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)

[wonkipop]

i'm unsure what type of EFI you have.

but i had a similar problem with a 74 1.8 L jet earlier this year.
guys here helped to point me in right direction.

with mine it was fuel pressure.
and it was heat related.
only happened when ambient temp here in aus got up into the mid to high 30s (centigrade) and beyond.

in the end i worked it out with clues from other members.

i had changed orig plastic fuel lines to steel.
i had replaced original fuel pump with newer in line unit.

all these added up.
i was boiling fuel in the steel lines after exit from firewall due to radiant heat from fan casting or just general heat build up under car while sitting at idle at the lights.
i was constricting fuel in the tighter bends in flexible fuel line from the fuel pump up to the engine bay due to fuel pump being longer and making that flexible bend tighter.
or all of the above together.

its cooler now in aus and the problem has gone away without me getting around to changing any of the above.

admittedly there was one other symptom that arose at same time which confused matters, namely a leaking throttle body gasket, but fixing that made the problem less severe, but did not make it go away a the time.

but i've got some insulation shielding for the metal fuel lines.
i've rebuilt my original fuel pump.
and i'm ready to change it all over.
i expect to ease both problems listed above.

but it would not idle steadily and hunted and it would cut out.
and i worked out it was fuel pressure problem that came in under hot conditions as heated air built up under the engine bay.
at all other times i was getting perfect fuel pressure.
i could hear the pump making beserk noises when it was stressing to get fuel and the problem was happening.

hope this helps. may not. the guys here really helped - they kept steering me to fuel pressure and ......but yours may not be that (IMG:style_emoticons/default/beerchug.gif)