Engine RPM vs Coolant Temperature...HELP

[sdigs]

I'm getting tired of running down the highway with the heater on when it's 95 outside. Here's my setup:

94 XJ 4.0 HO 140k miles
RE 3.5" superflex
32x10.5 mud terrains
AW4 tranny
stock gears (i'm guessing 3.55)

The motor is stock. I never had over heating issues before i did the lift and tires but i only had the idiot light. After the lift i started seeing the idiot temp light coming on during long uphill grades.

I swapped in a new dash unit with tach and gauges, i also put in new temp and oil sending units to match the new gauges.

When it is 70* or below outside it runs around 210. Above 75* it runs at 210 around town or if i stay below 50 mph. At highway speeds of 65 mph or more it runs at 220 on flat roads and any uphill grade will push it up to 235. turning the A/C off helps but not much. Running the heater keeps it below 235 but it wont take it below 220 unless i get a long down hill. I just did a 200 mile off road trip and even when crawling in 4 low i rarely saw a temp above 215, until i hit the highway.

I have replaced the thermostat and fan clutch. I run a 2 row aluminum rad from Radiator barn, all installed in the last year. All hoses are new and the electric fan works as it should. 50/50 water/antifreeze

At 55 mph i turn 1750 rpm, 65 mph = 2000 rpm and 75 mph = 2250 rpm. all RPM's are when in drive, speeds are checked by GPS as i have not corrected the speedo.

Does this sound like a low rpm issue causing the engine to "Lug" and run hot or can anyone sugest something else?

I am planning on putting the stock tires back on and trying it out. the only other thing i can think of is a plugged catalytic converter but i just passed my emissions test last month. I'm getting ready to build an adventure trailer but i want to solve my temp issues before i add the load of a trailer to it. Any suggestions would be greatly appreciated.

 

[xcm]

I had allmost zero coolant in the head itself one time, basically a huge air bubble. causing overheating for MONTHS...
did a lot of parts-chasing, turns out the head was dry! filled it via the t-stat housing, and ive been good sinse...
only time the heater comes on now is when offroading on a hot day.

 

[CanMan]

Sounds like a combo of a few things. 1 might be the lower RPM's and bigger tires causing the engine to 'lug' as you have said. The radiator may only be a year old, but how well has the coolant been maintained? It could be slightly plugged as well.

Try running the smaller tires and see what happens.

 

[sdigs]

Radiator and coolant are only two months old. I have tried to burp it but i dont think i had the heater on so there may have been air trapped there.

 

[CanMan]

Id try to get all the air out(if there is any). 4.0's, at least for me, all get rid of the air in the system quite easily(work on them almost daily).

If thats all good, wouldnt hurt to throw on the stock tires and give that a shot.

 

[badron]

Check for air on closed system. This is a big problem with them.
Are you sure you are in overdrive you speed/RPM/tires size don't sound right to me. Someone not drunk run the numbers and get back to us PLEASE.
Please check the cat and the rest of the poop pipe for restriction but if the power at high RPM is there. I would not hold my breath.
Check for ANY codes.
Check for EX gasses in coolant.
Check for blockage in front of rad. Sorry if you find this stupped but it happens a lot to offers.
32 inch tires is a lot for 3.55 and big hills but should be doable. If you find you ARE in overdrive. Try locking OD and the lockup TC out and see if the temp drops a lot, if so. Don't use OD on hills or regear.

 

[ChevelleSSLS6]

How do ya remove all the air from the cyl. head? I removed the air from upper coolant hose while running the engine I squeezed it and you can see it bubble up in the radiator... not sure if that helped the cyl head much.

 

[jeeperjohn]

I find on mine that under a load the tranny fluid heats up the coolant before it leaves the radiator causing higher engine temps. An aux tranny cooler would probably help.

 

[derider]

I find on mine that under a load the tranny fluid heats up the coolant before it leaves the radiator causing higher engine temps. An aux tranny cooler would probably help.

bingo

try running it in drive not od and it should bring the temps down, your torque converter is working too hard and causing coolant temps to rise(mine does this with same tire size and 3.55 gears)

 

[sdigs]

My shifter has 1-2, 3 and D. I assume D is drive and overdrive. Will locking it in 3 give me drive without OD? I tried 3 a few days ago after it had got hot and did not see any extra cooling. Maybe i'll start out in 3 and see if it stays down.

What's the best way to burp it. Park with rearend uphill, run it up to temp with the heater on and remove the tremp sensor at the back of the head? Should the engine be running or off when i pull the sensor?

 

[avnsteve]

What's the best way to burp it. Park with rearend uphill, run it up to temp with the heater on and remove the tremp sensor at the back of the head? Should the engine be running or off when i pull the sensor?

yes and no, your temp sensor needs to be the highest point on the engine, as you may have noticed 4.0's sit butt end low in the cradle, you need to position your rig so that corner of the engine is highest (air bubbles rise) and yes, I do run the engine, I try to start with cool engine so's not to burn myself, and loosen the temp sensor initially before start up, once fluid comes out I'll snug up the sensor, plug it back in and let her warm up, i've done it HOT before and it just isn't pleasant

 

[Suddenator]

When it is 70* or below outside it runs around 210. Above 75* it runs at 210 around town or if i stay below 50 mph. At highway speeds of 65 mph or more it runs at 220 on flat roads and any uphill grade will push it up to 235. turning the A/C off helps but not much. Running the heater keeps it below 235 but it wont take it below 220 unless i get a long down hill. I just did a 200 mile off road trip and even when crawling in 4 low i rarely saw a temp above 215, until i hit the highway.

I know you just changed the fan clutch, but you might want to try a heavy duty one from a Grand Cherokee. They pull more air and you can really hear it kick in. I had the same lift, trans, gears & 33s, with the same overheating issues. I changed to thermostat to a 195 and replaced a new stock fan clutch with the HD one and no more problem.

 

[sdigs]

Does the fam make that much difference at highway speeds? I would figure that at 70 mph you're just ramming air through the rad and the fan is free wheeling.

 

[Suddenator]

That's exactly what I thought too. I think its just that the engine & trans are working much harder at highway speeds. I also switched from a 180 thermostat to a 195. That keeps the coolant in the radiator getting cooler longer before the t-stat opens.

 

[CanMan]

Does the fam make that much difference at highway speeds? I would figure that at 70 mph you're just ramming air through the rad and the fan is free wheeling.

The fan is designed to pull air through the radiator, not push air through it. So it helps pull it through faster somewhat. That'd defeat the purpose of having one if it was that way.

 

[derider]

My shifter has 1-2, 3 and D. I assume D is drive and overdrive. Will locking it in 3 give me drive without OD? I tried 3 a few days ago after it had got hot and did not see any extra cooling. Maybe i'll start out in 3 and see if it stays down.

I meant the 3 position, I don't know what is says there on mine so I guessed

good luck