I have a 98 XJ Classic that is overheating in about 10 minutes when I drive it 50mph or more. I've let it get up to 235 degrees before letting it cool down to avoid any damage. It only overheats when under load and seems to stay around 210 on idle. This started when I replaced the radiator, water pump, thermostat and fan clutch.

Here is a parts list for reference

• Mishimoto Radiator
• ACDelco Water Pump
• Stant - 195 Degree; Superstat Thermostat
• Fan Clutch

When installing the Mishimoto radiator, I ended up snapping off the aluminum overflow outlet nipple so I had to drill and tap the hole for a slightly larger one. When I did this, I nicked the inside of the filler but I don't think it would be enough to cause any issues. I am not leaking any coolant. Could this cause the issue? See Photo

I am also wondering if the water pump has the correct impeller rotation? Rock Auto shows it to have a reverse rotation. I did not confirm if there was an "R" on the impeller when I installed it. I don't see any indication of a blown head gasket. The oil looks good. Also, there doesn't seem to be much pressure in the upper radiator hose when pinching it so I am wondering if its a coolant flow issue?

My next step is going to be to remove the thermostat and do a full coolant flush. When I installed the components I wasn't in a place where I could do a thorough flush and later noticed that my heater wasn't very hot. I have since back-flushed the heater core and a lot of gunk came out. The heat is working well now but I feel I should flush the entire system.

Any advice would be appreciated.

 

I just had problems with a new engine that just runs hot.
I had a two row CSF radiator, and a crappy three fan set up.

So I just added a Mishimoto radiator, and a factory e fan on the drivers side, and something different on the passenger side.
The Mishimoto rad made a HUGE improvement in my cooling (now runs right at 210 or lower under load).
The Mishimoto is a good 1/2" wider than the factory radiator.

Did you check your thermostat before installing it?
If you have no movement in your fluid, that would be the first place I would look before doing anything else.
Test it like this:



Use a meat thermometer, and watch as the temp comes up in the boiling water.
If it does not open up, well.............. Yeah that Tstat is no good.

On the nick in the rad cap area.......... Well yeah if it is not able to hold pressure around that area it could be a problem too.
Get a radiator pressure tester from the Mc Parts store, and check to see if it holds pressure. They loan them out.
The system should hold pressure at 16psi for about 6-10 mins.

You have done everything right, and it should be cooling better than before?
BTW
You do have the fan shroud on the mechanical fan right?
If not they are readily available as either a MOPAR replacement or an after market like Crown or OMIX.



EDIT: I have that same water pump and it works fine.