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  #1  
Old 08-01-2007, 07:01 PM
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rreed rreed is offline
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Angry Truck overheats at highway speed but not in town

I've been having fits trying to get this thing to stop overheating. I can drive around town up to ~40 mph all day long in temps up to ~90 (so far) and the needle hovers just left of the center line (I think it's either an '85 or '79 dash, not sure). No issues.

Hit the highway at ~65 mph and temp starts climbing, once it gets much past the middle of the temp range it doesn't come back down. I can get off the highway and try to cruise town for a bit 30-40 mph since it runs fine at those speeds but it still just keeps heating up until it just overheats (needle in the red, knocking under load, boiling into the overflow jug, all hoses, radiator blistering hot). RPMs are about ~2800 at 65 mph (as fas as I'm comfortable taking it for lots of reasons).

360, stock motor, stock water pump (that is a little loose and will soon get replaced), new 180 t-stat, aftermarket four core radiator, all new hoses, tried replacing fan clutch to Hayden thermal (which does not seem to tighten up much even at overheat; same as old stock I pulled off it), stock 7-blade fan, fresh Prestone mixed 70/30. Radiator is clean inside and out (no bugs, clogging). I don't understand, I would have thought at speed it would get plenty of air through it; people generally tend to overheat putting around town in hot traffic. I don't get it. On the highway once it starts getting hot it just keeps going until I bring it home and park it. Putting around town it's fine all day long.
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Last edited by rreed : 08-01-2007 at 07:04 PM.
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  #2  
Old 08-01-2007, 07:24 PM
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First thing that came to mind: low water throughput. How I think it could happen.... loose or slipping belts, or bad waterpump.
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  #3  
Old 08-01-2007, 07:27 PM
GWChris GWChris is offline
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The common thread with these overheating threads seems to be the 4 row radiator.
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Old 08-01-2007, 07:39 PM
joe joe is offline
 
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You have a fan shroud? If not you need one.
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  #5  
Old 08-01-2007, 07:40 PM
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Quote:
Originally Posted by GWChris
The common thread with these overheating threads seems to be the 4 row radiator.

I'd agree with that except the overheating is coming at speeds high enough to force air through the fins. If it was overheating at city speeds, then yeah...

I'll x2 the water pump being weak.
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  #6  
Old 08-01-2007, 07:44 PM
davisa davisa is offline
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Have you had the system flushed? That bit me once in the butt.

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  #7  
Old 08-01-2007, 07:44 PM
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No fan shroud but as mentioned it's fine at low speeds/rpms in town all day long, even on a rather warm day of near 90 sitting in traffic. At highway speed it should have tons of air blowing through it esp. w/ everything clear (no bugs, etc. clogging up the fins). Belts are all good and tight, no squealing.
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  #8  
Old 08-01-2007, 08:07 PM
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Quote:
Originally Posted by scantar
I'd agree with that except the overheating is coming at speeds high enough to force air through the fins. If it was overheating at city speeds, then yeah...

I'll x2 the water pump being weak.
Actually it doesn't work that way. At hwy speeds the air is so turbulent from the bumper, grille, front axle, suspension(especially if lifted) etc the fan pulls the easiest air which is from around the sides of the rad rather than through it. At slow speeds the air isn't as disturbed and the fan can pull the air through the rad.
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Old 08-01-2007, 08:08 PM
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The ONLY thing that I would point at the 4 row for would be slow coolant flow. At higher speeds you might be getting more air through the fins but you're also running the engine hotter too and if the flow is restricted it's going to overheat pretty quick.

I pulled my thermostat out of my '87 and was absolutely floored by the river of coolant that went through the radiator.
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  #10  
Old 08-01-2007, 08:25 PM
GWChris GWChris is offline
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I dunno what to think about the fact that this one is overheating at speed, and whether that is relevant in regard to the 4-row. Airflow around a brick like these could be pretty unpredictable and vary a lot with speed.

If you think about it at the extremes, you can see that if you had a radiator with huge coolant flow and lots of surface area, but no airflow, it would be worthless. On the other end, if you had a lot of airflow, but coolant was restricted or the surface area was very small, you would again not pull much heat out of the coolant. So it has to be balanced.

It seems to me that the 4-rows may just be too restrictive of airflow, and things like lift, removing the airdam, etc just reduce airflow all the more.

OTOH, this problem may be something completely different.

Also, a 195deg thermostat makes for more efficiant heat transfer, as there is a greater temerature difference between the radiator and the air - but then I guess your coolant is getting to 195 (and beyond) just fine anyway!
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  #11  
Old 08-01-2007, 08:54 PM
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Your post says you have a stock 360 but doesn't mention whether or not it has been rebuilt. 360's can't handle much of an over bore. The thin cylinder walls inhibit heat transfer to the rest of the block if bored too much. A fan shroud would be a good place to start I think.
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  #12  
Old 08-01-2007, 09:21 PM
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If you had that truck in any kind of mud I would take another look at the radiator. I thought I had mine clean from last fall until things started heating up this spring. I must have spent an hour power washing the rad trying not to bend the fins but in the end I had to take it out of the truck and power wash it in the drive way to get it to clean enough to run cool again. It would idle all day long and cruise around the city fine but would heat up within 10 minutes of going on the freeway.
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  #13  
Old 08-02-2007, 05:01 AM
RottenDog RottenDog is offline
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Consider this when all the standard/obvious solutions might not help.

I wrestled with this problem for two summers in a row with a before and after rebuilt engine. High performance waterpump, thermostadt, fan shroud, bigger fan, trimmed wheel wells, aluminum radiator, new hoses, etc.,etc.,etc. and still had your same problem.

Went up a size on my jets (Truck Avenger), problem is gone even with my A/C on.
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  #14  
Old 08-02-2007, 07:32 AM
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Could the lower rad hose be collapsing at higher RPM's? that is the "suction" side of the pump and the lower hose should have a big coil spring inside it to prevent it from collapsing.

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Last edited by Tripwire : 08-02-2007 at 07:46 AM.
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  #15  
Old 08-02-2007, 07:43 AM
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CROCKETTBOONE CROCKETTBOONE is offline
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That black rubber thingy that was under the front bumper from the factory is an air dam. It was installed to divert air away from the engine compartment at highway speeds. If your is missing then it may be causing the overheating.

Just a guess on my part.
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  #16  
Old 08-02-2007, 07:51 AM
FSJ Guy FSJ Guy is offline
 
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Put a fan shroud in. The cooling "system" needs all parts of the system installed in order to work correctly.

Let us know what happens.
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  #17  
Old 08-02-2007, 07:57 AM
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When was the last time you adjusted your carb? If you are running lean, you can quickly build up some heat at highway temps.

Also, have you checked and rechecked for any leaks in the system and also bled the system with the nose up in the air?
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  #18  
Old 08-02-2007, 08:52 AM
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Quote:
Originally Posted by jpcoutts
Your post says you have a stock 360 but doesn't mention whether or not it has been rebuilt. 360's can't handle much of an over bore. The thin cylinder walls inhibit heat transfer to the rest of the block if bored too much. A fan shroud would be a good place to start I think.
I don't think that's true, at least when compared to say a 304 or other 'conventional' engines from the era. Certainly the 401 can overheat if overbored more than, say 40 thou. However, the bore size of the 401 approaches the maximum bore spacing for the AMC block, making the as-cast cylinder walls thinner.
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  #19  
Old 08-02-2007, 08:52 AM
d4xycrq d4xycrq is offline
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What are your RPMs at highway speed?

Do you know what your RPMs are at highway speed? You could be flirting with redline if you are in the 4s' (4.10:1, 4.30:1) I have an early Bronco that does exactly what your truck is doing. It has 4.10s, and I suspect the valves are not staying closed long enough to bleed off the heat! (at that high RPM...)

Meanwhile, I have a J-Truck as well. It overheats at highway speeds, too! But the big 'ole snow plow in front is the culprit!

Ray
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  #20  
Old 08-02-2007, 09:44 AM
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I remember a clutch going bad on a Merc I had................replaced the clutch and it would do exactly what yours is doing. I could drive around the city all day long never a problem. Get it out on the highway and kick it, would not take long I was on the side of the road.............waiting for it to cool down. After many frustrating hours of trying to figure out what was wrong........................Turned out I had put the fan on backwards when I changed the clutch, so I had turned it into a pusher fan on the wrong side of the rad
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