I just bought a 99 f150 4x4 Bi Fuel AT 4r100 truck. The intake manifold was swapped out with a normal 5.4 alum truck manifold so all the lpg stuff was already removed. I removed the rest of the lpg wiring and computer. I traced wires that were in the lpg harness, most ended up "Y"ing into the lpg harness.

Problems: (exactly the same before and after removing the lpg harness)
If I clear the codes and start it I get:
p0155 Drivers side upstream O2 heater circuit
p1747 EPC short circuit
U1262 Comm failure (looks like this is comm failure between the pcm and lpg compuvalve)

If i drive it i will end up with:
p0135, p0141, p0155, p0161, p0443, p1747, u1262
Basically all 4 O2 heater circuits, evap purge, epc, and comm failure.

Things I have checked:
Fuse #23 is good, in fact all fuses are.
12v power and ground go to all O2 sensor heater circuits.
12v on both power wires on the trans solenoid pack.
We pulled both ends of the trans harness and checked it, no breaks, or shorts to ground.
EPC ohm tested perfect, same as a known good one i got, changed them out anyways...no change.

I honestly have about 20 book marks of help threads and 25 saved wire schematics. Ive been crawling under and over this truck probing wires for two weeks every spare second I have and I cannot figure it out. I'm sure it is something simple. Ive honestly put it back up forsale but I really need it.

 

All the codes are common to the harness going back to the transmission, the Evap system and Ox sensors.
Either opens, crosses or incorrect connections..
Since you done a conversion, anything anywhere can be the cause of the issues.
You just have to revisit your work with the harnesses.
Good luck.

 

I wholeheartedly agree, something in the harness. This being a Bi-Fuel something is a miss, BUT it did also run on gas.
I attached a picture, you can see what I mean by "Y". The lpg wiring was "y"'d or "T"'d off the existing harness. i did not re wire anything, just cut out and re connected what was already there. Before and after removing the harness the codes and problems were the same.

 

Little more info: the wires cut out are on the engine harness. There are the 4 front O2 signal wires, 2 signal, 2 return. the big red power wire, which seems to be the power wire for all the sensors. A black wire that went to ground, and the pink and tan wires under the brake reservoir. Nothing was rewired just the "Y"'s cut out.

 

your problem is still in a harness, but not at that spot. The harness under the truck to the O2- transmission has a problem / short/ burn area.......... it was there before, and still there........ not the TEE you are working on.

 

Is it possible to remove the whole trans harness as it sits?
The front O2 sensors run through the engine harness correct? And the rears through the trans harness?
Im running out to see what I can do with the harness til it gets dark.

 

First I checked the trans harness connector. It was kinda dirty looking to i cleaned them good and tried again. Same issues so I pulled the trans harness. No apparent breaks or problems. I'm in the process of ohm testing each wire for conductivity.

 

Unhappy to report, there are 0 problems with the transmission harness. I checked every wire for conductivity, all check out fine with low resistance. No shorts, I checked every wire against every other wire. I already checked the harness for shorts to ground before I took it out.
I tested the rear O2 heater circuits also, I have seen them short inside the O2 before. Everything is good. The front upstream O2 sensors are not on the trans harness, just the rear O2's.
When I get the trans harness back in I will most likely pull the engine harness.