Did you ever check out the 02 sensors?

 

When the blip occurs I see the FRP sensor drop to ~10psi and the front O2 sensors peg rich. The MAP sensor value stays steady, as does the MAF. I'm not actually seeing any change in RPM. Previously it would cause a very noticable stumble, but now it's barely noticable (if at all).

I never replaced them--they all seem to be responding normally as far as I can see. Since both fronts peg rich for a second when this blip occurs, I ruled out both failing in the same way at the same time. Still on my list of items to replace due to age, along with the knock sensors.

 

I think you are seeing the effect not the cause of the issue

Cheers
34by151

 

I'd agree with that. The fuel pressure dropout wouldn't have gone from happening 90% of the time to 5% of the time by fixing a vacuum leak if the issue was a faulty pressure sensor. Something in the PCM is likely commanding this condition.

So your suggestion is fix the grounds, replace the O2 sensors, let it relearn and see where it stands?

 

My suggestion is just do the grounds for now.

The ones to upgrade are G1, G2, G3 and G26 (LHD) / G30 (RHD)

The ones that could be effecting your issue are G2 and G26/30

G2 has the slave Ign Relay & O2 Sensors
G26/30 has the Ecu + sensors & Ign Coils

After that see how it goes

Also do a quick check of the exhaust pipe bolts where the pipe meets the manifold. You can get to then from under the car and its a good source for unmetered air.

Cheers
34by151

 

Update: a couple months continued with the intermittent stumble, no new symptoms/codes/issues, just the annoying very intermittent stumble. I finally decided to gamble on my original instinct and replace the FRP sensor based on the dropouts I was seeing in scan data. Also, the realization that the "random" nature of the issue had a feeling to me of a bad solder joint (I can see that the FRP sensor has some electronics in it, as it's a vacuum-pressure transducer, which is epoxy potted).

The FRP pressure sensor made an immediate and marked difference in the engine running. Been about a week and 200 miles, including one long trip (70 miles), I think this finally fixed it. It has seriously changed the way the engine runs (for the better). Smoother shifts, torque off the line under normal acceleration, no more stutter/stumble. Waiting a few more miles and will check to see how the LTFT have adapted.

After a few more weeks when I'm 100% sure that the base issue is solved, I think I'll go ahead and replace the O2 sensors and maybe the knock sensors to give the engine a refresh. It doesn't surprise me in the lease that the sensors on top of the engine aged/failed in such a harsh environment with so many heat cycles.