Posting this in case someone comes across it on google, will try to include lots of keywords.

Spent the better part of a day working on it, finally fixed it. 2003 Mazda Tribute LX V6 3.0L FWD
The goal was the replace the lower intake gaskets with part # MS96124. Part includes upper and lower intake gaskets, Idle Air Control valve gasket, and throttle body gasket; I replaced those too because why not.

Symptoms:

• Upon starting, car would idle roughly. RPMs would dip into the 500 range and cause noticeable shaking.
• Shifting into Drive, the car would shake even more and sometimes stall.
• Only occurs from a cold start (example: car has been off overnight).
• Symptoms disappears once warmed up to operating temperature; usually after ~5-10 minutes.

Diagnosis:

• OBD reader initially threw codes P2195 P2197, but failed to re-appear several days after clearing.
  • Codes suggested faulty O2 sensors, but the live data confirmed their voltages were fluctuating properly between 0.1v-0.9v.
• No OBD codes thrown afterwards.
• Short term fuel trim was high, fluctuating in the 20-30% ranges.
  • Upon increasing the RPM, the STFT % would drop; apparently this indicates a vacuum leak.
• Spraying carburetor cleaner around the engine bay while idling did not change the RPMs.
  • The leak was either not close to the surface or not part of the intake.
• After scouring the net for useful information with symptoms similar to this, I found a post by someone who had a similar symptom and changed a whack ton of stuff. His last post was him planning to change the lower intake gaskets at a mechanic, and then never posted again. My guess is that fixed it.
• Upon inspection, the lower intake manifold gaskets were completely flattened and squished; looked like web coming out the sides. Mating surface was also very dirty.

Plan / course of action

1. Use Youtube video guide in resources section.
2. Replace lower intake gaskets.
3. Perform other preventative maintenance while everything is apart.
   • Clean / replace PCV valve
   • Clean areas and components along the way
   • Replace upper intake gaskets
   • Replace IAC valve gasket
   • Replace throttle bottle gasket
   • Insulate fuel injector power cables with more electrical tape; some were frayed
   • Clean fuel injector power connector sockets with compressed air
   • Inspect hoses and replace or seal them. In my case, the right-angle crankcase ventilation hose that connects to the front was ruined. Replaced temporarily with clear PCV tubing until I can get high temperature rubber hose. The right angle rubber tube / hose is 5/8" on 1 end and 3/8" on the other; inside diameter.
   • Inspect and clean the ends of the fuel injectors.
   • The back 3 spark plugs and coils wouldn't be a bad idea either if you haven't changed those in a long time; those are a pain to get to.

Tools and supplies used / needed

• Blue Driver ODB scan tool; I used this to get codes and check live data. Has served me well over the years. A bit pricey, but it's paid itself off over the years. Wish I had gotten one sooner before I went shopping for used cars.
• Cleaners: electronics, carburetor, brake. Compressed air.
  • Electronics for cleaning plastic parts and any sensitive things like fuel injectors.
  • Brake cleaner for cleaning the metal mating surface for the lower intake manifold; spray on rag / towel and wipe, do not spray directly on the surface.
  • Carburetor for cleaning throttle body.
• Small pliers and plastic spudger / pry tool to remove gaskets without damaging the manifold.
• EGR valve gasket; apparently these shouldn't be reused.
• Socket set; 8mm and 10mm deep sockets for most bolts.
• AC / Fuel line disconnect tool and penetrating oil.
  • I struggled a bit disconnecting the fuel line. Decent amount of residual fuel still in the line, even after de-pressurizing, so be careful.

Resources

• Youtube video series (4 parts) on changing the upper and lower intake gaskets.
  You Tube
  • Note, the part where he removes the back 3 spark plug coil wires has an extra cable that also needs to be disconnected.
  • Note, the bolts holding down the wiring harness where he gets a jack to support the oil pan is optional; at least for me. I was able to remove and install the lower intake manifold without having to move that wiring harness completely out of the way.
• Chilton service manual for informative pictures and torque specifications.
  • Intake manifolds were all 89in/lbs (inch, not ft), just need to be careful with the order in which you install / remove the bolts.
  • The 2 bolts on the EGR valve (one with the gasket) are 18ft/lbs.
  • Throttle body and IAC valve are both 89in/lbs.

Results

• Short term fuel trim during idle has dropped to the single digit %, sometimes low teens, which is the normal and idle range and considering the age of the car.
• So far so good; no more symptoms.
• Added some Gumout REGANE (the one with PEA / polyetheramine) high mileage fuel additive to help clean the fuel injectors.

References:
Mine was a lot dirtier than this. The red circle is the PCV valve and tubing / hose; good to inspect that. The PCV valve can be broken loose with a 15/16 socket, be careful though, it's plastic. I didn't have a 15/16, so I used a 7/8 spark plug socket, or something that looks like it; had a lot more ridges.
I used electronics cleaner to clean the PCV valve.


The red circled right-angled tube / hose is where it had a hole. 5/8 into the engine; 3/8 into the black tube (inside diameter).


These fuel injector power cables were frayed on mine, some even had exposed copper wire. Re-insulated with electrical tape.

 

Well done.