I had this same exact problem. Drove me nuts, and I live in AZ, where it gets 110+ in the summer. At stoplights and drivethrus, my temp would creep up to 240! Here's what I did:

1.Installed B-Cool Radiator and overflow can.
2. Brand New Stock Water Pump, not high flow or anything like that.
3. Miloden 180 thermostat
4. Flexalite 210/220 dual puller fan (this was after trying highflow flex fan, HD Fan Clutch, New Viscous fan, etc etc etc - none of it helped)
5. Flushed and drained system til it ran clear, about 6 or 7 times...


Hope this helps, it worked for me. I didn't go through all the posts, but what do you have for an overflow can? This is important because when the radiator gets hot/pressurized, the excess fluid goes into the overflow, then is pulled back in when needed. I can't stress enough, check the small things. Make sure there's no air pockets or something like that.

Good luck, I feel your pain.

my charger with a 440 does a similar thing. moving it will be locked in at about 170-180. at idle it will creep up, but if i turn on my electric fan, it stabilizes..im running a mild 440, be cool direct fit rad, flowkooler waterpump, 160 tstat,fixed flex fan, shroud, and a 16' pusher electric on the front. tranny runs through a seperate cooler, and not through the rad.

i, too, am running a rebuilt, carbed motor. block was tanked, etc...


i still don't like it. I don't want to have to use an electric as a crutch. I have always suspected the flowkooler or any high performance pump. I don't know why, but maybe the stock pump in the stock housing moved the coolant the most efficiently. maybe slapping on a superduperdeluxbignuts water pump screws that up..maybe they cavitate or just move the coolant too fast. lots of r and d went into this stuff years ago and we go throwing these parts on because it looks nice in the summit catalog.

I read a story of a guy with a kitcar cobra or something like that years ago who was running a smallblock ford that would just not stay cool. ended up that he was running a 'not quite right" water pump. got the right one and his trouble went away.. so maybe it's possible to be running a "stock" pump and still have the wrong one?

what's real strange is there are guys here running big motors on puny radiators with no trouble at all. no shrouds, stock systems......

Cooling has quite a bit to do with cylinder wall thickness as well. If you are overbored this may be part of the issue. The thinner the walls are the more heat escapes through them into your cooling system. -Bob

Wouldn't most of the heat generated be up in the cylinder head? When you fire up an engine the cylinder heads get warm far faster than the block and there is coolant in both places.

I used to have a 350 in my truck that ran always ran hot. I used the parts store type water pumps and always figured it ran hot because I was told the engine was bored .060 when I bought it. I have since replaced that engine with a 400 small block. I took the heads off the old 350 the other day and it was only .030 over. So much for that excuse.

On the new 400 I built I was warned by several people that those engines can't be kept cool on the street because they are a siamese bore block. I had no experiance with them, but knew I didn't want to build another 350 so I ended up buying an edlebrock water pump as I was told they really work wonders. Needless to say the 400 runs cooler than the 350 ever did. I think my 350 ran hot just because the stock style water pumps are barely capable of cooling a stock engine. Add a cam, headers, etc and you are pushing your luck.

The edlebrock pumps have huge curved blades and no extra space within the pump cavity. Every bit of fluid that enters is pushed out the other side. The edelbrock pump works by moving as much fluid as possible.

Bob is definitely right on the overbore, it can be a huge issue, especially if you have any core shift.

One thing that hasn't been mentioned is that it appears that some of the pump manufacturers are putting non-mopar impellers in their pumps. I saw one a friend had bought, (small block) and it had an impeller that was between the diameter of the small and large Mopar ones, with 8 blades. I think it was a GM impeller. I don't think anyone knows if the flow and pressure out of a pump like this is correct for a Mopar, or what speed it should really run.

well, i have good news.....

I've done everything i listed in my last post, and the car ran at 175-180 with the new pump, ect....but still ran a little crappy after about an hour of driving....so i changed the coil, and the problem disappeared completely. By the way, this weekend in toronto, it's been extremely hot out, 39*deg C, and very humid. perfect testing grounds. I can see a huge difference in water flow in the rad at idle, where before coolant would just barley move past the top, now at idle it's a very nice stream of coolant. Huge difference in cooling, the car runs great now, last night at a cruise night i was idling for 15mins in this heat, the car did get to 190, but not higher, the electric fan really makes a big difference at idle in gear stopped, it really keeps things under control. Funny, this is the 3rd coil i've tested, 2 new ones, 1 old one that was a known good one, they all failed, and the one that works perfectly is the big old accell yellow plastic one, that was the coil on the car when i bought it and drove it from Minnesota to Toronto in retarded hot conditions, and the car never went over 200-205 in insane heat. I also changed the ballast resistor, hot i was getting 8-9 volts at the coil, but changed it for piece of mind. The upgrades I've done to the cooling system (high flow stat, and water pump) really made a big difference. I can't believe i have 3 crappy coils....I can officially say the car is ok now.....thank GOD>.....

Good to see you got the problem fixed. One thing to remember if your coil is mounted horizontal, is to make sure the terminals are facing down or bottom of the coil so the oil covers the connections. If the terminals are up the connection can get hot and cause failure from overheating if the oil is below the connections.

What water pump did you end up using????