Cool in town; hot on the highway

[Guest]

Long story, here's the short version:

I replaced the water pump, thermostat and both hoses two years ago. A couple of months ago I noticed that my radiator had developed a leak up near the top, so I purchased a two row aluminum radiator and installed it a couple of weeks ago. Ever since then the jeep runs cool in town, but heats up on the highway.

My first thoughts were thermostat or fan clutch.....but why does it run cool in town?

Any ideas about where to start would be greatly appreciated.

HOOYAH!!!
Dan

[Black & Tan TJ]

Auto or manual trans? The temp should run at 210 with a 195 thermostat, at least that's where mine runs once warmed up. Too low it will create fault codes. A thermostat is probably the cheapest to change and maybe a good place to begin. My radiator also started leaking under the label next to the cap so I probably got the same china radiator you did and replaced the thermostat at the same time. Since I have a manual didn't have to run the trans fluid through the radiator. Another thing to check if auto trans, is it heating up on the highway. Food for thought.

Tom

[Guest]

Thanks Tom....sounds like our radiator leaks were in the same place.
The trans is manual, the thermostat is a 195; the jeep runs at 210 in town, but when I get on the highway it heats up to around 230ish. (that's a guesstimate based on the guage in the dash)

Well, there are only a few things it could be. Thermostat, waterpump, fan/clutch. I'll start with the thermostat, then go to the fan clutch.

[6spdYJ]

[/QUOTE]
Well, there are only a few things it could be. Thermostat, waterpump, fan/clutch. I'll start with the thermostat, then go to the fan clutch. [/QUOTE]

Don't forget the cap. I had a Nova do the same thing. At low speed the fan pulled plenty of air, but at higher RPM on the freeway the radiator acted like an air dam and wouldn't let enough air past.

Also, when I did the radiator in my YJ, I got a bad Stant radiator cap right off the shelf. Again it did the same thing.

[Guest]

Ok...I gotta ask because I've never heard this before. How can a bad radiator cap make it heat up?

Here is kinda what I was thinking....the fan is really not a good design anyway...so I'm wondering if my fan clutch is starting to go, and at higher rpms on the highway it just won't pull air. It could also be that the water pump just won't pump water efficiently at sustained highway rpms. Either way.....GRRRRRRR!

HOOYAH!!!
Dan

[OU812]

Water and coolant's boiling point rises with pressure. So if your cap is not maintaining pressure in the system your coolant will boil. The air in the system is far less effictive at transfering heat than air.

[Jeeperator]

Mine does the same exact thing. Personally I think it is not enough airflow going across the radiator. I have a 3 row rad with a ac condensor in front of it and winch in front of that. Noticed It ran hotter on freeway when the winch went on also. I am thinking an aluminum single row rad would work better than either 2 row brass or 3 row because it will allow air to pass through a bit easier. Not to mention alluminum disapates heat quicker.BTW changed cap, thermostat, rad and fan clutch.

[snydercj7]

I thought on the freeway the fan is not really used much? Did you burp the system properly?

[Lumpy Grits]

Make sure the rad. fins are clean and you can see threw them.
My bet is your fan clutch isn't turning the fan fast enough. Your fan etc is OEM, right??
Your running a fan shroud I hope.
Respectfully,
LG

[Lumpy Grits]

SynderCJ7 You running a shroud??
LG

[snydercj7]

Yes sir, factory shroud bolted to an Active radiator. I did a similar upgrade to my Jeep as Dan. If he did not start having the problem till after doing this it seemed more likely that the system was not properly burped rather then a fan clutch going out.

[Lumpy Grits]

99% of cooling systems will "burp" themownselfs after a few hot-cold cycles.
How far into the shroud does the fan sit? How well sealed is the shroud to the rad?
Respectfully,
LG

[Guest]

Thanks for all the input. I appreciate it.

I'm running the same set up that Jeeperator is except I have a 2row radiator.
The cap is brand new. Thermostat is about two years old.

Burping....I do it the same way every time:
Fill the radiator, start the jeep, let it heat to open the thermostat, continue filling and watching. Take it for a ride, let it cool, 'burp' it again.
If you guys know a better way, I'm all ears. The engine isn't heating up enough to "worry" me, but it is hotter than normal by 15-20 degrees. Normally runs 210 on the dot!

HOOYAH!!!
Dan

[Jeeperator]

Oh yea mines ran the about same temp with a 2 row or 3 row rad 3 row didnt make a difference. Temp gets hot but not in the red probably ~ 225-230 max.

[6spdYJ]

Just 'cause a cap is new don't mean it's good. Make sure the fan isn't too far into the shroud. 1/2" is good. If it's out of the shroud you just pull a bunch of air from the engine compartment and circulate it.