1990 4runner 4x4 manual, 3.0 engine from a 94 pickup, using all 1990 manifolds and wiring.

The wires going to my 02 sensor have been cut since before I bought the truck. It ran pretty good without it so i didn't bother soldering it till today. I crawled under and soldered the 4 wires from the o2 sensor to the factory harness. It went smoothly. I went to start the 4runner to leave and it cranked fast and smooth but wouldn't sputter or attempt to crank. First thing I did was undo my o2 sensor wires hoping I had wired something wrong, nothing, then pull a plug and hold it near ground (tried the fender and intake manifold which I have used successfully before), no spark, I put a noid light between the coil and distributor, no light. Then i noticed while sitting in the cab with the key on that a sort of electric whining noise I usually heard wasn't there. Normally when the key is in accessory i hear this noise, I had always thought it was the fuel pump, and normally gas would come out of the main fuel line and one of the small lines at the damper whenever the key is on, but that wasnt happening when i pulled them. So it appears I lost spark and fuel. It was already late and the only part i could find in town was a coil, I ran over and bought it and slapped it in, nothing. I checked all the fuses, in cab and in engine bay, none blown. What could cause this? Knock sensor wire? Could this have fried my ECU? All lights come on normally on the dash, fuel gauge comes up, says half tank, radio comes on, acts normal.

 

Most likely you shorted the O2 Sensor Heater wires, which blows the EFI fuse. This time, use your multimeter to check it; ATO fuses have a pair of slightly recessed metal tabs, that connect directly to the blades of the fuse. With key-on, test for battery voltage to ground on each tab. You should get battery voltage on each tab; if on only one, the fuse is blown. If on neither, something is broken upstream.

You'll probably need to figure out what is wrong with your O2 sensor wiring (your prior owner CUT them, so you know he didn't give a darn). Start by listing the wire colors you see, and someone can give you the multimeter test.

 

It was the EFI fuse, it was blown down on one of the tabs, It looked fine from up top. Normally i wouldn't have made a post about this but this is my only vehicle until I'm done working on my daily and I gotta be able to get to work. I tend to go worst-case thinking when I try to diag something. I had always assumed something happened on a trail that yanked the wiring, but now I definitely think it was cut due to a problem. The wires on the truck are pink/green, white/red, brown, and a thick black wire with extra heat insulation. The o2 sensor wiring is 2 white, 1 black, and 1 gray. I found a pinout on 4crawler saying black=signal+ gray=signal-, and the white wires are for the heater. I found a post on toyotanation saying that on the truck pink/green is negative and white/red is positive. I'm not gonna mess with this any more until I have my daily back seeing as this means I'll probably be running new wire to the ECU. There's also a wiring issue with my reverse lights popping my gauge fuse, I just disconnected the switch for now, the truck is really just a backup/trail rig for me so this isn't an urgent matter usually. I'm guessing something got pinched in the harness that runs down toward the transmission.

 

On the truck side, your information appears to be correct.
- White/pink is low side of heater - switched to ground by ECU when heat is called for
- White/red is high side of heater - comes from battery via EFI fuse.
- Brown is system ground
- Black (fat wire) is O2 sensor signal.

On the O2 sensor side, I don't know what the colors are. The colors you give make sense. You can find the heater terminals with your multimeter by simply measuring ohms between the terminals. The two heater terminals should measure a few ohms between them, and should be open to the other two terminals. The other two terminals (signal and ground) should measure open between them, or at least a very high resistance.

If the O2 sensor you used was healthy, it's hard to see how miswiring the connections could blow the EFI fuse, unless you inadvertently shorted the white/red wire to the brown wire while making the connections. The other possibility "might" be that you applied +12V across the actual O2 sensor, causing it to fail shorted. In which case, you'll need a new O2 sensor to move forward. This is just speculation, however. I have no actual idea what happens to an O2 sensor if you apply an overvoltage across the sensor terminals.

 

It's the o2 sensor that was in it when I got it, I tried to pull it out and the nuts were starting to strip with the wrench I had. so I figured I would wait until I could use one of our tech's snap on 6 point sockets. I'll go ahead and grab a new sensor and try to check for shorts afterward. Like I said I have the same issue with my reverse lights and my gauge fuse so I think the harness is exposed and shorting somewhere. Hopefully I can get my Pathfinder finished this weekend and dig into this some more. It does run pretty dang good with no 02 sensor though lol

 

If the harness isn't shorting and blowing the EFI fuse while disconnected from the O2 sensor, it's unlikely the problem is in the harness. Assuming you've done your splicing properly, and it sounds like you have, it's more likely a shorted O2 sensor, which is probably why the PO took the shortcut way out and just cut the harness wires. Measure all of the O2 sensor wires to truck ground. They should all be open, except possibly the gray wire. (signal-).

It's possible for the engine to run well with the O2 sensor disconnected, but only if the VAFM and injectors are tweaked optimally for each other. However, without a working O2 sensor, the ECU has no information to make on-the-fly adjustments, so changes in temperature, altitude, etc. will result in non-optimum mixtures, resulting in lower power and/or decreased fuel economy, increased emissions, and possible damage to the cat converter.

BTW, it's a good idea to soak the O2 sensor nuts with PB Blaster or similar for a day or two before trying to remove them. Be careful putting too much torque on them. If you twist off the studs you'll be making a visit to your local exhaust shop to get a new mounting flange welded in.