My experienced friend and I rebuilt my jeeps engine. We ended up using a head off a 97, the original head was a 94 -95. On the back right corner there is some type of sensor that threads into the(94-95) head into where the water goes. Looks like a nut. The 97 head doesn't have it, so before its drilled and tapped can some tell me what it is and what's it for?

temp sensor for the ecu i believe

Temp sender for the gauge. Pipe thread. 1/4" IIRC.

That would be the temperature sensor to drive the gage in the IP - drill & tap 1/8" NPT, in the same location (recommend you centre drill - or at least a good centre punch and pilot hole - before you drill it out for real!)

Use never-seez on the screw threads, tighten finger tight, then an additional 1/4- to 1/3-turn with a wrench (you should not need more than that.)

The sensor for the ECU is found in the thermostat housing, it generates a signal for the ECU's reference, and for the ECU to energize the fan relay.

This sensor was omitted for OBD-II, and the IP gage is driven by the CAN bus instead (OBD-II data stream) I do not believe the water passage in the head was altered, merely that the boss went unmachined.

EDIT - Cruiser seems to think it's 1/4" NPT, but I'm pretty sure it's 1/8" NPT. To make sure, take the sensor with you when you go to get the drill & tap - stop off in plumbing and thread the sensor into a union of each size. It will only fit one - please confirm once you have done so, for the rest of the class...

Could be. That's why I typed IIRC!! Hmm. Not so sure now. Isn't the oil sender 1/8" NPT and the temp sender 1/4" NPT? I don't feel like pulling down the ladder to the garage attic and verifying right now. For some reason, my balance might be off this time of night........

I survived the trek up there. 1/8" NPT on oil sender and temp sender. CTS and IAT are 1/4" NPT.

It would be easier just get a new t-stat housing and put it in there since that's where the later models (97-01) have them. You can get a new housing that already has the threaded hole. I know that a few people on here have done this.

That was done 1991-up.

1987-1990 had three temperature sensor - driver's side of the block (behind the exhaust, for the ECU, M20-1.5 straight thread, I think - a Peugeot or Renault part also fits there with lower operating temperatures,) driver's side radiator tank (thermal fan switch, 3/8" NPT, I think) and driver's side rear of cylinder head (IP gage, 1/8" NPT.)

1991-1995 had two: the driver's side rear of the cylinder head (for the IP gage,) and the other two were combined into a single sensor in the thermostat housing (threaded 3/8" NPT.)

1996-up (OBD-II) rigs have a single sensor - in the thermostat housing.

Therefore, he should already have the later thermostat housing.

Also, the upper rear of the cylinder head is pretty much the hottest part of the cooling jacket - it's a sort of "flow pocket," and has a reduced flow rate for coolant. If you move the sensor to the front of the head (better coolant flow,) you'll wan to keep a better eye on it, since it won't tell you what the "hot-spot" temperature is. I prefer having the sensor at the rear of the head, since that will tell me much sooner if the engine is overheating!

CTS on the block, for the ECU, is 1/4" NPT. Radiator sensor is metric.

Ah - thanks for that. Most of the CTS units I've seen (in general) were 3/8" NPT, and I hadn't had an XJ one out yet.

Do you happen to know the threading on the RENIX drain plug? I want to put an elbow and a cable-operated valve in there in my refit, I just haven't had one out yet (I already know it's a 5/16" square drive to get the thing out - you can easily make a tool to do the job with a 5/16" or 8m/m cheap-O hex key and a grinder in about five minutes.)

I believe the block drain plugs are 1/4" NPT but with a 5/16 square drive.

To give you all a better idea

Yep. That's it.

Remove the remnants and put a 1/8" pipe plug in it's place.