Running hot at idle- rebuilt 454

Discussion in 'Troubleshooting & Diagnosis' started by Uiga, May 27, 2021.

  1. RS79

    RS79 New Member

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    May 21, 2001
    Blandford Ma.
    I just put a new crate engine in my 79 and had the same problem. Ran OK at cruising speeds but way to hot at idle. Switched to an aluminum radiator and that helped a bit but still was running hot at idle. The fan spacing from my radiator was the problem. I set mine so it is just inside the shroud, maybe a half inch. Problem solved.

    Your situation seems similar in that you are not getting enough air flow at idle. I notice in your photo’s that the fan is pretty close to the radiator. You really need to get that fan further back in the shroud. You might think that closer is better but that is not the case, you are not getting a good draw when it is that close.

    The thing is that it looks you are real close to that radiator but no spacer to remove to get it further back. On my 79 (small block not BIG block) I needed a 2” spacer to get me just inside the shroud. The absolute closest you what that fan to be is 1/2 way in that shroud.

    So try getting that fan further back, shorter water pump maybe??? I would try to avoid trimming that shroud because it will lose some of it effectiveness.

    Best of luck………
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  2. Zclassic

    Zclassic Veteran Member Lifetime Gold Member

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    May 28, 2002
    Walnut Hill, Florida
    I agree with a lot of opinions you have been given. One thing to note is a newly rebuilt engine is tight. That in itself will create more heat for your cooling system to dissipate. As you put the hours on it things will wear in and arrive at their comfortable clearances. I would also think your temps are a little on the high side but I'm always reminded of my '89 IROC when it comes to temp issues. The fans don't even come on until 220 degrees on that car. I believe it was designed that way to reduce emissions so if the general is happy with that setup then I am to.
    RS79 likes this.
  3. Brian H.O. Cornelius

    Brian H.O. Cornelius Breeze

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    Jul 1, 2001
    Bonaire, Georgia
    While I am no expert I had a similar issue and converted to a dual electric fan set up. The problem sounds like a lack of air flow when sitting still. With the fan sitting closer inside the shroud it reduces the amount of pull (air flow) across the radiator. It would slipping a smaller straw inside a bigger one and sucking on it, you will still get some drink but a much smaller volume. Other issues I have seen is low volume flow from the water pump or partially opening thermostat but if all that is new chances are slim.
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  4. carhead22000

    carhead22000 oldblue

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    Sep 5, 2011
    where are you getting your temp reading from?intake or block.did your builder use or block with restrictor plates.i had a builder did that to me.keeps pistons cooler.but you can get heat saturated at top.just t
    imho thats too hott.
  5. DennisG

    DennisG Member

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    Dec 26, 2016
    A problem with coolant temperatures is that several things affect the temperature and you can be fixing the wrong one of them. I will make a simple list of them.

    In general, only make one change at a time or you will never know what worked and try to not make changes that you can't undo if they don't work for you.
    I have battled high temperatures in a special bodied car that has little room for a radiator. I am happy if the temps stay under 220 as once they were up to 264.

    Air bubbles trapped in your engine cause higher temps. Water normally has air dissolved in it Boiling your water before adding it to the antifreeze will get rid of the dissolved air (let it cool before working with it).

    Squeeze an upper radiator hose to purge big air bubbles from your system.
    If the top radiator coolent inlet is not the highest point in your coolant passages,
    you may have to put the front of the car up on jackstands to get trapped air out of it. Adding a header tank cures this problem.

    Belt driven fans vibrate, make noise and soak up power. Your fan and water pump are doing their least at idle and that is where you are unhappy about engine temperature. I saw a mention of underdriving the water pump, higher temperatures could then be expected from doing that.

    In general, a fan is not needed over 30 mph. Electric fans make much less noise, are smoother and free up several wasted hp. If large enough, they work very well.

    A higher compression ratio generates more heat, if yours is stock you should be ok.

    Ignition timing makes a temperature difference. If you have a vacuum advance, is it working? you can change the advance curve, but be sure you can put it back as it was.

    Temperature senders and gauges can be inacurate so you might swap in new ones as a test.

    For better heat transfer, you could run less antifreeze and more water in the coolant. Another thing that improves heat transfer is to have a greater difference between the outside air temperature and the coolant temperature so higher operating temperatures have their place.

    If the car is running well and you still worry, move the temp gauge over to the passenger side where you can safely watch it as a passenger. Or you could cover the gauge with tape so it does not distract you.
    Last edited: Jun 3, 2021
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