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Overheating at freeway speeds, but cool on the trail...what gives?

Discussion in 'The Garage' started by zakk, May 28, 2003.

  1. zakk

    zakk 1/2 ton status

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    Location:
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    Truck Specs

    · GMPP 350 Crate Motor w/ headers, manifold, HEI, Q-jet (285 HP or so)
    · TH350 with Mild Shift Kit, 24,000# B&M Stacked Fin Trans Cooler
    · 5.13’s in the D60/14B 39.5” TSL’s
    · Stock radiator, still have A/C condenser in front of it.
    · 185* Thermostat, straight water in the radiator
    · Water pump is a Remanufactured Unit less than 2 yrs. old.
    · Clutch mechanical fan, shroud still on

    OK, after a fun weekend, the rig was overheating badly on the way home. Would get up to 210* before I would have to shut it down a wait for it to cool to try to limp it home. The trans temp would go up after the water temp had, so I suspect it is the radiator heating up the fluid.

    On the trail it was fine, ran nice and cool. No problems at all.

    What can it be? The motor turns about 2900 on the freeway doing 65-70MPH. It never overheated (the water temp anyway) with the 4.10’s. Is the motor working harder with the 5.13 gearing? I have always has trans temp problems…not water temp problems.

    One friend thinks I need a big block or diesel radiator. What does the Jury of CK5.com think?

    Thanks!
     
  2. yellowK20

    yellowK20 Well Lubricated Premium Member GMOTM Winner

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    If you don't have A.C. I'd get rid of the condenser it's restricting some of your airflow A bigger radiatormight help but it's Over haeting at Highway speeds and not at slow speeds Maybe your fan clutch is disengaging prematurely and not pulling enough air through
     
  3. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    You should be running some sort of coolant, not straight water. It cools better than straight water for one, and doesn't corrode everything...

    You may need a bigger rad, it could just be the engine is creating more heat spinning 2900 than the cooling system can get rid of.

    Rene
     
  4. BlazerGuy

    BlazerGuy 3/4 ton status

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    Well if the truck just started overheating on the way home from the trail, part of the cooling may have been damaged. Pull the thermostat and make sure it's still functional or just replace it if it's old. How old is the radiator? Since you said that you run water only in it, it may have corrosion build-up inside. Upgrading to a 4-row or a large 2 row rad is always a good idea, CK5 members get a discount on rads at Radiator.com I would also check the obvious crap like water level in rad, check the sending unit to make sure the gauge readings are correct, fan clutch etc. In the meantime I would bypass the radiator trans cooler and just use the aux cooler so the trans will not be affected by the motor. Also what was the ambient temperature, I don't think it would matter though since the temp was probably the same temp at the trail... My K5 always runs 200-210 in the summer since it's hot outside.
     
  5. RingMaster4x4

    RingMaster4x4 1/2 ton status Premium Member

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    [ QUOTE ]
    On the trail it was fine, ran nice and cool. No problems at all.

    [/ QUOTE ]


    My blazer does the same thing. I put dual elec. Fans on the radiator, trashed the condenser and put a remote tranny on it. All that help a little but not much.
     
  6. big pappa b

    big pappa b 3/4 ton status

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    It sounds like the thermostat went out if it just started on the way home. That's the cheapest place to start with
     
  7. BILLY RAY

    BILLY RAY 1/2 ton status

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    RADIATOR!

    this is not real uncommon. I have seen a lot of car in our shop that do the same thing and the radiator seems to fix them. You may run your hand all over the rad. to see if has some cold spots that would also tell you the rad. has a problem heck just use a lazer temp. sensor if you have one. I sure hope this helps a little. /forums/images/graemlins/tongue.gif OH! YES MAKE SURE TO USE COOLANT 50/50 MIX.
     
  8. zakk

    zakk 1/2 ton status

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    maybe have the radiator flushed?
     
  9. yellowK20

    yellowK20 Well Lubricated Premium Member GMOTM Winner

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    if your just runnin' straight water stick a hose in the radiator cap and let it run with the lower hose disconnected
     
  10. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    I'm no expert on thermodynamics or anything....but let me throw out a couple of ideas:


    To maintain a given highway speed requires a certain amount of horsepower (call it X amount!) it is a sort of "equilibrium" point between aerodynamic drag against the vehicle and the power reserve that the engine has available. If you produce more power than "X amount" the truck will accelerate until the aerodynamic drag creates an identical amount of resistance and equilibrium is achieved again.

    I suspect what all that means, is that the amount of POWER it takes to go 65 mph in YOUR truck still has not changed. Aerodynamically, the truck is the same.

    With new gearing, you have effectively taken some of the strain off the motor to produce the "X amount" of power needed, because now the gearing is giving you better leverage. HOWEVER, you are now spinning more RPMs to do the same thing. More RPMs (to me at least) means more combustion cycles, and more combustion cycles means more fuel burning, and thus more BTUs of heat created. So even though the engine is less heavily "loaded down", it could still be producing more heat???

    The answer DOES seem to be RPM-related. Low speed crawling doesn't require the revs....and everything is groovy. My guess is that if you could run some tests at different RPMs on the highway (drive for a while at 2000 RPM, 2500RPM, 3000RPM, etc) you would see that the temps are directly correlated to engine RPM.

    That said (and assuming I'm actually right! /forums/images/graemlins/grin.gif) it's probably time to think about a HUGE radiator and fan setup. If it's a BTU issue...the more surface area you can put hot coolant into....the better! Aluminum is supposedly even better at removing heat, but it has the side-effect of also removing large amounts of cash from your bank account. /forums/images/graemlins/tongue.gif
     
  11. CHEVY 4WD

    CHEVY 4WD 1/2 ton status

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    Isent 2900RPM to high to be crusing at? /forums/images/graemlins/confused.gif
     
  12. BlazerGuy

    BlazerGuy 3/4 ton status

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    my brain hurts... /forums/images/graemlins/thumb.gif /forums/images/graemlins/ears.gif
     
  13. 55Willy

    55Willy 3/4 ton status

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    I'd flush the radiator out and run 50/50 mix.
    and maybe a thermostat ($3) if this doesn't help I'd look into adding on a aux. elec. fan
     
  14. zakk

    zakk 1/2 ton status

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    Right, but the motor doesn't spin above 3100 at freeway speeds. The variable is the MPH, my motor is spinning the same RPM's.

    so a 4 core radiator, huh? anyone know if those are plentiful in junk yards?
     
  15. yellowK20

    yellowK20 Well Lubricated Premium Member GMOTM Winner

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    I just got off the phone with Radiator.com and they Quoted me 139.00 shipped for a 4-core for my yellow pickup ....It has a 4-speed though so one for an auto would be alittle higher unless you just Run an auxilary cooler and not even bother with the cooler in the radiator
     
  16. RingMaster4x4

    RingMaster4x4 1/2 ton status Premium Member

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    I orderd my 4 core radiator for a manual transmission from Radiator.com and all they did you put some steal plugs in the connections for the Transmission cooler
     
  17. dyeager535

    dyeager535 1 ton status Premium Member

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    Sounds to me like your fan clutch is locked up.

    Pull it off and go for a cruise, see if its solved.

    The fan can't move air nearly as fast as you can drive, so it actually will act as a restriction at higher speeds if its turning. (which is why stock fans are clutched)

    tRusty missed this one

    BTW, Prestone shares the same boiling point (straight) of all the other antifreezes out there (approx. 334*) so I don't know as I trust whoever at the company replied with 275* 50/50, since no other company I could find claimed boiling point that high without pressure.

    I should mix some of this stuff up on the stove and see what temp it boils at /forums/images/graemlins/smile.gif
     
  18. arq

    arq 1/2 ton status

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    here's my shot at it. it could be that your thermostat is staying open all the time. as your speed increases your water speed through the pump and rad also increase. it could be that your water is not in your rad engough to get cooled down. i had this happen on a camaro i had once, i rand it without a thermostat. when i was in the city i had no problems, once i got on the freeway it began to over heat.

    try putting a 190 or a 210 deg thermostat and see if that helps.

    ARQ.
     
  19. dyeager535

    dyeager535 1 ton status Premium Member

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    [ QUOTE ]
    here's my shot at it. it could be that your thermostat is staying open all the time. as your speed increases your water speed through the pump and rad also increase.

    [/ QUOTE ]

    I had a 307 Cutlass with good cooling system, except the PO didn't bother to put in a 'stat. The coolant never got hot enough to build pressure, whether city or freeway.
     
  20. marv_springer

    marv_springer 1/2 ton status

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    I think it's coincedental.... Something in your cooling system has taken a dump on this last trip.

    ...or, you bumped the distributor and advanced the timing by accident while doing that last round of mods /forums/images/graemlins/confused.gif..

    Marv
     

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