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Hello everyone!
I have a 01' MDX and I been the owner of the vehicle for 5 plus years now. The engine had 220,000 miles but I replaced the motor in may of this year with one that was purchase from a used auto parts store and it has 160,000 miles on it.

Ok, so I'm getting poor gas mileage and I do feel a misfire when at idle, after vehicles warns up, I don't fell the misfire but I do fell the engine with a very light shake. Under a load the vehicles runs great. I did have a P0300, P0301,P0302, P0303 and P0304 codes about three weeks ago but the codes clear after I purchased some fuel injector cleaner and always making sure I use 93' octane gas. I don't have any trouble codes since I used the seafoam.


So today I started to check the fuel pressure. I pull up some information (pictures are posted here) I started the test. With the engine running at idle the fuel pressure reads 54psi. Now the flow chart tells me to make sure there's vacuum at the pressure regulator? There's no vacuum at the regulator hose. If you look at the picture of the diagram it shows the hose for the vacuum regulator is connected to the duck work before the throttle body, so there should not be any vacuum at the point because the throttle is closed, my guess. I did unplug the vacuum hose and the fuel pressure maintain at 54 psi.
I went on and apply 10-15 psi of vacuum to the regulator and the fuel pressure did start to drop like it should. Once I release the pressure from the vacuum gauge the vehicle quite on me but started right back up. Right after my fuel pressure test I put a vacuum gage on the intake and my vacuum reads 22 psi, I snap the throttle and the vacuum drop to zero and back to 22 Psi, Great! The vacuum looks good so I don't think I should just jump right into a valve adjustment job just yet, or should I?

So my question is how does the vacuum regulator work and what should I do next?
I know I may need a valve adjustment but before I get in that area I want to make sure my fuel injectors or something else may be my problem.
If I'm missing something please correct me and ask me any questions.
 

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Why do you suspect a problem with fuel pressure? I bet you don't have a problem there.

Why not just adjust the valves? There's a 99% chance you'll fix your misfire (it's pretty much necessary over 100,000 miles, and a LOT of Honda / Acura 3.5L motors don't get this done.
 
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