Hot Rods New engine build overheating help

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??

 

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

 

Vacuum Advance working correctly is a must for cool street driving

 

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?

 

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.

 

What are you using for a fan ?

...

 

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.

 

You checked with a temp gun? I thinking thermostat.

 

Standard bore or has it been oversized ?

 

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

 

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

 

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.

 

Did you check your new thermostat in a pot of water on the stove?

 

Fresh motor (more heat), more cubes (more energy) temp goes up under load which is pretty standard sounds like it just needs more radiator .

 

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.

 

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.

 

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,

 

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

 

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?

 

I didn't see if you pulled the thermostat out and tried without it.

 

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.)

 

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.

....

 

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.

 

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.

 

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).

 

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

 

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.

 

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

 

Thanks so much for all the replies. I’ll post on here if I get it licked

 

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