Nissan Repair: 95 maxima fuel injector test, haynes manual, fuel rail

Question
Von,
One of the previous thread here (on how to check the fuel injector) I did the following test according to your instruction:

1)I couldn't find the valve on the fuel rail you mentioned. Therefore I did the test connecting the gauge to rail directly which comes out [not the fuel regulator end, other end rather] of the fuel rail. Anyway. here are the readings: car running(regulator hose on)-34-35psi. Took the regulator vac hose off while running-44-45psi. Put the reg hose back on-34-35psi as before. As the engine running with vac hose on, Now I turned the engine off - pressure went down IMMEDIATELY to 27-28psi.  
I think, I got one or more injector might be bad am I right?.

I did the Ohm meter test on the the injectors on the front part (underneath the NISSAN aluminium cover) as these were easily reachable and all of theem came out 12-13 ohms. So I think problem MIGHT be with one/more injector(s) under the intake manifold.
I heard when there is problem with any of the injectors, it's recommended to replace the whole set of injectors rather than just 1 or 2. Is that correct? That will cost me big $$$. I found remanufactured injectors online $240 ($40 x 6).

2) Where can I find good instruction on how to replace the injectors myself (especially the ones under the manifold)?

3) There is a mild low "humming" noise I can hear when I wait at the red light. I don't hear it when the car is on parking(P) gear. I don't know what could be causing that. Any idea?

Thank you Von.

Answer
Viv,

You only have to change the injectors that are defective.  The intake manifold removal instructions can be found in a Haynes Manual at the local parts store.  It cost about $20 bucks.  The upper intake is only held on by six bolts and a couple of braces.  It looks more difficult than it is.  The injector replacement requires the removal of the fuel rail.  So, purchase a set of injector o-rings prior to doing the job.  If you really want to save money go to a wrecking yard and source yourself a set and ohm them out and use the good ones to replace your leaky injectors.  The easy way to figure out which injectors are defective and leaky is to remove the sparkplugs and look for the really black and wet electrodes.  Those will be where the leaky injectors are...  The running fuel pressure is great from your readings.  The noise you hear should be normal if you have worn motor mounts.  Check the engine when it is loaded, like as in drive, and see if the exhaust is touching or something like that.  Basically, any metal to metal contact.