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51 GMC still overheating

Discussion in 'The Hokey Ass Message Board' started by Mike in Tulsa, Apr 11, 2011.

  1. I have been fighting this for a while and I'm running out of ideas. The truck has a 1958 Chevy 261 inline 6 with a 200 4r trans. The radiator is the original 51 GMC that I had cleaned out. 7# cap. I also had the heater core cleaned out at the same time. I'm running a 235 848 head that was completely rebuilt the same time the motor was rebuilt - I have 1000 miles on this motor. They drilled the head for the steam holes too. The truck will stay cool when the air temperature is cool. But when it gets warmer, say 70 to 80 degress it boils over after about 5 to 10 miles. I checked the t-stat and even put a new one in it. It's a 180 degree. I put on a new water pump with no change. I pulled all the hoses off to check for blockage. I replaced the lower radiator hose with a heavy duty one, so it's not collapsing. I ran water through the radiator to make sure it wasn't blocked. My fan is the stock fan on the 261 and it's about 3/4" away from the radiator. I was running a smaller 235 carb and it was running lean. Now I have the correct 261 carb (thanks Poverty Flats) and it seems to be much better. I've adjusted the timing with no results. I don't want to run a fan shroud and I don't want to run an electric fan. I really don't want an aluminum radiator either but I'm totally out of options. I'm taking the truck to my mechanic in the morning but I'm afraid he's gonna tell me to get a new rad and shroud and... Anybody else got any ideas? They used to flog the heck out these truck motors back in the day. Why won't mine stay cool? Thanks for any help and sorry about the random thoughts.
  2. JohnEvans
    Joined: Apr 13, 2008
    Posts: 4,883

    from Phoenix AZ

    Take it to a shop that has a "Block Check" tool to see if you are getting combustion gases into the cooling system. The symptoms sound like that is the problem to me,cracked head bad head gasket.
  3. Thanks. I forgot to mention that I rented the block checker from Autozone and it didn't detect any gases in the antifreeze.
  4. Brad54
    Joined: Apr 15, 2004
    Posts: 6,006

    from Atl Ga

    Stock 4-blade fan? Without a shroud.

    That's a big part of your problem.

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  5. At highway speed isn't the fan pretty much unnecessary? Since it's still overheating on the highway wouldn't that eliminate the possible need for a fan shroud?
  6. JohnEvans
    Joined: Apr 13, 2008
    Posts: 4,883

    from Phoenix AZ

    Yep, with normal air flow through the radiator with no tail wind a fan is normally not needed above 40-50 MPH. Which also rules out the need for a shroud. You have confirmed the dist mechanical advance is working ? If so 2 things remain water pump not moving water or a bad/clogged radiator.
  7. historynw
    Joined: May 26, 2008
    Posts: 806


    Are you sure you don't have an air pocket? My buddy's T-bucket would do this time after time we found it had an air pocket. A couple trys later and a couple burps it went away.
  8. Black Panther
    Joined: Jan 6, 2010
    Posts: 1,440

    Black Panther
    from SoCal

    If you are overheating on the road when going over 35-40 mph like you said...the fan is not the issue...its a water flow issue...something is blocked....
  9. Truckedup
    Joined: Jul 25, 2006
    Posts: 3,838


    Heats up on the hiway is usually radiator,get a new one,GMC radiators are kinda pricey....
  10. I've drained and refilled it a couple of times. I burped it by letting it idle with the cap off. It burped, so I don't think it's that. I'm thinking new radiator too and yes they are pricey. Thanks for the help.
  11. Cymro
    Joined: Jul 1, 2008
    Posts: 666


    Could be the result of an air lock, do you have a heater fitted? if so set to hot let the engine idle and squeeze all the rubber pipes, a couple of burps later may solve your dilemma, is your engine coolant thermostat functioning correctly? as a sticky thermostat will pressurise the system similar to overheating,although not universally recomended try a run without a thermostat to see what happens, ( on early Minis we used to replace the thermostat with a metal plate with several holes drilled in it to restrict the waterflow mimicing the action of the open thermostat in the hope that one end of the block didn't get much hotter than the other). Water pump pumping ok? rad clear? crap in water passages? did you flush the engine and rad with a hose in the opposite direction to the normal flow, this will usually shift a fair bit of rusty flakes and crap from a block.
  12. ddawg16
    Joined: Apr 10, 2011
    Posts: 60

    from So CA

    Those 261's are tough little engines.

    My gut feeling says blown head gasket...but you said you did a block check for gases.....

    I would do this....with it cold, take off the radiator cap. Start it up....monitor the heater should feel it get hot....once it reachs about 180 (t-stat temp), you should start to see flow in the radiator. Water should be flowing by pretty good assuming everything is working should sit there all day at idle and not over heat.

    If you want to eliminate the radiator, use an infrared thermometer and measure the water temp on the top of the radiator and at the will be coming in at over 180.....but can't be over 212 (otherwise the water is boiling). The bottom should be at least 10-20 deg cooler.....if it's not cooling down...radiator is not doing it's can confirm it is or is not air flow by taking an old house fan and putting it in front to blow air across it.

    Most engines I have seen on a mild day will not overheat at idle if you leave the cap off....the creep up...but should stay out of the red...

    Note....not all blown head gaskets push gasses into the some cases I have seen where they suck water....main issue is that you can't build up enough pressure to prevent the water from boiling. That was the problem I had with my old I6 in my jeep. I never did find a problem with the gasket....I'm pretty sure the head had a crack on one of the intake ports...

    Over heating issues can be hard to track down......I would be suspecious of not having a shroud....they make a bid difference.....especially when things become marginal....

    Start with basics....and make temp measurements.....
  13. oj
    Joined: Jul 27, 2008
    Posts: 6,074


    I've had similiar problems and bought a radiator filling tool from lisle tools. It is like a big funnel and holds a couple quarts, you remove your radiator cap and there are adaptors for this funnell so that it'll twistlock onto the radiator neck and make a seal. Just dump some antifreeze in there and you'll see the air bubbling out of the motor, after about 10 min or so start the motor and let it sit there and idle for a holf hour or so and air will find its' way out. When you are done shut the motor off and let it sit for a few minutes, stick this plunger into the funnel to seal it and take it off the radiator - good to go!
    It is Lisle #24610, google it and check it out, slickest tool i've bought in ages.
  14. Mr48chev
    Joined: Dec 28, 2007
    Posts: 25,019


    Are you leaving about an inch and a half at the top for expansion? If you over fill those radiators they tend to want to puke coolant when they get warmed up.

    Other than that the first thing I'd do is pull the radiator and take it to a good radiator shop and have them flow test it. See if the thing is flowing like it should be or if it is plugged up.

    The other things I can think of right off are:
    The possibility of the thermostat not opening like it should. When you run the engine and burp it can you feel the thermostat open by holding your hand on the upper hose? When the gauge gets to 180 the hose should get hot to the touch rather quickly.

    The possibility that the waterpump isn't moving the coolant as it should.
  15. chaddilac
    Joined: Mar 21, 2006
    Posts: 13,799


    It's gotta be either a air lock or the radiator... but the heater was working fine down to Austin.

    Coincidentally, the Temps outside were consistent with the water temp in the motor, when it was cool Wednesday when we left, the motor ran fine, thursday morning little nipply and it ran fine, half way to Austin it started warming up outside and that's when the water temp started climbing on the gauge.

    Also we installed a new water pump when we were down in Austin, also a new thermostat, and neither changed it, plus we ran it without a t stat and no change.
  16. dvzdeathtrap
    Joined: Nov 30, 2010
    Posts: 125

    from austin, tx

    have you opened the petcock at the rear of the engine? those engines sit at an angle leaning towards the back. a common over heating issue is from deposits collecting in the rear bottom end. this will inhibit flow aswell. sometimes you have to take the whole thing out and put a cathanger or peice of metal in there to get all of that loose enough to get flow from it. remember, it is a deep insert too- like almost a foot of travel. are you on the stovebolt forum? there are a lot of knowledgable people on there that eat, breathe, and sleep those inline 6's. (they eat engines?) -DVZ
  17. I would use a cathanger as well...LOL
    You have a restriction in your cooling system. If you took out the T-stat AND still had the run hot condition then its a restriction. Warm it up and run your hand over the radiator to see if there are any real cold or real hot spots. A temp gun will work better.
    If you have even temp over the radiator then it has to be in the block or head as you have changed every thing else....wait a second....what about pulley sizes!!! have you changed your pulleys from the stock ones, crank or water pump?????
    I would then pull the drain plugs from the block, and the easy access soft plugs (I can't call them freeze plugs down here) and check for rust buildup.
  18. 50Poopbox
    Joined: Mar 5, 2011
    Posts: 69


    I would say try pressure testing your coolant system. It is under pressure because water under pressure boils at a higher temp then 212, therefore if you use a pressure tester in place of the radiator cap and it doesnt hold pressure you have a leak somewhere. SOmetimes the leak isnt always head gasket so you wont always get exhaust gases in the coolant.
  19. Babyearl
    Joined: May 23, 2008
    Posts: 610


    I haven't seen anything on retarded timing or heat riser being checked, or exhaust system checked for blockage.
  20. I never saw a petcock on the back of the engine. The block was tanked for 2 days. Surely, it's not blocked. The radiator was tanked but I'm pretty sure that it wasn't rodded out. Since the GMC motor was longer than the chevy, I was able to use a stock water pump and pulley for the 261. I have checked the t-stat. At 180 degrees it opens and the top hose gets hot. The way the filler is on the GMC rad you can't see down in the rad but it's flowing I'm just not sure how well. I'm thinking the best thing to do now is to pull the rad and have it rodded out. That's the last thing. I just didn't want to go there unless I had to. Sounds like it's time.
  21. I have adjusted the timing with no help either.
  22. hemimerc
    Joined: May 22, 2010
    Posts: 21


    Well mines not a chevy but here it goes I had the same issues on my 36 dodge truck with a 230 flatty after months of headaches trying to figure it out why I was overheating it ended up being my new water pump. There was a casting flaw in the inlet where the bottom hose connects to pump opening it was blocked by left over casting that wasn't knocked out there for restricting flow causing it to overheat ? Go figure, Good luck
  23. FityFive
    Joined: Aug 9, 2010
    Posts: 283


    Just a thought...

    Make sure your carb isn't running too lean. This will definately cause the motor to run hot.

  24. :p:p:p

    best use for those leftovers from the drycleaner that I've ever heard!!

    "I suggest not telling your wife how you REALLY feel about her cat." - Sigmund Freud
  25. 52pig
    Joined: Jun 9, 2007
    Posts: 436


    7# cap? That's not much pressure. Is it actually overheating, 'cause I didn't see any numbers in your post when it boils over. Cars tend to do that when being pushed down the high way, that's why there are puke tanks and over flow canisters.
  26. Petejoe
    Joined: Nov 27, 2002
    Posts: 9,953

    from Zoar, Ohio

    Run 100% vinegar through it for a week and drive it. See what kind of crud you get out of that block and radiator. Use baking soda to neutralize it when your finished and happy.
    I've run vinegar in a flathead three times like this before it cleared up. It doesnt damage a thing and smells like french fries :)
  27. the-rodster
    Joined: Jul 2, 2003
    Posts: 6,499


    If you need it, I can give you a nice clean, stock 49 GMC radiator.

  28. narlee
    Joined: Dec 7, 2009
    Posts: 240


    You say you adjusted the timing but what is it set at? You also say you put on another carb but are you sure it isn't lean? (Jetting)
  29. Thanks fellas! When I was building the truck I took the radiator and the heater core to a radiator shop to have them cleaned out. The radiator shop I used to go to went out of business, so I took it to one I found close by - an old radiator shop on Route 66 that "has done these old radiators for years". Well, after trying everything I could think of, I decided to pull the rad and take it to another shop that was recommended to me by my mechanic. Turns out that the rad wasn't so clean after all. As a matter of fact it was almost completely clogged up. He pulled the tank and rodded it out and now everything is working like it should. That's what I get for just picking someone out of the phonebook! Thanks again for all the replies. If anyone in the Tulsa area needs a good radiator shop try Johnny's Radiator on 3rd St. - 918-582-6610. I highly recommend him.

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