SOLVED! Dreaded Code 43! Pulling my hair out!
 

SOLVED: See post below.

I have a 90 hatch with a JDM 96-01 B18C (GSR) engine in it. Converted to OBD1. The original swap was done my a very reputable shop and has run very reliably for 13 years.
I have recently started getting the dreaded code 43 (Fuel supply system) C.E.L. and I am racking my brains trying to figure it out.
The code comes on randomly when cruising, usually in the 3-4k rpm range for some duration. There are NO noticeable symptoms when the CEL comes on.
The engine pulls VERY strong through the entire RPM range and I barely ever notice any smoke in the exhaust, however, there are occasionally some other minor symptoms that have been occurring for longer than this new CEL, I will list them below.

Things I have already done:
Replaced gas cap (was suggested in another thread)
Replaced Fuel Filter
Replaced O2 Sensor (Denso)
Replaced and Calibrated TPS sensor
Checked Fuel Pressure (40psi at normal, 50psi with vacuum removed from FPR *exactly in spec)
Hand-inspected fuel injectors. Cleaned with pressurized cleaner through the injectors, all spray patterns looked good and uniform.
Checked for vacuum leaks multiple times, can't find anything.
Checked compression: 150 psi across all 4 cyl. I know this is low for this engine but meh, shes old and they are all uniform.
I have tripple and quadruple checked timing and valve clearance, they are perfect.
I have gone through the engine bay and ECU harness jiggling every wire/cable I can find while the engine is running, to see if there is any noticeable changes or CELs popping off, so far nothing.

Strange behavior:
When the engine is cold / still warming up, transitioning from throttle to no throttle (engine braking) causes the engine to stumble a little when you initially take your foot off the gas. This becomes less noticeable once warm.
Occasionally engine has "jerky" on/off throttle response, this is usually most common after a warm-start (ex. drive to store, sits in lot ten minutes, restart). The throttle behaves as if it is either OPEN or CLOSED, no "middle" throttle at all. This can make driving in parking lots at slow speeds much less pleasant.
Under the same warm starting conditions, you can not hold the throttle at 1800 rpm, it is very strange. Idles normal and drives normal, but if you try to hold RPM at 1800, they just keep dropping and going back up, this only lasts a few minutes after a warm start.
While the engine idles normally at 700 rpm, after giving it some gas, it tends to hover a little high before dropping down to the normal 700. Maybe only for 3-5 seconds, this is a newer symptom.

Like I mentioned before, the engine runs STRONG and generally feels great. The CEL can some times go 50+ miles without popping off after a ECU reset. Other times it will come on right after I leave my house, if anyone has any ideas on this I would be truly greatful!

Re: Calling All Honda Pros! Dreaded Code 43! Pulling my hair out!
 

Does the car have an O2 sensor?

Diagnostic Trouble Code 43 may be a problem in the Heated Oxygen Sensor circuit, or a problem in the Fuel Supply System.

Re: Calling All Honda Pros! Dreaded Code 43! Pulling my hair out!
 

Yes it has the standard upstream o2 sensor just like a normal OBD1 GSR. I have already replaced this part.

SOLVED! ECT Sensor connector was damaged.
 

SOLVED!

This is an old post but I thought I would share as I found the problem and maybe it will help someone in the future.

After Inspecting or replacing just about the entire fuel system, and multiple attempts to find a vacuum leak with no luck, I decided to start looking for wiring problems.

I removed the distributor to inspect my Cyl. head sensor wiring and I found that the wiring to my Coolant Tempurature Sensor was badly damaged, almost to the point that it looked like a full on short. Surprisingly, the computer never gave me a code 6 which would point me to that sensor, but none the less I do believe this was the main culprit to my problem. I replaced the sensor connector for $10 and put new plastic conduit on most of the wiring down there. No more engine stumble, and I drove her 100 miles last night with no check engine light to speak of!

Even if this helps one person, it will add to the already awesome feeling of fixing my ride for the millionth time.