Intermittent lean issue, causes stalling / unstable idle *Update: Bad ECU?* Options

[rjames]

Forgive me for starting a new thread that is really just a continuation of a thread I started last year for this same issue. I've tried a couple of new things since then with no progress and figured I'd start a new thread to consolidate everything I've tried so far on one post. (I also put a link at the end of that thread to this one).

Symptoms:
Car will intermittently go into a very lean state causing the power to fluctuate, RPMs to fluctuate at idle and often stall if not opening the throttle a fair amount. Usually happens when engine is warm, but will go away mysteriously on it's own- usually after over enriching the air/fuel mixture and running in that way for several minutes.

I’ve determined it’s going lean because when this happens unplugging the TS1 / Air temp sensor and the surging / staling will cease. (Unplugging the TS1 according to pbanders site enriches the fuel mixture)
Or I can dial the ECU or MPS for a richer mixture which will also solve the issue to certain degrees, but when the issue mysteriously goes away the fuel mixture is too rich.

Symptoms are similar to when an MPS diaphragm fails. In fact, the car was running fine up until about 3 years ago when the MPS failed. I bought a rebuilt / recalibrated one from Jeff B. The car wouldn’t start with the rebuilt MPS because it was too lean (or rich, I can’t remember). I adjusted it, it started, I adjusted the AFR using a wide band sensor and ever since it’s had this intermittent issue.

Things I’ve done to try and solve the issue:
-Ensured correct matching ECU, MPS and CHT sensor.
-Have tried a correct # spare ECU
-New CHT
-Verified CHT is in spec both when engine is cold and warm. (I checked the CHT when the lean condition happened and it measured 78 ohms).
-Installed 123 distributor (issue also happens on stock distributor). I’m running retard on distributor, but have tried without.
-Installed new coil with the 123 distributor. Issue happens with old (stock) coil, too.
-Replaced all vacuum lines and verified they are hooked up correctly.
-Cleaned and flow tested injectors
-New injector seals
-Installed fuel pressure gauge in engine compartment- ~28psi. Reading is constant even when surging/stalling happens.
-New intake runner seals
-New intake runner gaskets
-New throttle body gasket
-Swapped throttle body with NOS model.
-Verified no vacuum leaks with smoke test.
-Verified AAR is opening/closing correctly.
-Verified the decel valve is opening/closing correctly.
-New injection harness
-New ignition harness
-Swapped relay board and verified continuity on all connections.
-Tried a spare rebuilt MPS that hold vacuum
-Manifold vacuum is constant at 15-17 HG
-Smoke test didn't reveal any leaks.
-Verified decal value is functioning correctly
-Monitoring the AFR using wide band sensor.
-Calibrated throttle position sensor. Tracks look good. (Unplugging the TPS has no effect when issue is happening)
-Verified grounds from the FI harness, under the relay board and ground strap on tranny is good.
-New odyssey battery (verified voltage is good)
-Verified timing and valves are adjusted to spec.

Misc info:
-Setting the idle above 13:1 at idle or thereabouts puts the idle speed at 1100rpm, and it will hunt unless you richen up the mixture, either with the ECU knob.
-Throttle body idle screw is all the way in to keep the idle at ~ 1000rpm. Verified that air gets past the idle screw from above the butterfly. Apparently not an uncommon issue according to others. Others report having a hard time getting the idle below 1000 with the 123 ignition.
-Compression is low on cylinders 1 & 2: 110, On cyl 3 & 4: 131
-Open AAR valve doesn't increase engine RPM when engine is cold. Verified AAR is opening. However, plugging the hose that runs from the AAR to the airbox will stall the engine until it's warmed up. (AAR closes as it should then)