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I have a Mazda CX-9 SUV 2008, 3.7L engine version - just turned 300,000 KM. The CEL come on recently and the code pointed to the VVT solenoid on bank 2. There was also a misfire on Cylinder 2. I Ohm test both VVT solenoids and the one on bank 2 was 14.1 Ohms, so almost at max of spec; bank one was around 7, well in spec. Resetting CEL, it came back, same codes.

To replace, I had to remove the valve cover, etc. Did this, no longer have CEL for VVT solenoid. However there is still a slight misfire - never stalls, and CEL comes back on misfire Cyl 2. Assuming the misfire resulted in an advancement request, which the ailing VVT could not fulfill... or two things happened to break at once.

Removed the Plenum again to get a Cyl 2 coil and swapped with Cyl 6. Few days later, CEL back on, misfire Cyl 2. So coil is okay.

So I am left with injector, plugs or compression. Now the question. All the examples I find for checking injectors involve the vehicle running and back probing injectors, etc. I cannot do this though, because this Cyl 2 is right at the back of the engine directly under the Plenum. When I remove Plenum, I can access, however can I then perform compression test and/or injector testing? With the Plenum off there are obviously large gaping opening in the top of the head.

I guess the low hanging fruit is just to pull the plugs and check gaps and condition, especially on Cyl 2.

Any advice of diagnosing Injector versus Compression versus Spark plug with the plenum removed?

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There is a tool that I use for such things. https://www.amazon.com/Automotive-Injector-Tester-Diagnostic-Pulse/dp/B01MTSPCOX

This tool allows you to fire an injector while the engine is not running. First prime the fule system by cycling the key. Usually, they settle at some set pressure, let's say 35psi. Then use the tool in pulse mode to fire the injector. This fires the injector for a fixed amount of time. Record how much pressure is lost. Let's say the pressure dropped to 28psi giving you a difference of 7psi. Reprime the fule system and repeat it. Do this for all six injectors. The key is to always start from the same pressure. The tool has different time lengths. You may have to play with which one works best. It will depend on the flow of the injectors and how much residual fule volume the fule system has.

What you are looking for is an odd man out. If all the injector drop 7psi and #2 drops 5psi then you are on to something.

You can also swap injector 2 with an injector on the front bank and see if the misfire moves.

You can also do a leak down check of the cylinders.

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  • Interesting did not know such a tool exist, that could work. I like the idea of moving the injector first. Think I'll pull it to bits again, check the plug and then swap the injector. Should be able to rule out spark and fuel. Cheers
    – Chris
    Oct 9, 2019 at 14:36
  • I swapped Injector 2 with Injector 5 and spark plug 2 with spark plug 6. I only swapped, nothing else. Now I only have the misfire 0.00001% of the time, never engages the CEL. I can feel it fish biting every now and again but some days I don't feel it at all. Can moving an injector (mostly) fix it ?
    – Chris
    Nov 5, 2019 at 3:46
  • @Chris Maby dirt got cleaned out. That is all I could think of.
    – vini_i
    Nov 8, 2019 at 17:43
  • Finally got CEL. Cylinder 6 - getting there. Just picked up a set of plugs. Weird that moving a plug makes it work better. Perhaps the operation of the bad plug is minimized or with a different coil that is perhaps strong in some way
    – Chris
    Nov 12, 2019 at 22:39

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