1968 318 temp

1968 family wagon (high top) with the 318 and a 727.
short : What temp should this engine be running at? (80+ degree weather, driving on country roads)


long: I got this beast running and didn't really worry too much about temp. Took it for a nice long drive with some highway to stretch its legs (after city driving for a few weeks). The overflow filled up and sprayed boiling coolant everywhere, so I pulled over and got towed.

After inspecting things, I see that my belt was slipping and pump wasn't spinning. this all makes perfect sense, but now I'm on edge. I replaced my alternator and belt and snugged it all tight.

I have no guages right now (trying to decide whether or not to go original or new). So I bought one of those caps that go on the radiator.

Driving on a hot day, I'll get a little bit of fluid into my overflow, but it doesn't fill up like that horrible day. The radiator cap will sit between 200-220.

Does that sound ok for temps? Seems hot to me from a bit of research, but people allso say these radiator cap guages show hotter. I have no idea.

Do you have a belly pan under the engine to direct the flow of air? I'm sure some of the Dodge guys will chime in.

I do have the belly pan in place.

I prefer 180*

when running to grab groceries and junk, it tends to sit closer to that 180 mark.

I installed a proper temp gauge and took it for some drives. Close to home, no problem, sits between 180-190 ish (gauge isn't super exact).

Then I took it down a stretch of highway and it crept up toward 210. The overflow filled up and foamy fluid gushed out of the overflow again. I'm suspecting a very slight head gasket leak or tiny crack in a head. That would make sense right? I have a test kit coming to test the antifreeze for exhaust.

Interesting... let us know what the test reveals.

well, I did the fluid test thingy and it didn't show as having exhaust in the coolant.

with the cap off I can look down in there and see coolant moving after it gets warmed up, that means my thermostat is working, right?

I guess that leaves timing, fuel/air mixture, or exhaust leak right at the head. I'm no expert, but these are the things I'm considering. Does anyone have any more ideas why it would overheat when hitting the highway but be fine around town (more or less).

You can remove your thermostat and test it in a pan of hot water to see if it opens up.

First I would put in a good quality 180 degree thermostat, yours may be stuck open, they are cheap.

Next I would check you hoses, if old the spring inside the hose maybe rusted out causing the hose to partially collapse at highway speed.

Lastly I would check the radiator and see if you see a bunch of buildup in the internal passages. It may be flowing enough for around town driving but not enough to cool it when you are generating more engine heat from freeway speeds. Does it also do it if you are going up a steep hill?

I don't think it is a timing, cracked head, or air mixture issue.

My opinion...

If your overheating on the highway, then based on my experience you may have a cooling capacity problem; either not enough water flowing or not enough air moving or both.

What i mean is this: when you cruising along ' in town' you're likely not sustaining higher RPM's over long period of time.  That leads to the engine producing less heat because it not working as hard as it is under sustained load, ie highway driving.  So, less need for cooling; usually, some vehicles hate stop and go and will overheat worse that way.

I'd start with the basics, as other have noted.  Make sure your hoses are new(er), fan clutch is engaging, radiator is not gummed up (have you used a 'good' radiator cleaner and followed the instructions exactly to clean your radiator?), air flow to radiator is not restricted, try a high flow T-stat (their inexpensive)    

It's a pain to trouble shoot such issues.  But, if you don't eliminate all the 'easy' stuff you can, you may throw money at parts that are not required.  

BTW, good 3-4 core radiators are pretty inexpensive nowadays, might be worth considering plus you get reliability of new radiator.  

I chased on overheating issue in my wagon for a year or more.  Finally, got a high flow water pump, new al 3 core radiator, electric fans, high output alternator and high flow t-stat and issue is much better.  I'm still not driving it in stop and go traffic this time of year.  

Good Luck!

Agree with others, radiator likely not fully functioning due to scale buildup. I have a 1969 G10 with a mild 350. It would get to 210-220+ and keep going the longer I drove it. An infrared temp scan of the radiator showed the lower half to not be flowing. I installed a 2 row high-efficiency aluminum radiator and problem is solved

thanks for the feedback folks!

Welp, I adjusted my timing and took it for a spin only to blow a tire a few blocks from my house. I popped on my spare and took it right to my local shop.

While it is in getting new tires I'm having them do a full radiator flush. We'll see what that does. If I can identify that the radiator is the problem, I'll happily toss a new one in there.

while it was getting new tires, they flushed the radiator and tested my radiator cap. They said that my radiator was nearly plugged, and I could do better to get a new one, but a flushing helped. They also said my radiator cap was releasing pressure at roughly half the pressure it should have been, so I got a new one of those. it has been great since then, though now I think I'm being paranoid about temp. (still creeps around between 180 and 210 when driving around in 90 degree weather).

Sounds like an aluminum radiator may be in your future.

Maybe. I actually made an offer on a small farm and they accepted so it may be the next owner's first upgrade. We'll see. My heart won't be broken if it just happens to not sell and I'm forced to keep it.

Can you post the Craigslist ad? I’d like to see pics

I listed it pretty high, figuring I can negotiate and come down over time https://springfield.craigslist.org/cto/d/springfield-reduced-1968-dodge-a100/6984261279.html

Get an infrared temp gauge and measure real temperature first.

I get approx. 180 with no/few load on the engine (around town) and if I go highway with sustained speed of 65-70 (approx 3200rpm with my 3.23 rear end, ambient approx 75F) I get approx 200 - and I'm still fine with that number. Above that, I'd start to think about it. First would be to flush the system and check the radiator core.