Megalodong

1995 Civic EX coupe with JDM B20b block, Integra LS head, P75 Intake, GSR Throttle body. Currently has P75 OBD1 ECU. '95 Civic wiring harness. Previous owner did all swaps. I purchased it five months ago. Stock automatic transmission has been/is converted to an Integra LS manual 5-speed.

Engine has a hunting idle since purchase. I recently depowered the steering rack and now experiencing stalling and lean idle issue.I'm getting help for the lean issue on a another forum from a long time Honda dealer machanic. Also, I have a Hondash OBD1 bluetooth scanner and app to view live data with. We're still narrowing down possibilties and the ECU is one of them. The person who sold it to me is good guy, straightforward and always answers any questions I have about to this day.

My question is what ECU or ECU + tune combination is best for my setup? Seems to me if it had a hunting (without stalling) idle issue since before I purchased it ..then maybe it's related to the ECU. When I say hunting idle it idles around 700 to 750 rpms (no load) for the majority of the time but then it will drop to around 300 rpms for a few sceconds and then return to 700/750. It does this sporadically but consistently enough to know it's not normal. When it drops it in idle I feel as it's searching for a basemap but can't find one. This and now the current stalling a idle issue leads me to the P75 ECU may not be idle for it?

Reading through info online there seems to be mixed answers...some say P28 (obd2) + Hondata 300 chipped and tuned. Others say OBD1 p73 itr map is best. The previous owner told me it has never been tuned so, maybe that's where the issue lies? All suggestions appreciated.
Related read: http://hondaswap.com/threads/ecu-for-b20-vtec.514606/

HondaPartsHero

You don't need a P73 baseman that's for a VTEC ITR. It should run fine with the ECU you have albeit a tad lean. Have you tried cleaning the IACV?

Megalodong

Yes..cleaned IACV, iacv screen and TB a month ago.

Megalodong

Lean condition stills persists as shown in datalogging video below. Several test performed to track down the issue and it seems the only thing left is the ECU. Here's a list of what's been tested, checked or replaced:
1) New o2 senor Denso part# 234-4099
2) Used, tested MAP sensor Denso part#
3) Tested injectors by pulling each injector harness off one at a time and listened for drop in idle: all dropped evenly
4) Removed and cleaned injectors by adding pressure + bake cleaner + engaging/pulsing with a 9 volt battery. Reinstalled injectors but no change in lean idle issue.
5) Smoke tested intake & exhaust but no leaks seen, I pushed air into tail pipe and sprayed soap water on flanges/connection/donut seals and no leaks seen.
6) Sprayed TB cleaner around intake and vacuum sources: no leaks detected/idle didn't rise.
7) Compression tested an all cylinders are between 205 and 210 psi. Leakdown test performed day of purchase and engine is healthy.
8) Checked valve lash adjustment: all are in spec.
9) IACV and TB removed and cleaned
10) Mechanical and Ignition timing checked: in spec
11) Power balance drop test : all cylinders drop equally in rpms when wires lifted from plugs. Can hear good spark with wires lifted slightly from plugs.
12) Adjusted idle screw so that the engine is no longer stalling.

New Denso o2 sensor (installed a month ago) cost $30 and is made in America. I crossed reference NTK,NGK, Desno and Honda Genuine and the sensor I purchased is the correct one for a 95 Civic EX.I would like to source one made in Japan if possible but Honda Genuine are $250 and really not sure if it will make a difference? The o2 sensor readsup to .9v at WOT. I'm bringing it to an exhaust shop to weld in a temp exhaust (CC) test pipe and inspect for leaks pre-o2 sensor..I'm not expecting the test pipe to solve the issue.

This is a response from (HT member) Civic Service three weeks ago when I posted a prior data-logging video:
" Your O2 sensor voltage isn't oscillating. I'm not familiar with earlier than '96 models but on '96 and above, that O2 sensor oscillates between 0.1 and 0.9 V when in closed loop mode.

Also, your short term fuel trim is way out of range and increases with engine RPM (> 40%). Anything more than ~10% is cause for concern. That could be caused by a number of things. If your O2 sensor is bad, that could do it. If your O2 sensor is good, then a fuel delivery problem could do it.Given what you said about the questionable origin of the O2 sensor and its flatline voltage, I would look at getting a better one before moving any further."

So, this brings me to the possibility of an ECU/ fuel delivery issue causing the lean condition. Also, inquiring if a chipped/tuned ECU with the option of o2 sensor disabled/open loop will solve this problem?
Looking at HA motor sports, Phearable or Xenocron as the supplier.

Datalog from a few days ago. Longer engine idle times near end of video:

Megalodong

Update:

Purchased an OBD1 chipped basemap from Phearable that constantly runs in open loop (o2 sesnor disabled) and engine seems to be running much smoother now with no dips in rpm at idle. Only downside is my Hondash scanner no longer works with this chipped ECU. Next upcoming repair is passenger side CV axle half shaft replacemnt..torn outer boot.