It has always run at the correct temperature. Winter or summer - its always gets to about middle on the guage and stays there.

Well, that has changed. The only thing that has happened was an oil change (i changed from 10w to 5w) but that shouldnt have ANY effect on my engine temperature... i wouldnt think.

So now it does this deal where I crank it... and it stays cold... and stays there. Until I get it out on the interstate and run it at 2300RPM for a while... if I do that it will actually start to heat up. But as soon as I am back on the surface roads, and I take it down from a constant high rpm, it cools back off.

So whats wrong, how do i fix it, and how much does it cost.?

 

It may just be the gauge has started to wear out. The thermostat may be sticking also. Check coolant levels/hoses just in case.

 

After one of your highway runs, pop the hood and feel the upper radiator hose, it should be too hot for you to hold onto for any extended period of time.

If it's hot, your thermostat is fine. If it's not hot, the thermostat's probably stuck open.

 

thats what it sounds like is the thermostat is stuck open

 

Ok, the hose is cooler... so the temperature guage is reading correctly. How do I solve this stuck-open thermostat?

 

Replace it. No other way to fix it.

Since it's a 4.0L, the procedure is actually pretty easy. Buy a replacement from your local auto parts store. Don't go cheap on it, get a good one. Make sure it comes with the rubber gasket around the edge!

Remove the upper radiator hose from the thermostat housing. It's got a squeeze clamp on it, squeeze it with pliars and slide down the hose, then get a screwdriver gently under the edge and pry it off.
The thermostat housing is held on with three bolts in a triangular formation. I seem to remember them being 10 or 12 MM. The problem with these bolts is that if they're dropped, they are jammed in a VERY inconvenient location and need a LONG magnet to get them out. So be careful and go slow. You'll need a U-joint to make it any sort of decently fast operation. I've done it with just a wrench and it's taken 15 minutes to get the bolts off, 1/8 turn at a time...
Once the bolts are off, just remove the housing. It may take some force if it's never been replaced. Go straight out as far as you can with it then up. Antifreeze will spill out. Yank out the old thermostat, replace it with the new one, making extra sure to install it the correct way and with the vent up, clean the sealing face of the housing, replace housing, bolt back in the housing, reattach the upper radiator hose, clamp it down, start truck.
Let it warm up fully, confirm the thermostat opens by the upper radiator hose getting hot, and then top off the coolant as necessary.

 

mine is going too on a four banger i drive it on the highway and it reads cold if i let it idle she warms up partially until i drive then it only goes up half way if i shut it down and let it sit it goes to normal til i start running it again

 

Yep ranger1999 Bob, sounds just like a thermostat going out. Lots of things can cause an overheating engine, but an overcooled engine typically only has one cause, a bad thermostat.

 

Ok, i looked up replacement OEM parts... seems there are different temperatures at which some of the thermostats engage... some are 180, some 190, or 195. There's even a 198 in there. So what temperature thermostat do I buy?

http://www.partsamerica.com/ProductL...OE+Replacement

 

Factory is 195.

 

Stay with 195 or 190... I ran a 198 for a while and it got scary at times, would happily run up to 205 or so before coming back down in traffic. I run a 190 now without issues.

Don't run a 180, that's entirely too low. 195 or 190, take your pick.

 

Alright, I think 190 is what I'll do. The Motorcraft™ Thermostat is a 190, so I guess that makes sense.

 

I know this is in my directions up there, but I just wanna say it again, make SURE the vent is installed facing up.