Hello everyone... I just recently put my 1970 340 on the road after doing much other work on it, so I'm finally driving it. However, I am having problems with the cooling system and it seems to be coolant flow. The car is all stock, new 180 t-stat, not new 16 lbs. cap. The radiator is a 3 row 26" unit. It's a Chrysler radiator, but I believe it is not an actual 'cuda radiator. The coolant is clear, no debris. While driving it runs great and stays cool for about 5 minutes, then the temp climbs to "H". The new POS heat riser is adjusted to full open position. At rest and while at operating temperature with the radiator cap off, I see no movement of fluid. The top hose is hot, the bottom hose is warm. The heater blows cool air and the heater hoses are cool. Could the system be air bound? I say this because I thought for sure I had flow a few nights ago, but this was with the cap on and I did not verify. But then had to drain down the system to re-seal (change out) the t-stat gasket. Or, could the water pump be shot and only operating intermittently? I checked for water leaks and bearing play at the water pump and it's all good. My first guess was the radiator being plugged and perhaps loosening debris and giving me some flow, but wouldn't I get flow to the heater core regardless? I pulled the heater hoses and blew air through them and through the heater core, they appear to be clear. I'm really stumped. I was thinking of flushing the system, but without flow, there is no point. Before I tear into this tonight (pulling the radiator and water pump, if necessary) I was wondering if anyone had any words of advice. All thoughts would be appreciated. Thank you all in advance.

and.....Thank You Moparts!

Tell us more about the motor, what cam, what is the timing set at?, what carb, what jets, how cubic inches? Got a shroud? What fan?

Doubt that there's anything wrong with the water pump or radiator.

1st thing I'd do is pull the stat/refill it/take it back out for a run. On the heater see if the cables on the heater box are positioned right and that the heater hoses are on the right nipples. When it reads H might borrow an infrared gun & find exact temp or just feel it & see if it's anywhere near "hot" and I have a wacky eng temp sender that erratically reads hot & there was no problem and a new $15 sender from Joesixpack" will (soon) solve that.

For what I know, the Motor is a stock 340, nothing fancy or radical. I do not have specs on the cam. The carb is a fairly new Edelbrock 650 AVS. Plastic stock shroud. Fan is a spring thermostatic, sitting 2" from radiator. I tested the fan at operating temp and it pulled paper tight to the radiator. I also tested it placing a magazine against the fan while operating, and it stopped. I feel no difference in the fan resistance from engine off to operating temp. It appears fairly new.

When I thought I had some flow I used a laser thermometer and took the following readings:

top hose 185, bottom hose 125, by-pass 182, heater hose 133/146, top of radiator 199, bot of radiator 105, water pump 152, engine temp sensor location 218... the radiator was fairly consistant in temp from top to bot/side to side

Give it more fuel and intial timing and I bet that solves it.
Be sure the thing is full to the top and then drive it hard to purge air and then let it cool and remove rad cap and top off. Keep the cap tight because when the thermostat opens for the first time it's gonna gush out if you leave it loose.

Only other thing i'D BE CONCERNED ABOUT WOULD BE THE CLUTCH FAN, THEY DO GO BAD. kEEP THE CLUTCH FAN FOR SURE BUT THE CLUTCHES DO STOP WORKING AND WILL CAUSE THE motor to run hot at low speeds. It should not spin easy hot.

Run it with the cap off until the thermostat opens and then see if you have flow. When the thermo opens it will blow some water out but not much. Have the radiator level just above the core.

thank you for all the suggestions... A couple of things I want to mention, one; when I returned home from a ride out, I parked the car and heard a very small hissing sound from the radiator cap, so yesterday I cleaned the gasket seat point at the radiator, it had some paint/etc. on the surface. The engine was very hot and I noticed the gasoline in my visible fuel filter was evaporating real quick. It has also been very hard starting after running hot. Another thing is that yesterday I ran the car up to operating temp with the water level about an inch below the neck. The temp started to climb, and I still saw no flow and nothing in the heater hoses. The coolant started to steam pretty good and suddenly it overflowed out of the top of the radiator pretty good. I shut it down. Wiped down the car and said good night...

Just some thoughts.Rad caps are cheap get a new one.at least 16 I prefer 18 lbs 340's seem to respond better cooling wise to the 8 blade pumps rather then the 6 including the 6 blade police pumps, When I use the 6 blade I use the flow cooler plate on the back of the blades. There seems to be a good possibility your stat is not opening fast enough even new I have had bad ones. What is your anti freeze ratio? should be no more then 40% with a high comp 340. You can add more in the winter but if its warm out keep it to around 10-20 below max. Also condider a max cooling declutching drive with that 3 row rad. Try to stick to 180 stat min I use 195's in all but dead summer weather. By the way the factory gauges are often wrong due to a sending unit I use a mecahnical Temp gauge.

put a new stat in it.. your new one is bad.....

I'm trying not to think the worst (block/head cracked). Lets do this: take off the thermostat housing and the thermostat and fill it with coolant to the neck brim. Take off the fan belt so the water pump is disabled. Have a helper start it and put it in gear/foot on the brake & give it a LITTLE gas to load it and you under the hood see if there's any turbulence in the coolant. With the pump immobile there must (should) be a dead calm, no bubbles and definitely no gushing. Do not let it run for more than several minutes max w no pump, but just for this test & holler back when you can

I would first check and make sure you dont have the stat installed upside down. I would also drill a couple of little holes in it.

Easy way to check the stat

Boil water and use a candy cane thermometer, watch the temp and just lower the t stat into the water and watch the temp as it opens.

Yeah if you have a rad pressure pump you can pump it up to 15 & see if it holds & even without a helper tomorrow you could pull the stat/housing & do the bubble test & let it idle & you eyeball it (several minutes max). If it has a blown head gasket or block or head crack it'll make the water gush out in short order.

Maybe the lower radiator hose is collapsing but it sounds like the radiator is not flowing. At operating tperature with the cap off you should see movement in the coolant. If it doesn't flow with no thermostat, I would be thinking about rodding or recoring the radiator. I had an A12 with an original radiator that used to get hot at idle or if I ran it hard but ran fine as long as I was moving at highway speeds. It pressure tested OK but it didn't work. I had it recored and it solved the problem. If the bottom hose dies collapse wgen it's hot, it means the coolant us not flowing from the eadiator thru the bottom hose. I think it's the radiator?

There is no spring in the lower hose. I felt the hose and did not feel any depressions. At operating temperature with the cap off there is no movement at all. no bubbles, nothing. However, with the cap off eventually the coolant will heat up and as mentioned above at one point boiled over. I hope to work on it tomorrow.

Thanks for your help

imo No that's not the problem... you never did say what the timing or jetting is.
Be sure the lower hose is not sucking closed like RoadrunnerJD mentioned above. Make a spring for it to shove in the hose if it don't have one.

I have been running 340s since 1976, how long have you been running them. I drove mine last night and this past summer to top of Pikes Peak and then 5 days in Vegas in 112 degree heat. Does anyone who responded to this thread have that kind of experience?