My Heep overheats on the highway, but runs great in the city. I can get off the exit at 30-40 degrees hotter than normal, and jump into stop-and-go traffic, and the gauge will go down to about 190-200 every time, and stay there.

It could be the gauge, maybe, but it's way too consistent and predictable. My cooling system was shot, so I've replaced nearly the entire thing:

New rad

New water pump (not rebuilt)

Thermostat (modified, cut out the spring and pellet just to make it into a reducer for the opening)

Fan clutch

Upper and lower rad hoses

Flushed system

I don't boil over, or lose water in another way (I'd been thinking head gasket, maybe? Oil's clean). The only thing I can think of is that I accidentally mixed different color coolant when I did the water pump (put in one bottle of orange, the rest was green), but it has since been flushed and it's all green now. Besides, it was overheating the same way before. How big a deal would this be, anyway?

The timing's fine, too.

I'm stumped. Any ideas? I'm gonna try flushing it yet again, but that's all I can think of. On the highway, I turn on the heater and it cools down most of the time.

1987 YJ, 4.2/auto. TIA

Check your upper and lower radiator hoses for areas that could pinch together when the vehicle is warmed up... The hoses get very pliable when hot, and the lower hose could be getting "sucked closed" at higher, or steady-continuous RPMs. Driving around town your RPMs are going up and down, so maybe it never builds up enough pressure to impede flow... Likewise, The upper hose could be fine until it starts getting some serious flow and that could be causing a bend to close off...

I dunno. Worth checking I suppose...

If this does turn out to be the problem, you can fix it with one of those springs that goes inside the hose to keep it open, or puting a hose clamp on a spot that is being pinched and just clamping it enough to keep the hose round. I had to use a spring in the bottom hose and two extra hose clamps on the upper hose in order to keep my hoses round enough to cool the Ramjet when I did my swap... Cools like a cucumber now.

It could also be a problem with engine timeing (too far retarded) or air/fuel mixture (too rich) causing the engine to run hot, but if it runs fine and/or you haven't messed with the timing or carb recently, these are unlikely to be the root of the problem.

Good luck.

HTH,
Jake

I had the same problem last year...

I have a Chevy 305 in my scout with a 5 core triple pass radiator..ran cool on the trail and around town but on the freeway(especially uphill:eek: ) it would climb to 240º...turns out that my extremely-small finned radiator (you can't even see through it!) turns into a wall at speed. I put some matting around it to force air through it and it solved the problem. Could this be your problem?

Thanks for the replies ...

Scout Dude, what do you mean by matting? Like a bigger fan shroud?

Jake ... both hoses are new, but the bottom one does have a bit of space in it without a spring (it's too short), so mauybe I'll try a clamp in the middle of that area. The timing chain was recently replaced, but it was overheating before that.

He means airflow. Is your radiator getting PLENTY of air across it? Is there any major gaps around the radiator where air would go around the radiator instead of through it? Basically a fan shround on the other side of the radiator funneling the air into the radiator.

Originally posted by The Rockslut
He means airflow. Is your radiator getting PLENTY of air across it? Is there any major gaps around the radiator where air would go around the radiator instead of through it? Basically a fan shround on the other side of the radiator funneling the air into the radiator.

It's got the A/C condenser (ya I know, but it came with it and it still works so I'm leaving it on for now dammit :D ) in the way, but that was never a problem before.

What vehicle are we talking about here? If its a CJ/YJ/TJ the radiator mounts where air has to go across the radiator. Do you have a winch up front blocking air?


The A/C condensor will not cause this problem unless it is plugged with road debris and bugs, or the fins are pushed over

Rockslut, it's a YJ with those big-a$$ slots for a grill ... no winch (yet), condenser has seen better days but isn't so bad.

I'm thinking it's a pressure problem, but if it's an airflow thing then I'll stick a universal electric fan in there, in addition to the motor-driven blades, and the hell with it. This is driving me crazy ... I gotta drive out of town for decent wheeling, and I can't 'cause it won't make it through a road trip :(

Well I am an idiot and didnt read the bottom of your first post. YJ 4.2

If you are not losing fluid I would highly recommend running an aftermarket gauge to see if you are truly running that hot. I have seen factory gauges that were 10* off.

I have seen a 4.2 that was too advanced do about the same thing that you have described. Fine everywhere but on the highway.

You didnt list the radiator cap in your replacement parts, did that get changed also? Correct pressure?

Mate - my 97TJ seems to be doing the same thing... around town etc it sits below 100degC (say 200 F), but on the highway it is at about 105degC (or around 220 F).

Not that I can help you at all, cause I have no idea what is going on. I have changed my radiator hoses, and the radiator was changed only about 12 months ago.

I do have a winch, but I also raised my motor 1" and have a 1" body lift so that helps to clear the winch for airflow. I have no boil over etc either.

B]Is this dmaging my motor??[/B]

Originally posted by FireTruck
Is this dmaging my motor??

No. 220 degrees won't damage your motor.

I've had the same problem and it turned out to be the transmission that was causing the overheating at freeway speeds. The fix was to add an aftermarket tranny cooler and disconnect the lines through the radiator.(don't plumb them in series like most aftermarket instructions will tell you to do) I guess with the extra burden that big tires, winches and accesories put on the trans, it works so hard that it actually heats up the coolant at freeway speeds. Took a lot of trial and error to figure this one out ,but I have not overheated since.

Did you replace the stock water pump with a high flow one? Some times modifying the cooling system by installing a high flow water pump or high flow thermostat etc. will actually increase your engine temp slightly by not allowing the coolant enough time to cool in the radiator at high RPMs'. Just something else to think about.

The only thing I can think of, is taking some of the burden off of the radiator by installing an aftermarket tranny cooler and/or oil cooler.
Oldmanscj7 may have a point with the hi-flow water pump theory, I've heard about that before. (bigger pully to underdrive it a little maybe? I don't know.)

Well, after having a bit more of a critical look at my air flow...

I do have an ARB bar, and the middle loop blocks a little of the grill. Then there are the 2 spot lights, and the winch. To protect my winch in deep water and mud, I mounted my low profile number plate in front of it (like a sheild).

Beats the crap out of me how any air gets in there at highway speeds. Time to shuffle some stuff around I guess :rolleyes:

get a heat gun and next time ur running down the highway.. and it starts to over heat get out and shoot ur engine.. it will tell u exccatly howmuch heat is comming from ur engine..

repeat this step after city driving ..
bad gauge orconnection sum where.. bad temp line.. etc.. this is the cheapest way to actually check the temp!

Originally posted by Bandit
Thermostat (modified, cut out the spring and pellet just to make it into a reducer for the opening)
Upper and lower rad hoses


My first thought is the thermostat is allowing too much coolant to pass through and by doing that it doesn't let the coolant to stay in the radiator long enough to give up heat. Next does the lower hose have a spring in it? If not it may be partially collapsing at higher rpm restricting flow. Next I agree with the aux trans cooler.
:cool:

OK, looks like it might be fixed ...

The timing was off by about 5 degrees, too retarded. That'll do it:rolleyes:

I had not checked this properly before since my mechanic did the timing chain and it'd been running fine. Just to be sure, I called him and asked to use his timing light (a friend has mine and he's up in Jersey right now), and he says - get this - "I did it by ear, 'cause my light is broken". WTF?? The only reason I even took it in was 'cause of school and work - I had no time to be digging into the Jeep in the middle of the week. I've done timing chains before and it would've been a cinch on the Heep. I should've just done it myself like I normally would .. live and learn.

Anyway, I splurged on another timing light and sure enough, that was it. It still runs a little hotter than it used to when on the highway, but no big deal at all, maybe 10 degrees over. I imagine that, as has been pointed out, the modded 'stat doesn't let the water cool off as well anymore. I'll swap in a 160 soon.

Thanks for all the help, especially the Rockslut who mentioned the 4.2 that overheated due to timing. I was gonna dismiss this entirely since in my experience, timing that's off would cause the motor to run hot all the time, and not as much as 40 degrees higher. But my cooling system was shot anyway, the rad had clogged slots and the fan clutch had some play in it - I was boiling over and spewing coolant out before I replaced them.

I need to get me a damn driveway with good lighting so I can work on the Heep at night if need be ... Anyway, thanks again.

Originally posted by Bandit
Thanks for all the help, especially the Rockslut who mentioned the 4.2 that overheated due to timing.

Hey!!! Check the last paragraph of my first post!

:flipoff2: :flipoff2: :flipoff2:
Jake

Originally posted by Jakesteramalamajama


Hey!!! Check the last paragraph of my first post!

:flipoff2: :flipoff2: :flipoff2:
Jake

LOL, you're right. Sorry:D :flipoff2:

Actually there are several good ideas here (aux. trans cooler, clamp on the hose to prevent pinching, underdrive pulleys, etc). Really good stuff. I can't believe it was something so simple, though ... too bad you guys can't hear the phone call I'm gonna make to my mechanic this morning, ya'all might like it.

Originally posted by Jakesteramalamajama


Hey!!! Check the last paragraph of my first post!

:flipoff2: :flipoff2: :flipoff2:
Jake


HEY! Dont take ALL of my glory :flipoff2: