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Hot Rods New engine build overheating help

Discussion in 'The Hokey Ass Message Board' started by boy_named_sue, Jul 21, 2025 at 9:05 PM.

  1. boy_named_sue
    Joined: Apr 9, 2006
    Posts: 145

    boy_named_sue
    Member
    from Dayton, OH

    Freshly built 350 in my 48, got it broke in, started and timed and running good, but it’s overheating…weirdly. It take a good long while to get up to temp, and when idling it will sit there and stay pretty stable. AC on or off, doesn’t seem to make a difference. While driving, it will verrry slowly creep up and up and up, and never really go back down. Not with speed or anything. On the highway it will come down a little and hold temp, but around town it’ll start pushing past 240 and that’s when I cut it off. But even there, I’ve arrived home at 240 and let it sit and idle for a minute to see what it will do, and it doesn’t move when I stop. But every so slowly even will come down a little (220 after about 20mins).

    engine is a 1995 tbi block, I put a small RV cam in it, lowkey port job on the stock heads (just knocked of the casting marks mostly) and an Edelbrock o take & carb. Now I know you’re gonna say “there’s a million factors” but here’s where I’m at: I pulled an old tired 305 out that never overheated once in 4 years, and changed nothing but the engine. Same aluminum rad, same electric fan, same thermostat, water pump, intake, carb, headers… literally I just undressed the 305 and put its clothes on the 350.

    I have checked and confirmed coolant moving through the upper hose at temp (therm is also brand new, just in case). Burped all the air out of the lines, heater hoses are flowing fluid. Electric fan is working fine. Radiators not blocked.

    Any suggestions? With Frog Follies coming ip I’m getting close to just yanking it and putting the 305 back in. But would love ideas for what else I could check first. It really feels like there’s some kind of fluid restriction to me, but I can’t think of a way to figure out where. Any of you ever had something like this??
     
  2. boy_named_sue
    Joined: Apr 9, 2006
    Posts: 145

    boy_named_sue
    Member
    from Dayton, OH

    And just in case it helps to be extra clear, same distributor, timing is correct, and no it’s not a reverse flow pump.

    again the weird part here is that it’s FINE at idle, and it seems to heat up when under load. That’s what’s got me boggled. Unlike all the other things I’ve overheated in my lifetime.

    thanks again for any help or ideas
     
  3. deathrowdave
    Joined: May 27, 2014
    Posts: 4,827

    deathrowdave
    ALLIANCE MEMBER
    from NKy

    Vacuum Advance working correctly is a must for cool street driving
     
    osage orange likes this.
  4. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 15,057

    Bandit Billy
    ALLIANCE MEMBER

    Did it get hot during the break in? Is it getting hot enough to puke? Same or different temp gauge? Are you checking temp gauge against an IR thermometer? Did you do the rebuild?
     
  5. jimmy six
    Joined: Mar 21, 2006
    Posts: 16,653

    jimmy six
    ALLIANCE MEMBER

    Did you degree the cam? A retarded one will cause added heat and run a little sluggish from idle up to 2000. Also some rebuilder knock the freeze plugs inside instead of removing them this also doesn’t help. Good luck.
     
    bschwoeble likes this.
  6. lostone
    Joined: Oct 13, 2013
    Posts: 3,437

    lostone
    Member
    from kansas

    What are you using for a fan ?

    ...
     
  7. RodStRace
    Joined: Dec 7, 2007
    Posts: 7,575

    RodStRace
    Member

    Reading thru this, it shouldn't. Same parts, fresh engine. I assume it wasn't mud in there before and you didn't pull a "DOH!"
    One thing stuck out to me. New thermostat and you thinking poor circulation, which sounds right. But before you said old tstat. Was the change an attempt to repair? Same results?
    Especially these days, new doesn't equal good, it just means untested.
     
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  8. primed34
    Joined: Feb 3, 2007
    Posts: 1,531

    primed34
    Member

    You checked with a temp gun? I thinking thermostat.
     
    osage orange likes this.
  9. TA DAD
    Joined: Mar 2, 2014
    Posts: 1,559

    TA DAD
    Member
    from NC

    Standard bore or has it been oversized ?
     
  10. Radiator IS to small. OR partially plugged. If my hillbilly math is correct, the cubic inch has increased by about 13%. Radiator may have been ok, but on the ragged edge, with the 305.

    I overheated my Buick badly. I had gone from a 248 ci to a 263 ci bored to about 290. The original size radiator cooled the 248 + AC ok. It could not handle the 290 + AC. A radiator for a 320 cools so well the engine STRUGGLES to hit 190 on a 100 day.

    Get a larger capacity radiator

    Ben
     
  11. 19Eddy30
    Joined: Mar 27, 2011
    Posts: 3,370

    19Eddy30
    Member
    from VA

    I say , If you have correct water pump
    For for direction you are using
    I am assuming standard rotation,
    Timing & carb tuning ,
    If you know your TDC is correct 0
    Your timing , engine will be around
    14 deg BTDC plus @ Idle . Rv
    Maybe as much as 18 ish
     
  12. Magneto Bryan
    Joined: Jun 22, 2016
    Posts: 6

    Magneto Bryan
    Member

    I would check that the lower radiator hose is not collapsing. If you do not have a spring inside of it, or if it is old, and does not have a spring in it. That could be the issue you are having.
     
    BJR and Moriarity like this.
  13. alchemy
    Joined: Sep 27, 2002
    Posts: 22,264

    alchemy
    Member

    Did you check your new thermostat in a pot of water on the stove?
     
    Just Gary likes this.
  14. TA DAD
    Joined: Mar 2, 2014
    Posts: 1,559

    TA DAD
    Member
    from NC

    Fresh motor (more heat), more cubes (more energy) temp goes up under load which is pretty standard sounds like it just needs more radiator .
     
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  15. pirate
    Joined: Jun 29, 2006
    Posts: 1,218

    pirate
    Member
    from Alabama

    Not mentioned and a long shot but could there be air trapped in the water jacket. Is the radiator fill cap the highest point in the cooling system? You could try jacking up front of car, starting engine with radiator cap off and see if it purges any air.
     
  16. jaracer
    Joined: Oct 4, 2008
    Posts: 2,904

    jaracer
    ALLIANCE MEMBER

    The symptoms point to not enough cooling capacity. Have you tried spraying a hose on the radiator when it is hot? That will at least tell you if the cooling system is working.

    It looks to me like this engine wants a bigger radiator. When you ask it to make power, the radiator doesn't have enough capsctiy to keep it cool.
     
    firstinsteele and osage orange like this.
  17. FRANK GRELLE
    Joined: Oct 15, 2018
    Posts: 141

    FRANK GRELLE
    Member

    Is the thermostat in backwards?? sounds obvious but we all have the ability to screw up , It may be from the 2 beers wile working on the car on a Sunday nite or anticipation of firing up a new build and rushing to get done,
     
    porkshop likes this.
  18. Tow Truck Tom
    Joined: Jul 3, 2018
    Posts: 3,151

    Tow Truck Tom
    Member
    from Clayton DE

    I believe @Bandit Billy called it.
    In a case like this I would use ( I can hear the sighs ) I look to, , , ,
    Marvel Mystery Oil.
    Often debated but it works, if the engine may be a little tight, causing heat from friction build.
    Start with a quart split Pint in the gas and Pint in the engine.
    Take the night off get on the hi-way ( aka 4 lane )
    Start with the speed you had troule before and subtract 10 MPH,
    In time, you'll know, give it more throttle, eye on temp.
    Keep playing with slow and faster for a while.
    Repeat if neccesary it may take more time and MMO
     
  19. Joe H
    Joined: Feb 10, 2008
    Posts: 1,785

    Joe H
    Member

    Was the engine tight before you started it? Thick old school rings, or thin modern rings? Rough cylinder hone, or extra fine? Most new motors run hot for awhile, how many miles does it have on it?
     
  20. leon bee
    Joined: Mar 15, 2017
    Posts: 1,149

    leon bee
    Member

    I didn't see if you pulled the thermostat out and tried without it.
     
  21. tubman
    Joined: May 16, 2007
    Posts: 7,940

    tubman
    ALLIANCE MEMBER

    I had the engine in my Corvette completely rebuilt about 15 years ago. It must have been tight, because it ran hot from the git-go. I could live with it most of the time, except you couldn't let it sit and idle for more that 5 minutes or it would boil over. After about 2000 miles of this crap, I decided it was time to pull it and tear it down. All of a sudden, it quit the boiling and has been a model of decorum ever since. (Except for having to replace the "High-Pressure/High-Volume" oil pump for a stock replacement unit to keep the oil pressure gauge from being constantly "pegged". I think the two were related.)
     
    Last edited: Jul 22, 2025 at 3:12 PM
    osage orange likes this.
  22. lostone
    Joined: Oct 13, 2013
    Posts: 3,437

    lostone
    Member
    from kansas

    Like I stated in other posts before.

    I built all my engines tight, they'd get hot and this is how I broke all of them in. Cruise the local main drag until the motor got up around 220 or so, pull into car wash and with motor running hose down radiator until 140 or so then off and do it all again.

    Heat it up, cool it down. A night of cruising doing this. Once I was done cruising and after many heat cycles, I'd cool it down one last rime and head straight home. The next day was a new day and never had an engine run over 180° no matter what I put it thru.

    ....
     
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  23. boy_named_sue
    Joined: Apr 9, 2006
    Posts: 145

    boy_named_sue
    Member
    from Dayton, OH

    vacuum advance interestingly makes the car stumble and respond poorly to throttle, so I have it capped off currently. While it’s a puzzle to me, it Drives great that way. The 305 really liked the vacuum advance to the port side of the carb.
     
  24. boy_named_sue
    Joined: Apr 9, 2006
    Posts: 145

    boy_named_sue
    Member
    from Dayton, OH

    yes I did the rebuild myself. First one ever, but I took my time and went “by the book” and so far I don’t seem to have any kind of failures. Just this creeping temp. Yes it’s the same exact gauge and sensor and wires and everything I was previously using with no issues (and also fairly new, less than 5yrs old). Yes I compared temp reading with IR therm shot into the water at the radiator, as well as at various points along both hoses. Everything agrees.

    during initial break-in, temp was just fine. But it was only about 20-30mins without moving. I am still technically in break-in phase as I’ve only gone for about half a dozen short drives with it since then due to overheating.
     
  25. boy_named_sue
    Joined: Apr 9, 2006
    Posts: 145

    boy_named_sue
    Member
    from Dayton, OH

    it’s a 4yo aluminum radiator rated for a v8, I’d have a hard time believing it can’t handle a very plain 350.

    and to answer others: yes I’ve checked, re-checked, and triple checked the thermostat. Even tested it in the wife’s pot on the stove with a thermometer in the water. I am 100% positive thermostat is working properly (although it’d be awful convenient if that was the problem).
     
  26. boy_named_sue
    Joined: Apr 9, 2006
    Posts: 145

    boy_named_sue
    Member
    from Dayton, OH

    same 3000cfm electric fan I had in there previously. It’s less than 3yrs old and feels just as strong as the day I installed it. Also can’t believe it’s the fan because at speed is when it heats up, not cools down. It actually behaves pretty nicely at idle
     
  27. boy_named_sue
    Joined: Apr 9, 2006
    Posts: 145

    boy_named_sue
    Member
    from Dayton, OH

    lol I am very not beyond something stupid. I do all of my work after 10:00p so anything is possible. But no, I am sure the thermostat is ok.
     
  28. boy_named_sue
    Joined: Apr 9, 2006
    Posts: 145

    boy_named_sue
    Member
    from Dayton, OH

    I’ve richened up the carb and messed with the timing til I’m blue in the face. I am 99% sure it’s not timing. Swapped out for a known good one just to see if maybe the mechanical advance was acting weird and it drove exactly the same. So almost definitely not timing
     
  29. boy_named_sue
    Joined: Apr 9, 2006
    Posts: 145

    boy_named_sue
    Member
    from Dayton, OH

    Thanks so much for all the replies. I’ll post on here if I get it licked
     
  30. lostn51
    Joined: Jan 24, 2008
    Posts: 2,867

    lostn51
    ALLIANCE MEMBER
    from Tennessee

    My dad told me he used to crank a new one up and drive it without any water in the motor to seat the rings. I guess it’s the hot/cold/hot/cold thing and it seemed to work well for him. Me on the other hand I build mine with a little slop because for the most part it’s going to be run hard and put up wet right from the get go.
    The last 347 I built I punched it out to match each piston with a torque plate attached, gapped all the rings to fit each cylinder, miked everything basically I went overboard with the assembly. That was one of the most reliable motors I’ve built, no issues (not that any of my other motors had any) but I knew that I could send it to the outer limits ever so often and not worry about picking up parts on the street.

    I kinda miss those days :D
     
    ekimneirbo likes this.

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