I am having an over-heating problem with my 1969, Roadrunner. I have been working half-heartedly at solving this problem over the last three years. Since this car is a trailered show car (driven 60 miles per year) I didn’t put a real high priority on resolving the problem. Plus, if the air temp is 80 degrees or less, there is no problem. The overheating problem only exists when the weather is very hot or when the car is stopped and idling or in a “stop and go” traffic situation where fresh air is not moving through the radiator. Once the car is rolling, even 20 or 30 mph, the engine will start to cool down.
Vehicle specifics: stock 1969 383 ci, 4 speed, 22” radiator w/fan shroud, new/tested 190 degree thermostat, new 16lb radiator cap, hood to radiator seal is in place, gravel shield at bottom of front bumper is in place, radiator has been flow tested and validated by Glen-Ray Radiators, Inc., stock OE 7 blade fan and spacer is in place, 50/50 Anti-freeze/water mixture, heater blows hot.
I don’t know the exact temperature when the engine is overheating. I am using the OE dash mounted temp gage which simply has a “cold” mark, then what I would call a “normal Operating” mark, to far right of that another mark which would have to be a "do not exceed mark" and at the extreme right is what must be a “ruined” engine mark. The engine generally operates about a needles width to the right of the “normal” operation mark. However, the overheating is real. I have been caught in traffic and had the engine get so hot that it melted the foam firewall steering column gasket, the exhaust heat put a burned spot on the white plastic steering column bearing retainer at the bottom of the steering column, and the engine began to “spark knock”. So the engine is really getting hot, it is not just a bad gage or sending unit that is making me think it is hot.
Knowing I was going to ask for professional help I detailed the overheating of the engine on Sunday. The ambient air temperature was 93 degrees, 11:00 am. The engine was started with the radiator cap removed so that I could see coolant flow when the thermostat opened. At no time during the test event can I really say there was positive coolant “flow” through the radiator based on what I saw viewing through the cap opening. After the temp needle pasted the 2nd mark (normal operating) I would periodically rev the engine to see if I could see positive flow through the radiator indicating that the water pump was doing its job. Through the entire test I saw very little, if any, flow through the radiator. I owned an AAR Cuda prior to this roadrunner and I remember viewing the fluid flow in the cuda (340 cid engine, 26” radiator) and the coolant level would drop and the coolant would “rush” to the side of the radiator. That didn’t happen here. However, the coolant would “swirl” a little but I could see bits of dirt in the coolant that just “hovered” round the cap opening. Still, even at 93+ degrees, the engine took 40 minutes from dead cold to reach a point where the engine was clearly getting very hot, temp needle was vertical on the gage in the 12 O’clock position. Just to make sure the same thing would happen with the radiator cap installed, I installed the cap after a short cool down period and the results were the same, taking about 30 minutes to reach the same temp but the engine was started at a higher beginning temp.
My suspicion is that there is something restricting the coolant flow through the block, remember, it doesn't overheat if it is moving). I believe some coolant is getting through the block or it would overheat much quicker and it would not run normal temps when moving. I did have the water pump removed during restoration but I don’t remember any problems with it (loose impeller, etc). I did not drive the car in hot weather prior to restoration so I don’t know if the problem existed prior to restoration. At restoration, the recently rebuilt engine was stripped of all paint, disassembled to the short block, regasketed, new freeze plugs installed, and reassembled. The engine runs very good.
I need help diagnosing this problem, I'm getting desparate! What should be my next step? Is there a way to effectively “back flush the cooling system that may dislodge debris that could be restricting coolant flow? What would be the best way to check for a blockage in the engine block short of disassembly? Should I remove the water pump and inspect it to look for a loose impeller? Should I remove the engine, remove the freeze plugs, and see if I can find any obstructions (I would really, really hate to do that).
Thank you for reading this long manuscript but I felt to properly make suggestions you needed to know the facts and everything I have tired. I will consider each suggestion. Remember though, it doesn't overheat when constantly moving.

The BEST fan I have ever seen to move-air is the Mopar flex-fan with the stainless-blades.

I would start with a new water pump and a good flushing of the cooling system. Swap to a 180 T-Stat as well. Attach a real temp guage as well and see what the real temp actually is.

Are you loosing water? Any in the oil?

Added: Check the lower rad hose for collapsing as well.

If it has got as hot to melt that stuff, I would also be worried about the engine. It does not take a lot to distort bores and/or to start swapping metal.

a good investment would be an infrared temp gun, allowing you to get actual temp reading, by pointing at various locations. it sounds like the block needs to be flushed, and verify the waterpump is the correct one, for the pulley. (or visa versa) the 180 stat sounds like a good idea, and drill a hole or two into the plate on it, maybe 1/8" to allow it to "burp". radiator sounds good, since you had it tested. you might pressure test the cap, just to ensure it's actually working. (new doesnt always mean it works)

getting hot enough to melt nearby items sounds like a fuel issue. are you sure the carb is set correctly? float levels okay, seats not leaking, etc? and finally, are you sure fan is sitting properly in the shroud? remove the belt, and spin the fan to check for free operation of the pump. no noises or resistance? just a couple of easy to check ideas. the temp gun, is money well spent, and you'll find tons of places to use it.

When an engine gets hot it overheats and spews coolant. Has your engine ever done this? If not it's not really overheating. The gauge would also go far to the right if you were really overheating. Your car appears to fail these tests, but this is good.

Like GOLDEN ARM said..

There are so many things to look for. Chrysler did their best to engineer everything to work together. If you've changed one thing that can upset the balance.

You should have the correct:

water pump pulley diameter
crank pulley diameter
T-stat or one slightly cooler.. 180. I like 160s
clear radiator, head and block passages
good water pump. I like the 6 vanes pumps
proper ignition timing
spark plugs
carb jetting/settings
heat riser working properly(open all the way when hot)

The newer head gaskets have tiny(slotted) cooling holes versus the original gaskets were round. I drill the head gasket holes to 1/2" for more flow.

Drain some coolant and look down into the radiator. Look to see if gunk has clogged it since the last cleaning/test at the shop. I've had a lot of sand come out of the block and make a home in the cores over the years.

Check your timing and make sure it is where you think it is.

From the OP I don't see an overheating problem; assuming the factory gauge is accurate, half way up isn't overheating it's just hot. With a 16 psi cap you're good to over 240°F

I wouldn't take any actions other than installing a mechanical gauge to verify the temperatures.

The radiated external heat might be due to something besides the cooling system.

I also agree that he does NOT have an overheating problem. He isn't running hot while moving and only while stopped. Our old cars just run hotter at idle, and during stop and go, and it seems hotter than it is, or could be.

I run two gauges. The factory gauge, and an Autometer gauge. I use the second guage to measure against the original gauge just for piece of mind. The Autometer gauge I believe is more accurate, and the original gauge appears to measure higher temps

I had a problematic '68 GTX 4 speed where I experienced constant overheating in the summer as you describe. After 3 yrs and a lot of new and upgraded parts we never did find the culprit nor made any noticeable improvements via mechanical changes. Changed out the stronger aftermarket cam to a Mopar Performance stock one. Still didn't help. In my case the temperature gauge was verified accurate and it would go nearly all the way to the right when the car idled on a 90+ degree day (approx 230-245 deg F which was too hot for me). After several antifreeze belches I installed an overflow bottle and kept the fluid level just barely above the radiator core. A cooking thermometer placed in the open radiator is a quick method to verify gauge accuracy as the temp heats up to 212F.

The final thing I did was to go to a much weaker anti-freeze mix in the summer (June-Sept) and add a bottle of Red Line Water Wetter. Water is a better heat transfer agent than anti-freeze due to a higher specific heat capacity and allowing more nucleate boiling...but it also has about a 13 deg lower boiling point with a 50-50 mix (a trade-off). That got the car's running temp about 10 deg lower. Though on hot days I'd still get that temp. overshoot when coming to an idle after an extended run. Prior to using the Water Wetter it was normal to run around 195-215 while driving. After that the range was about 185-205.

Is your engine a fairly new rebuild to stock parameters? Does it ever ping? If you're using a stock V belt for show points those may not grab as well as a cogged belt, esp. at idle speeds where it's more important. Anything you can do to improve the "grip" from pully to belt will help. The belt "tightener" on these old Mopars is pretty poor. I feel your pain. A second GTX I owned had an overheating problem but that was an original engine probably in need of a rebuild after 45K miles. When I got that car it had a damaged piston and ring which were replaced. However it still ran hot and certainly could have used a fresh rebuild.

RR69s

It sounds like it is certainly running hot, but like what was said above I would first hook up a mechnical temp gauge. My 70 road runners stock gauge is about 3/4 of the way up while sitting at a stop light. But my mechnical gauge only shows about 190. I would first figure how how things actually are before going any further.

Did your post say you did not see coolant flow through the radiator? Take the thermostat out and see what happens? Or is the thermostat installed upside down?

Thank you for all of the good ideas. As far as a different fan or electric fan goes, the roadrunner is restored to OE specs and an electric or aftermarket fan would not be an option. The OE heat gage on my car will go all the way to the right if I let the car get hot enough. For testing purposes, I just let it get real hot then shut if off, that happened to be at the mid point of the gage range. Believe me, I've had it go almost to the 2 O'clock position before. When the car gets really hot it will belch out coolant, believe me, the engine is getting hot, it isn't a gage problem, it does spark knock when hot as well and the engine sits there and creaks in a very un-nerving way as it cools down. Getting a separate temp gage so I know the actual water temp seems like a good idea as well as an infared heat gun. If I find a specific "hot spot" that may help diagnois as well. Loosening the drive belt and spinning the water pump seems like a good idea although it doesn't squeak or leak so I don't expect that will reveal anything amiss. I've checked the lower radiator hose in the past to see if it was colapsing and it wasn't. However, I didn't check that the other day, I will check that next time again. Someone mentioned to do a "flush" of the system which sounds logical. I assume a reverse flush would be what I want to do. The service manual indicates the reverse flush should be done through the lower radiator hose outlet by attaching a speacial tool and forcing water back through the block, radiator, and then I guess out the bottom of the lower radiator hose inlet. That sounds effective, but has anyone seen a sepcial tool (they give a tool number in the service manual) that would seal the lower radiator hose outlet on the engine and allow for the intorduction of water into the cooling system of the engine and back flush it through the block? That I would be interested in. I will work on using these ideas and may be back with some updates. Thanks again for the ideas.

60 miles a year is severe duty. It invites this stuff. Go drive it, that's what they are for.

DennisH, I agree...It's reached the point I don't need anymore trophy's. But after performing an OE restoration for show purposes, you either have to show them or drive them...you can't do both and be competitive. Now I've reached the point where it time to drive and enjoy (cruises, etc.) but I can't very well do that until I get the over heating issue solved.

Can someone tell me for sure, if the water pump/thermostat, etc is working properly, shouldn't I be able to see a definite "coolant flow" through the radiator cap opening when the thermostate opens?

Thanks,

I say as someone else did above, pull the thermostate first.

see if it is a flow issue. I think we have seen 2 or 3 like that in the last few months where the thermostate was bad, right out of the box.

It only takes a few mins to do and see if the flow characteristics change. if it doesn't help, then just one more thing to check off as checked.

also I doubt it is the lower hose, as you were at idle.