[taylor2010]

i have a 68 with a smallblock 350 4 speed trans. if the car idles even for a little bit it starts to get hot and fast. i ran water through the radiator and the block. i flow tested the radiator and everything was alright. i bought a new thermostat. I also put new antifreeze in it but it still overheats. someone please help thanks.

[Fred Ficarra]

Whew, tough one. OK, things wrong to check: reverse flow water pump. (very wrong) Large diameter water pump pulley. Clutch fan drive (if equipped) failed. Cardboard in front of radiator. Joke, sorry. I was running out of ideas.

[Satatic]

Thermostat upside down?

[click]

what temp. stat did you install? should be 180 degree.

[zlek131]

You didn't say but I am assuming fan/fan shroud are running/on, right?

[69SS 350 5 Speed]

If everything is in working order in the cooling system, as mentioned above, have you checked to make sure your A/F ratio isn't too lean. Do your headers start to get really hot and glow at all?

[Steptoe]

If you have correct thermostat, water pump, shroud and fan
It will be the radiator
how old is it how often has it been serviced?
I strongly suspect that you will find too many cores are blocked or partly blocked even thu a flow test says it is ok.
OR the thermostat is not correct and doest have the bleed hole in it causing an air lock

You do have a shroud and the fan sits inside the shroud?

I assume from you description you dont have a over heating issue when driving?

[taylor2010]

yes fan shroud is in and the fan is working.

[Skeeter55]

Is the water pump by-pass hooked from pump to the intake. Ive hared that it can cause this kinda of problem if not hooked up. Also drilling 2-small holes 1/16 in the thermostat should help or its a bad stat.

[Z15CAM]

Not enough timing? Wrong Head Gaskets? Collapsed Lower Rad Hose?

With the Rad Cap OFF and your hand on the Upper Rad Hose you should feel when the Thermostat Trips and see the Coolant flow in the Rad.

[Johnny B]

Is the gauge accurate? Is it actually overheating, boiling over, and puking coolant out or does the gauge just say it is hot?.....Jb

[knightmoose]

Sound like your water pump is the issue to me before the radiator as your saying it gets hot quick right at idle... Thermostats are easy to replace and its unlikely you put it in upside down ... on top of that 6 mos ago i replaced my water pump and right at the start not even 5 minutes on she would over heat in a cold arse morning lol its the pump.

[Steptoe]

Will not be collasped low hose..that happens at higher rpms on the highway
Will not be timing if the radiaror is in good condition...may run warmer but not hot...and again this is at idle
Will not be lean it is on the idle circuits and again these radsiators are way over built, may fun warmmer but not over heat.

[Z15CAM]

Will not be collasped low hose.

I've seen the lower hose block coolant flow on several occasions when the inner wall lets go, probably because of repeated collapsing at higher revs - that's why most Rodder's put a coil spring in them. Retarded Timing will cause the Exhaust Manifolds to turn cheery red.

Perhaps someone did a lousy job of installing an intake Gasket and filled the Water Crossover of the intake with Silicon or plugged the Water Pump Outlet and Manifold Inlet and never installed the Hose or put Frost Plugs in the Blocks Water Pump Mounting Holes. Is the Fan Belt on - LOL

My guess the Water Pump Shaft may be sheared or Vane is damaged; however, the Lower Hose maybe be blocked - With the Rad Cap off, after the thermostat Trips - Check for Coolant flow by looking into the Rad. Either the Coolant is Flowing or it's Not. Perhaps the system isn't Burped and he has an Air Trap.

[ace's68]

I'm betting the reason is a fan set up that is insufficient.
If It runs normal going down the road, but not at idle that is a red flag.
Air coming in at cruise cools the radiator, at idle standing still there is no air coming in.

Can you tell us your exact set up?
How many blades on the fan? Mechanical fan, how many blades, clutch fan, or are you running an electric fan(s)?

Also, what is your definition of over heating? 190*-210* is perfectly normal.