check the color on the wires. On the 88, the wire colors are:

fuel injectors:
(Blue and White) wires or (White/Red and Yellow) wires.


Cold start injector:
Green(?) and Black

 

Yeah... #2 injector looks like is has the wrong plug on it... all four should be the same grey socket.

My guess is that the mystery plug goes to injector #2 and the one on injector #2 goes to the cold start injector.

 

The mystery plug is the air conditioning idle up vsv wire, stated once before. You don't have a/c so you don't need it. It used to be taped back to your harness till you ripped it off to try to plug it in some where. It will not plug into the injector.

The injector connectors are normal to be grey and blue like that.

 

I have this exact same problem, but I looked at my before pictures and the plug is sitting there not plugged into anything before I began wrenching.

 

do a continuity check from the computer to the plug to be shure. if you can get a breakdown for the pinouts on your year computer.

 

Awesome - thanks for the confirmation.

 

Unofficial story I once read said this (and other) mystery plugs are there because the wiring harness may also be used on different platforms, with common accessories located in different places. IE Alternator locations.

I can't quote a source for this, but a search will find a few other threads related to other mystery plugs.

Turn the key and hit the pedal.

 

Here's a crop from one of my before pictures. I put a circle on the plug to help you see where it is. Also, I checked my brother's 1990 4runner with a 22RE this evening and he has the same plug there as well. It isn't connected to anything. It must be that A/C plug.


My headgasket/rebuild hit a snag. My timing cover leaked like crazy as well as my water pump. DON'T CHEAP OUT ON GASKETS! I bought all my gaskets from the dealer except these ones because they came free with my eBay used timing cover that I picked up. Plus, I didn't use any sealant because the FSM says to just use the gasket. Well, if you're using super thin cheap-o gaskets, it's probably a good idea to use sealant. Mine started up fine and then misted the engine bay with coolant. Needless to say, I've ordered some new gaskets and I will have to remove the timing cover and put it back on with the better quality gaskets. I will be using a little bit of FIPG sealant when I put the timing cover back on as well. Still, it was neat to hear it start up again.

 

My truck has a third vacuum controlled sensor or modulator (whatever those things are) on top of the valve cover, notice how the plugs are color coded. You just don't seem to have the light gray thingy that looks a lot like blue and reddish ones. Hows that for a technical anwser

 

I checked a 94 4x4 Toy PU EXACTLY like mine (accosted the owner in the Schuck's parking lot and asked if I could look under his hood) and it too has that plug unoccupied, so I am 100% confident that it is unused. Thanks a ton to all who responded here.

 

That's funny, I just had the same problem a couple of weeks ago. Check out my thread:
https://www.yotatech.com/forums/f116/i-dont-know-where-these-plugs-go-help-112527/
Before I found out where it went, i was driving out the truck just fine. When I plugged it in, it ran the same. You may as well plug it in. Go to the end of my thread, there is a picture of the plug plugged in. It's hard to feel the plug in the engine, and almost impossible to see. Good luck!