Just bought a 2000 jeep cherokee xj sport 4 door 4.0 inline 6 motor ran fine from Denton to rockwall it's like an hour and a half drive. Temp was fine whole way home. Cranked it back up to move back into drive wat and the oil pressure gauge went to zero oil pressure was reading fine whole way home. After that my temp sensor went to red over heated to 270 the whole way home it was fine thought it sat at 190 to 210. All my radiato guild went to overfill. Also steady drip from transmission at the top of the transmission where the engine and transmission meet the is my first vehicle and would really like to find out problem and get it fix ty for ur Time

 

Sigh. Another one. Some things to verify first:

The oil pressure senders can fail on these Jeeps, giving a false 0 psi reading at idle. The way you test that is to get a mechanical oil pressure test gauge ($25 at Harbor Freight) and verify your oil pressure. Did it make any rattling noises from the top end of the engine when it showed low oil pressure? If so, the sound likely verifies the oil pressure issue for you without having to test further.

Likewise, temperature sensors can fail. You can test that by getting an IR thermometer (the kind with the laser) and point it at the thermostat housing.

Unfortunately, you may have bought a lemon.

The 2000 and 2001 XJs had the infamous 0331 head. If you search the board, you'll find volumes written about it. This head had a casting flaw that causes some of them to crack between the #3 and #4 cylinders, allowing coolant to mix with the oil. If it's allowed to go long enough like that, the coolant will destroy the bearings. One of the symptoms of worn bearings is low oil pressure.

The crack also allows hot exhaust gas to enter the cooling system, boiling the water out of the coolant and leaving a nasty brown sludge behind that clogs up the radiator and eats the water pump. That crap sticks to everything. The head can crack without having been overheated, but then the sludge subsequently causes overheating problems by gumming up the cooling system.

A lot of 2000 and 2001 XJs are hitting the market these days, and in some of them the heads are cracked and the sellers have masked the symptoms long enough to get them sold by flushing the cooling system and changing the oil right before someone test drives the truck. After the new owner drives it a couple hundred miles, enough coolant has gotten into the oil to thin it enough that it starts showing oil pressure problems again.

I'm not saying definitively that this is what has happened to you, but it sure does sound like it.

Some things to check:

Open the radiator cap (while the Jeep is cool). Is there brown sludge in the neck? Look in the plastic radiator overflow bottle on the passenger side. Is there brown sludge in there? If so, you'll need a cooling system overhaul at a minimum.

Pull the oil filler cap in the center of the valve cover. Is there milky crap on the bottom of it?

Look down into the oil filler hole with a light. Sometimes you can actually see the crack in the head right there below the filler hole, or you can see coolant weeping up through it.

Check the the oil on the dipstick. Sometimes coolant in the oil can cause it to become lighter in color or get a milky look, but it's not always obvious, especially with the newer oils. If coolant is getting into the oil, the oil level can also rise on the dipstick. Is the level higher than it should be? The only sure way to know if you have coolant in your oil is to send off a sample to Blackstone for analysis, but I suspect you won't have to go that far to know.

If the head is cracked, at a minimum it will need to be replaced. If the bearings are worn, you're looking at a full engine swap or rebuild. How handy are you with car repair? If this is your first car, you may be biting off a little more than you can chew.

Out of curiosity, how much did you pay for this thing?