi
Cylinder 5 223 psi
Cylinder 6 221 psi


91 NSX with 121,000 miles on it.....

 

[Old Guy]

Yes - it looks like my advice on the #18 fuse was bad. In addition to killing power to the main FI relay, a little further deep dive into the wiring diagram shows that it also provides continuous power to the security control unit. The service manual cause/problem chart for the security control unit does not list non operation of the starter motor as one of the problems associated with the #18 fuse. Despite that, it does appear to depower the starter cut relay resulting in an inability to crank the engine. My error!

 

[Lude_2_NSX]

Hey Lude et al.,

Here are the results of the compression test. Btw, I had to follow Honcho's technique of disconnecting the fuel injector resistor after letting the car warm up until the radiator fan kicked on. I tried Old Guy's Technique of pulling the #18 fuse and my car wouldn't even turn over....I freaked out a bit, so I plugged the fuse back in and did Honcho's technique. Old Guy did write back to me to inform me that this might be the case in the 91 NSXs as it cuts power to the starter, hence why my car wasn't turning over......I did a timing belt change prior to this test and this is what I got:

Cylinder 1 226 psi
Cylinder 2 233 psi
Cylinder 3 237 psi
Cylinder 4 226 psi
Cylinder 5 223 psi
Cylinder 6 221 psi


91 NSX with 121,000 miles on it.....

Those number look pretty good to me!

 

[AcuraNSX]

Thanks Lude! Yeah they looked kind of good to me too. Seems like this car was driven but well taken care of. The hoses were in excellent shape as was the timing belt and spark plugs....I just worry about the automatic transmission because I have no data on the longevity of Honda's V6 automatic transmissions. I know Honda had major problems with anything V6 auto transmissions up until the early 2000s with their Odyssey minivans! As for OLD Guy, no sweat, thanks for taking the time out to at least help a fellow prime member out.....I tried paying it forward by letting people know it's possible to do a timing belt change without ever tilting the engine!