I thought I'd share this with you guys in case it's helpful, hopefully it if you have a fuel related misfire and it doesn't throw you off onto the wrong track!

I randomly had a cylinder 5 misfire after my 2002 Chevy Blazer drove just fine the night before. It was a complete dead miss, you could hear it from the exhaust.

I was watching the o2's on my ELM327 scanner with the Torque app, and on bank 1 the o2's were switching lean and then rich way sooner than bank 2 was. LTFT on bank 1 was pegged at +25 (STFT were high as well). Bank 2 was spot on, LTFT at 6 and STFT hovering around 0. There were no change in fuel trims or o2 activity when the #5 spark plug wire was removed. At that point I was pretty certain that the #5 poppet valve had completely clogged.

I had an extra SCPI spider from a 2001 s10 so I threw that in, buttoned up the engine and fired it up... still misfiring (P0300), except much worse and the exhaust reeked of unburned fuel. Bank 1 was spot on this time, and bank 2 was WAY rich. Turns out the fuel lines going to the poppet valves were leaking and flooding the intake with gas. A trip to O'Reily and 400$ later, I installed the new Standard Ignition MPFI spider, and new plenum/TB gaskets. Car runs great now!

This is what I've learned:

• If you do go with a used unit, be very careful with the spider lines / clips because they are probably very brittle
• The instructions for the MPFI spider mention you need a new bracket for the assembly. You don't need one, as long as you route the lines carefully (it will be a tight fit)
• Oil the gasket on the new spider, it will help seat the unit in the plenum
• Spend the extra money on the intake plenum gaskets, they might look ok, but my TB gasket fell apart when I pulled the old gasket out
• The fuel lines do actually snap into the fuel spider, but they can be a bear to get them to stay in place

And the biggest lesson learned:

• If you get a P0200 and one or more felt misfires after you button everything up, check the electrical pins on the spider injector unit. I had a hard time initially getting mine on and that should have been a tell tale that I had bent the pins on the unit. I didn't even push the connector in at an angle, I pushed straight down but the only picture I took of the entire project illustrates beautifully what happened:
  
  

I wasn't watching my fuel lines and it had bent the pins on the unit. After I straightened them out, car runs like it's brand new again. LTFT's on bank 1 are around 7 on a warm idle, 6 on bank 2, and STFT's are around 0 again, and o2's are switching seemingly normally.

Hopefully this helps you if you run into some of these issues, especially that issue with the injector circuit open code if you install one of these. I know there are probably a trillion threads on this but hey, why not?

 

This is a helpful writeup and advice. Being careful of bent pins on the spider is an easy thing to miss or accidentally cause. Thanks for sharing your experience!