Truck overheats at highway speed but not in town

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts
  • rreed
    350 Buick
    • Aug 21, 2006
    • 1472

    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.
    Last edited by rreed; 08-01-2007, 06:04 PM.
    47 Willys
    75 CJ-5
    81 Scrambler
    76/79/80/81/85 J20 (all the same truck)
    86 Grand Wagoneer - FOR SALE!!!
    96 ZJ

    Member, FSJ Prissy Restoration Association

    You can't drink all day if you don't start in the morning.
  • Brown Bear
    304 AMC
    • Apr 09, 2000
    • 2334

    #2
    First thing that came to mind: low water throughput. How I think it could happen.... loose or slipping belts, or bad waterpump.
    NathanielButts
    IFSJA Member #18
    RIP-USS Minnow-The three hour tour is over.
    MacGuyver - 1985 Grand Wagoneer
    TFI, Edel Intake, Holley 80457S (yeah, it's shiney), K8600, K&N, Taylor Wires, Ford HD springs, and way freakin' cooooooool

    Comment

    • GWChris
      304 AMC
      • Jan 22, 2005
      • 1798

      #3
      The common thread with these overheating threads seems to be the 4 row radiator.

      Comment

      • joe
        • Apr 28, 2000
        • 22392

        #4
        You have a fan shroud? If not you need one.
        joe
        "Don't mind me. I'm just here for the alibi"

        Comment

        • scantar
          AMC 4 OH! 1
          • Jul 16, 2005
          • 4296

          #5
          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.
          '76 Waggy, TH400, Quadratrac and a fourOH!one Sold

          My other car is a Viner Pro Team...no wait, my other car is a Masi Speciale...nonono, my other car is a Bianchi 928SL

          Comment

          • davisa
            232 I6
            • Mar 17, 2006
            • 130

            #6
            Have you had the system flushed? That bit me once in the butt.

            Davisa
            71 Gladiator, 350 Buick th400.
            A coat of many colors, mostly rust.

            Comment

            • rreed
              350 Buick
              • Aug 21, 2006
              • 1472

              #7
              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.
              47 Willys
              75 CJ-5
              81 Scrambler
              76/79/80/81/85 J20 (all the same truck)
              86 Grand Wagoneer - FOR SALE!!!
              96 ZJ

              Member, FSJ Prissy Restoration Association

              You can't drink all day if you don't start in the morning.

              Comment

              • joe
                • Apr 28, 2000
                • 22392

                #8
                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.
                joe
                "Don't mind me. I'm just here for the alibi"

                Comment

                • scantar
                  AMC 4 OH! 1
                  • Jul 16, 2005
                  • 4296

                  #9
                  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.
                  '76 Waggy, TH400, Quadratrac and a fourOH!one Sold

                  My other car is a Viner Pro Team...no wait, my other car is a Masi Speciale...nonono, my other car is a Bianchi 928SL

                  Comment

                  • GWChris
                    304 AMC
                    • Jan 22, 2005
                    • 1798

                    #10
                    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!

                    Comment

                    • jpcoutts
                      304 AMC
                      • Jun 28, 2002
                      • 2107

                      #11
                      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.
                      Jim C
                      '67 J3000 Dually
                      '86 J20
                      '79 CJ5(in pieces)
                      '86 CJ7 Doesn't need anything(so why do I have it?)
                      Correction- it just needed a water pump!

                      Comment

                      • djmac
                        350 Buick
                        • Aug 19, 2003
                        • 968

                        #12
                        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.
                        82 J10 SOA 6" SPRINGS D60-D70 W/DETROIT LOCKERS 38x15.5 TSL'S on 12" M/T CLASSICS 460 V-8 C-6 CUSTOM EFIS EFI AND HYDROBOOST

                        Comment

                        • RottenDog
                          258 I6
                          • Jun 13, 2001
                          • 254

                          #13
                          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.
                          77 J10 Fuel Injected 401/700R4/20 with Thorleys D44s (ElecTracs+4.10s+4wheel discs)35 BFGs A/C and 17 dash toggle switches

                          72 Commando = my next Money Pit

                          Comment

                          • Tripwire
                            AMC 4 OH! 1
                            • Jul 30, 2000
                            • 4656

                            #14
                            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.

                            Trip
                            Last edited by Tripwire; 08-02-2007, 06:46 AM.
                            Abort? Retry? Ignore? >

                            86 GrandWag. Howell fuel Injected 360. MSD Ignition + Dizzy. 727/229 swap BJ's 2" Lift and 31's

                            88 Wrangler 4.2, Howell TBI and MSD - Borla Headers w/ Cat-back + winch and 31's AND a M416 trailer (-:

                            Comment

                            • CROCKETTBOONE
                              350 Buick
                              • Jan 14, 2005
                              • 924

                              #15
                              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.
                              An opinion is just that, an opinion. A fact is a fact, and thought to be true. Please dont confuse an opinion with a fact. Life: Its sexually transmitted and always Fatal.

                              Comment

                              Working...
                              X