My Nightmare 5.0

Solution: Both O2 sensors needed replacement!

Alright, this is gonna be a lengthy post. I've done some troubleshooting so I'll try to list things I've done in chronological order for ease of reading.

1995 Convertible 5.0L w/ T5, bone stock aside from a cobbled together "catback" exhaust consisting of flex pipe and old glasspacks.

The Problems (before yesterday):

• Nightmare misfire (cold or at operating temperature)
  • Rough Idle (bounces anywhere from 400 to 800 RPM, occasionally stalls but sometimes recovers before it dies and jumps briefly to 1000 RPM before going down again)
    • Check engine light will sometimes turn on and off (not flashing) at idle and while driving.
      • CEL on: Car runs better.
      • CEL off: Car misfires.
        • This isn't consistent, but happens enough to show a pattern.
  • Constant misfire with anything less than WOT applied
    • Under WOT, car pulls hard and doesn't seem to have a miss. Immediately after, car will run without a miss regardless of where the throttle is. After a minute or two however, vehicle starts misfiring again. Very evident when going from WOT to activating cruise control.
    • Sometimes goosing the throttle a couple of times will let the car run without a miss for a minute or two much like after a WOT pull, but starts misfiring again while cruising.
    • Backfire and popping coming from exhaust when starting from a stop, struggles to get up and go. Also happens on deceleration below 45-50mph.

• Inconsistent 3-Digit Codes/Failure to Read
  • Not my first rodeo with OBD1, both Ford and GM. Started by using the paper clip jumper method.
    • First KOEO, multiple codes appeared.
      • "113" - IAT sensor is/was high or open
      • "157" - MAF signal is/was low or grounded
      • "158" - MAF signal is/was high or short to power
      • "172" - Oxygen sensor not switching – system is or was lean
      • "173" - Oxygen sensor not switching – system is or was rich
      • "176" - Oxygen sensor not switching – system is or was lean
      • "177" - Oxygen sensor not switching – system is or was rich
      • "542" - Fuel pump open, bad ground or always on
    • Second KOEO a couple days later, the CEL flickered multiple times before giving a "111" code. Code flashed "154", then flashed twice before CEL turned off and remained off.
    • KOEO again, "111" code then gave codes "217", "317", "417", "617" and "717" which, to my knowledge, mostly aren't proper 3 digit codes to begin with. Maybe this was my own mistake by blinking as I was writing down the codes, but after this, attempts to do more KOEO tests failed as the CEL either remained on or would turn off and stay off.
  • Purchased the Innova Ford OBD1 code reader out of desperation
    • KOEO, "111" code then "542" and "543". Code "10" (separation code) then "542" and "543" repeated. No other codes from before were present, and I didn't clear any of the codes to my knowledge.
      • Code "542" - Fuel pump open, bad ground or always on
      • Code "543" - Fuel pump monitor circuit shows no power
        • These codes surprised me as the fuel pump was priming and seemed to be functional, but after scanning these codes, the car had difficulty starting. It wasn't until I held WOT and continued cranking that the car started and idled, then the car started just fine a couple of times after that and still does.
    • KOER, codes "40" and "50" appeared.
      • According to my research, code "40" and "50" show issues with Cylinders #4 and #5.
      • A strange occurrence; during the KOER test, white smoke started coming out of the exhaust from the rear of the vehicle. After disconnecting the code reader, the white smoke stopped. There is no white smoke during engine operation, so I found it strange that it happened during the KOER test.

• Aftermarket alarm system
  • There's a six pin port with a non-stop flashing red light installed underneath the steering wheel column trim, which has wires connecting to a larger black box underneath the steering column. There is a thick gauge red wire and black wire coming out of this box, but the box itself has no markings to identify itself aside from a serial number that doesn't reveal anything after searching on Google.
  • It's not for keyless entry to my knowledge, the vehicle didn't come with a fob for it.
    • I believe, but cannot confirm, that the alarm system is bypassed as of now. The wiring runs deep into the column/dash, and it'll require a day of disassembly to see where it all leads.

• Yesterday/Fuel Pump Failure
  • Yesterday, after I left work, the car would crank but wouldn’t start. Pulling codes gave me “542”, so I assumed the fuel pump was dead and gone. Started to get desperate as I have a 1+ hour commute back home from work and wasn’t looking forward to the tow charges. I did some research, and a stray forum post suggested the following:
    • “Have you tried hitting the gas tank with a rubber mallet a few times?”
  • To my surprise, after the third or fourth hit during KOEO, the fuel pump started to prime. Jumped in, started the car, and I hit the road. Unfortunately, the “nightmare misfire” became even worse. Unless I feathered the throttle, or pushed it wide open, the car would backfire, pop, miss, etc. severely. Luckily I got to the highway, went into 5th and was able to limp it home.
  • After returning home, I turned it off and then tried KOEO to try and read codes. The fuel pump primed with no issue, but my scanner wouldn’t connect to the PCM. Tried the paperclip method, and I got a “111” code before it flashed “13” and then the fuel pump primed again, which turned off the CEL.
    • It seems like the fuel pump wants to prime in 30 second or less intervals, which is interrupting the code reading process. So I can’t pull codes from the vehicle anymore.

Conclusion:

This car hasn't been taken care of. Lots of broken tabs on sensor pigtails, the interior is shot, the shifter itself is duct taped together along with the rear window of the convertible top, and the air filter was removed in lieu of using mesh wire hose clamped to the MAF housing.

So I'm inclined to believe that these issues are arising from the IAT, MAF, O2 sensors and the fuel pump. A few notes, the MAF appears to have been replaced at some point as there is a peeling sticker on the MAF itself that says, "Remanufactured" on it. I wouldn't be surprised if these issues were present with the previous owner, but I'm open to purchasing a new (not reman) MAF as I've already tried cleaning this one out.

Another thing I've noticed is that the electric fan comes on when the vehicle's key is turned on, and stays on nonstop. I've read that this could be a problem with the ECT, and some articles/forum posts have suggested that a faulty ECT could cause some of the issues I'm facing here.

I'm also a bit nervous about the alarm system, and have it in the back of my mind that it may be causing some of my issues with drivability. I have very little alarm/security system experience, so I'm not exactly eager to go digging into it's wiring or attempting to remove it.

I got the car for dirt cheap, so I'm not at the "this sounds like a lemon, get rid of it" stage just yet. I'd rather bite the bullet and do an EFI to Carb conversion and run the powertrain standalone from the body harness and PCM before I'd sell/scrap it. Plus it's a Rebuilt Salvage title anyhow, resale value is pointless to consider.

So, how would you guys proceed?