This has been a major pain in the butt over the past few weeks. I'm assisting a local friend and we're drawing a blank on what in the world happened, and what to do to fix it.

Background
A simple clutch slave cylinder rebuild. Due to rounding of nuts at the slave cylinder and at the soft clutch line, the entire hydraulic assembly to the master cylinder was removed in order to have better access to get the slave cylinder itself off. Annoying for sure, but not a problem, really. Slave cylinder was reinstalled, bled, and so a test was needed to verify function. The car is started, all is well. ~15sec later, the car begins to hunt aggressively then dies. Restart, same results. The car will not keep itself alive. The car is running a Haltech Elite 2500 ecu. The tune on the unit has been good for weeks (the tune itself is in-progress, but it runs (/ran) very well). In order to remove the hardlines for the clutch, the connector to the Haltech wideband was disconnected along with the MAP line to the cabin and the starter wire.
The car is a 1996, 16-bit harness, running factory twins.

Initial tshooting

• Verify all connectors reconnected and vacuum hoses in their proper place.
  • no change
• Revert to an older, known-good tune
  • no change
• Monitor via NSP as the car is running and dying
  • Car starts fine then goes very lean. As it hunts it transitions from an indicated 15 to 22+ AFR.
  • What is up with fuel?
• Restart car, add 20% fuel to table
  • Car runs on its own and will hold idle. However, watching short terms, more fuel is continuously added. at an additional 15%, the car was keyed off.
  • Suddenly it's asking for an additional 35%+ worth of fuel once it gets warm?

Troubleshooting fuel system

• Fuel pressure. Due to this being an almost entirely stock car, we were short on sensors. First assumption was that perhaps fuel pressure was unable to be maintained causing the lean state for the same duty cycle. With a known good OEM fuel pressure regulator on hand, we swapped out what was on the car.
  • No change
• Fuel pump. Okay wasnt the FPR. Perhaps it's the pump. Maybe it's not getting enough voltage? The connector at the rear harness to the pump sub-harness as well as the connector on the hangar itself have been issues in the past. But we decided to first tackle the easy thing that was an issue on another friends car after he drove through a puddle; Fuel Pump relay. Instead of swapping it out with a known-good at this point, we just made a jumper and bypassed it.
  • No change
• Fuel pump continued. Well it's getting power. Maybe the fuel pump is finally dying? What a coincidence, but, okay I guess.... So we pulled out the hangar, swapped out the OEM pump for a Walbro 450 that was on the upgrade list already.
  • No change
• Spark plugs. This is annoying at this point so. Screw it. Let's check out the spark plugs and see if one or more might have fouled out, sending unburnt fuel out into the exhaust that the wideband doesnt read. The plugs themselves were in decent condition. But, a little cleaning later and they were in significantly better shape. Reinstalled.
  • No change.
• Spark. Lets make sure that the leading and trailing coils are firing. Trailing coils disconnected. Engine can still run. Cool. Get trailing plugs wires near plug, observe spark jumping to plug. All coils appear functional.
• Spark plugs, 2. Maybe, maybe they got messed up during cleaning. Replaced with new plugs.
  • No change
• Injectors. Perhaps the injectors themselves are failing, one or both of them. Obtained spare known-good set on another rail, swapped.
  • No change
• Fuel pressure, 2. Perhaps the solenoid that controls vacuum to the OEM regulator is buggered. Screw it. Removed OEM fuel pressure regulator and swapped in a SARD unit and added a fuel pressure sensor into the Haltech. Base fuel pressure now set at 300kPa (up from what should be the OEM 250kPa, from what I gather from the FSM). Start car, observe fuel pressure drop with vacuum (which is now connected on the UIM next to the boost gauge, solenoid nipple on black box capped).
  • No change. Fuel pressure maintained adequately, following vacuum.

Upon contact with a regional shop, the owner was advised to jack up the rear of the car and verify that he was not experiencing "clutch drag" which may be applying load to the engine at idle that it's not expecting. Test performed, Wheels turn. Local shop contacted with positive results where he was then informed that this is normal behavior and not indicative of "clutch drag" which he'd also never heard of.

On the ECU side of things, we've also attempted to rule that out as best as possible by doing the following:

• Unplugged E2500, adapter subharness, adapter box from this car, transferred it to another car of the same series. Loaded up the other car's tune. It runs just fine.
• Took the other car's ecu, subharness, and adapter box, installed it in this car.
  • No change. Still dies.
• Attempted multiple permutations of ecu, subharness, and adapter box.
  • No change
• Attempted multiple variations of O2 controller box/CAN network and sensor connected/disconnected.
  • No change

And our favorite part of the whole thing. Reinstalled the OEM/Knighsports tuned ECU. The car runs. It doesnt die. It's not happy with no MAP sensor hooked up, no CTS or IAT sensor or narrowband connected. But it runs. Can even give it some throttle and it doesnt die coming back to idle.

Between my circle of garage mechanics here that have been around for a while, two local shops (one specializing in rotaries), we're at a loss on what the heck is going on with this car. So I reach out to the hivemind for any additional ideas.

Honestly, I'm guessing it's going to be something extremely asinine. It'll probably be an "are you effing kidding me right now....." thing. We are just out of ideas right now.

P.S.
Grounds are not corroded.
Do not have compression numbers, but plugs out and cranking, pulses are even and strong. Car starts up every time. Car was solid with hot starts prior to all this as well.

 

Just a guess, but it sounds to me like a vacuum line or electrical connection got knocked off or broken during the slave work, causing and extremely lean condition or bad voltage reading at the ECU, resulting in all the issues.

 

Get a data log and post it. Sounds like you've been through the gambit but because you're so deep in, this is the point where simple things potentially get missed. In the log, try and keep it from stalling.

 

These are the two most recent logs I ran, and we definitely have been through it.. I am the owner of this headache and man it's been keeping me up at night, we really tried to attack this from every angle before reaching out for help.
"fuel pressure" was just confirming that it was ok and "o2check" was me keeping it alive for around a minute or two, hopefully these uploads work since the other haltech file isn't supported for upload.

 

Some additional troubleshooting that has been performed over the past couple days:

• Coil packs. Even though we knew we were getting some spark, maybe something was out of spec causing it to be weak or die out as it idles? Checked resistances per FSM, all three checked out. Also checked out plug wires since we were right there, why not? They were in spec as well.
• Quick rats nest "test". Perhaps something was being erroneously triggered and allowing a bleed that is causing our issue? All solenoids unplugged.
  • No changes
• IACV. Maybe there's a failure here that is either causing it to run away or a leak maybe? Unplugged.
  • No significant change. Starts up fine, idles a smidge lower at the mechanical stop. But the hunting followed by dying persists as before.
• Haltech Firmware. Maybe we found a glitch in the current build. Downgraded firmware to 3.10.1, imported tune.
  • No change
• Haltech Tune. Attempted to load Haltech-provided base map and give that a go. No luck. Made enough changes between that and our current tune and it was a shot in the dark, wouldnt fire.
• Haltech Firmware, 2. Updated to latest firmware (3.11.3 from starting 3.10.3), imported tune.
  • No change.
• Vacuum lines. While UIM was off to inspect coils, all vacuum lines checked. All remain flexible, no cracks noticed. Shouldnt be an issue in this region.

At this point, we will perform a compression test to verify. We're confident this is not the issue, however. After this, we will be pulling the engine harness to inspect pin by pin for any breaks in wire. Figure something is old, heat-hardened, and it failed when it got bumped at some point during install. But may not be visible with the ABS still in and all that mess. So, we'll just check it all and make repairs as necessary.

 

When you hear hoofbeats, think horses, not zebras...

 

Is the coil harness hooked up correctly to the coils? Are the spark plug leads going from the correct coils to the correct plugs?

In 1996 and later they moved the mounting order of the coils.

But it is surprisingly common that people hook things up according to the USDM FSM, which will put your connections in the wrong place.

Also check for bad gas.

I went through a similar exercise as you about a year ago.

Likely cause was bad gas and week coils.

Also I had to watch the tech like a hawk because he was indeed about to hook up the new coils in the wrong order.

In the last year we have had two local cars blow their engines at the dyno shop, when the leads were hooked up incorrectly.

 

When we were trying to get the car started, we flooded it many times.

We figured it was flooding too quickly each time we went to start.

We removed the IAC and let the UIM suck in gulps of air (controlling the opening literally by hand).

That got it started.

The benefit of the 30 hours work testing everything and replacing a lot of things, is the car has been running beautifully since we finally got it started.

Ironically, later, I got a new coil harness, and a different tech installed it incorrectly. It is very easy to reverse the hook up order. The car ran but was backfiring intensely.