Slant 6 Overheating

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dartman59

dartman59

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Ok, folks: 1971 Dart, 225 /6, non-A/C. Never had an overheating problem; in fact, in the wintertime I complain my heater doesn't blow hot enough air (new core, so no problem there...).

Yesterday, on a 15 mile ride, I noticed after about 5 miles that my temperature gauge was up around 1/3-1/2. Not quite to the half-way mark (non-rally, stock dash & gauge). After about 10 miles (cruising at 65-70 on the interstate), I started smelling coolant. After 15 miles and when I pulled into my driveway, I had pretty much lost all my coolant through the puke bucket, which doesn't have a cap on it. Coolant all over the engine compartment. Big mess.

This car has NEVER overheated before. Why this all of a sudden? So, this morning, I filled up the radiator (with plain water), fired her up and let it sit until the thermostat had opening and I could see that things were flowing, put the radiator cap on, and took her for a spin. Everything was fine, temperature gauge at the 1/8-1/4 mark as per usual, and after about 5 miles the gauge crept up to 1/3-1/2 mark, and I headed back to the house, where I found that I had again lost my coolant through the puke bucket. When I stopped the car I could hear a humming noise (engine still running) and when I popped my hood the puke bucket was boiling over and the radiator cap was doing the humming and vibrating, as coolant was boiling out of the radiator and into the puke can.

Ideas? Here are mine: Water pump, or thermostat, or radiator cap. But I don't quite know how to evaluate the problem and would rather not replace the water pump. I guess I could start simple--do the radiator cap first and see what happens, then if needed the thermostat, and lastly if necessary the water pump.

But if anyone sees a clue here that I am missing, I sure would appreciate your advice ! ! ! Thanks.
 
Thermostats are varible open close gates that regulate coolant flow so the same slow to warm issue can also be the overheating issue. A stuck open thermostat is possible fault.
 
Also... Alt/wp belt tight ?
Clutch type or fixed fan ?
 
Put a new radiator cap on it.
 
OK--I put in a new radiator cap. Filled it up with plain water and took a drive. The motor warmed up and for the first ten minutes the temp gauge was right where it always is--at the 1/8-1/4 mark. Score! But then, at 15 minutes, the temp climbed up to the 3/8-1/2 point, and I said, "Here we go again!"

So, I headed home, expecting to find my coolant boiled over. But no! I hadn't lost a drop of coolant. Hmmmmm... So, I put in a new 195 thermostat (the old one turned out to be 195 also). Filled it up with plain water again, took it for a ride, and within 10-15 minutes I was up to the 3/8 mark. Arggh--but I figured new thermostat--maybe this is where this one sits. So, I kept driving. It stayed rock steady at the 3/8 mark for 15-20 minutes. I stopped at the grocery store, and when I got back in the car the gauge went up to the 1/4 mark and stayed there. Well, almost. It crept up a bit before I got home.

You know what? I stopped and got two gallons of distilled water, and tomorrow a.m. I gonna drain that radiator, put in a 50/50 mix, and say, "To hell with it!" I think it's O.K. and I think it was a defective radiator cap (thanks for the vote RustyRatRod).

A few days down the road I'll check back in and give an update.
 
Jones County. About halfway between Gray and Gordon. Am I readin this right? You're worried about the temp with the gauge at 1/3 the way up?
 
Hall County, myself. But up in Massachusetts since 1984. :wack:

No, I am not worried about 1/3 gauge. Not at all! But I don't like it fluctuating. Maybe it was air bubbles. I think you were correct--it was a bad cap. Probably didn't need to do the thermostat, but it was only a half hour and $12, and now it's done.

Anyway, thanks for the tip.
 
One guess is your thermostat was sticking shut, maybe erratically. Could be that when you saw good water flow, it wasn't stuck. My dad's 63 Valiant slant overheated on the freeway when I was a kid. A road worker looked at it, saw no flow, pulled the T-stat and found it stuck shut, said we didn't need one in FL (not really true) and my dad kept driving it w/o T-stat for 6 yrs. I was never able to see water flow in my 69 slant or 65 BB since the rad has a baffle around the inlet.

A bad rad cap gasket can slowly lose coolant as you drive, but probably not in 10 min. A lot of shops make money by routinely using a cap tester and replacing caps that are OK (good gasket, spring slightly weak).
 
I overheated in my truck, it was a bad cap. No spring left, slightest design pressure and it puked, and started the cycle, no coolant, hotter temp, more puke etc. funny thing was my gauge never made it past 1/2! Well, turns out my sender was broken, the brass contact seperated in the body to show no/false continuity, or what we call a high resistance short in the telco biz. Now I have a new factory spec 195 stat, new cap, and a bottom feeder puke tank with a vented cap. Rad seems ok. I think I was running really lean too...Well see.
 
Yeah, that's kinda a rule I have with old stuff. If I open the cooling system for any kinda maintainance, it's gonna get a thermostat. I usually flush the system too. You might do the same. No tellin when it was done last.


dartman59 said:
Hall County, myself. But up in Massachusetts since 1984. :wack:

No, I am not worried about 1/3 gauge. Not at all! But I don't like it fluctuating. Maybe it was air bubbles. I think you were correct--it was a bad cap. Probably didn't need to do the thermostat, but it was only a half hour and $12, and now it's done.

Anyway, thanks for the tip.
Click to expand...
 
Apparently, my problems are not over! Replaced the radiator cap and put in a new thermostat. Flushed system and added coolant at 50/50. Took a 15-mile highway run and the gauge got to the 5/8 mark while cruising at 65-70. Cooled down to 1/4-3/8 when I got off the highway and was making my way through traffic lights.

I wouldn't want to drive this thing a long way until I have a better understanding of what is going on. I do think I had a bad rad cap, but I also think it wasn't causing any problems until a few days ago when it overheated for the first time. What I don't know is why is it now running hot? And variable temperature--used to be the temp gauge was always rock steady at 1/8-1/4.

If my water pump was going, wouldn't you expect the temp to go down at highway speeds and get hotter in stop-and-go traffic? I seem to have the opposite problem.
 
dartman59 said:
Apparently, my problems are not over! Replaced the radiator cap and put in a new thermostat. Flushed system and added coolant at 50/50. Took a 15-mile highway run and the gauge got to the 5/8 mark while cruising at 65-70. Cooled down to 1/4-3/8 when I got off the highway and was making my way through traffic lights.

I wouldn't want to drive this thing a long way until I have a better understanding of what is going on. I do think I had a bad rad cap, but I also think it wasn't causing any problems until a few days ago when it overheated for the first time. What I don't know is why is it now running hot? And variable temperature--used to be the temp gauge was always rock steady at 1/8-1/4.

If my water pump was going, wouldn't you expect the temp to go down at highway speeds and get hotter in stop-and-go traffic? I seem to have the opposite problem.
Click to expand...

If you're water pump was going....it would leak or have a broken impeller shaft :lol:
If you really want to test the water pump, start the engine with the thermostat out and upper hose unhooked from the radiator, then rev the engine...you'll see instantly if it's working ;)
Overheating at speed is a flow problem. How long will it idle before it overheats? Mine used to do that, overheat at speed ( hwy driving ) and normal operating temps at long idle in traffic ( 15-20 min of idling )
It ended up being a plugged up radiator...and it just started doing it one random day, it wasn't a progressively worsening condition.

http://www.forabodiesonly.com/mopar/showthread.php?t=43837

After digging into the radiator I made several hwy trips of 3 hrs or more and it never got above the second left temp mark, even at 75-80 mph with the A/C on

You could always get one of those little dentist mirrors and have a look inside the radiator, but my money is on a bunch of plugged passages...
 
player1up said:
If you're water pump was going....it would leak or have a broken impeller shaft :lol:
If you really want to test the water pump, start the engine with the thermostat out and upper hose unhooked from the radiator, then rev the engine...you'll see instantly if it's working ;)
Overheating at speed is a flow problem. How long will it idle before it overheats? Mine used to do that, overheat at speed ( hwy driving ) and normal operating temps at long idle in traffic ( 15-20 min of idling )
It ended up being a plugged up radiator...and it just started doing it one random day, it wasn't a progressively worsening condition.

http://www.forabodiesonly.com/mopar/showthread.php?t=43837

After digging into the radiator I made several hwy trips of 3 hrs or more and it never got above the second left temp mark, even at 75-80 mph with the A/C on

You could always get one of those little dentist mirrors and have a look inside the radiator, but my money is on a bunch of plugged passages...
Click to expand...

Totally agree, probably a flow limitation.
 
Well, well, well. Guess what? I found out the reason for overheating. And it had nothing to do with my radiator cap, thermostat, water pump, or radiator.

My motor was running waaayy lean! I had a small rubber cap blocking off a vacuum port at the base of my carb. It mustof been a real cheapie because it basically rotted and split. Didn't fall completely off, but was not sealing off the port any longer. So, I've been sucking lots of air at high rpms. :banghead:

I'm a bit surprised that I didn't notice it in the way the engine was running... I did notice something a little abnormal (running a little rough at idle, and cold start manners were off a bit), but it wasn't too bad.

ANyway, I got a kick out of this solution, and I thought it would be worth sharing. That explains why it starting running hot all of a sudden--the thing finally ruptured and the vacuum leak started abruptly.

Thanks everyone for the input and ideas!
 
Hey dartman, glad you were able to solve your issue. Is it cool to hijack a thread that's been dormant for a few weeks? :p

I also have a /6 225 (71 Scamp) that for the life of me I can't get cool under load. I can drive around town all day regardless of outdoor temps, but if I get over 50-55mph, the radiator pukes. The motor is (to the best of my knowledge) completely stock with 20k miles on it. I drove it from its original home about 100 miles and had no problems, let it sit for a year, and then it just kept overheating. Now I'm determined to figure out what's going on.

Anyway, I've set the timing to 0*, checked the dwell (can't recall what it is, but it was within spec), adjusted the carb, replaced the radiator and cap (16#), replaced the water pump, replaced the thermostat twice (first with a 180, now with a stock 195). The radiator shroud is missing (if it ever had one?), but since the engine is not overheating in traffic, I don't know if it could be contributing.

The car feels like it lugs just a little bit at steady cruise, but it still has get up and go if I stomp on it. Should I advance the timing from stock? On the off chance one of my cylinders isn't firing, I got a spark tester yesterday and plan on checking that as well. At idle, the car bounces a little bit more than I would expect from a smooth idle.

Thanks, guys. I really want to start/continue driving my 71 to work. My parents bought it new and special ordered it with a really horrible color combo that according to the trim tag required special authorization - LOL. I remember hating to get dropped off at school in that thing, but I'm really attached to it now. :)

-John
 
Mine hits the half way mark sitting in traffic. New water pump, and 180 t-stat. I'll be trying a new radiator here soon and better fan. I always get a little worried when I see her starting to climb.
 
I had the same problem in my old Camry, turns out it was only a bad cap. It looked fine, just would not hold pressure.
 
I just spent a few minutes checking the timing and dwell. Dwell was at 39 instead of the 41 to 46 - I don't think this is far enough out of spec to cause overheating. Timing was at TDC, and correct according to spec. Idle was a hair low, maybe just below 750.

Having seen a number of folks post that they advance the timing a bit, I tried 5* advance. That bumped the idle up to about 800, which I've seen posts use as a rule of thumb.

Took it for a 5 mile spin, stomped on it, had it up to 70mph and no pinging, good power. No dieseling when I shut it down, no boiling over. ?????

So even if I "fixed" the overheating issue, can someone please tell me why I would need to advance the timing to get it to run properly? Am I just masking another issue here that will bite me in the long run?

Thanks,

John
 
FrozenCaveman,

Next time, start a new post so less confusing to readers, and always tell us your year, model, and engine, as relevant. Nice to have another slant owner nearby. Better to post this in the slant six forum. The more advance you can get without pinging makes more power, better economy, and less pollution. Pinging depends on gas quality and temperature, which is why all modern engines have knock sensors.

You can take the setup off an 85-95 GM V-8 like I did for all my 60's cars. The knock module has a single output "5 V knock, 0 V no knock" which you could rig to an LED in the cabin. The knock sensor (1-wire) screws into your block drain, directly for a SB or BB or with a 3/8 to 1/4" NPT bushing for a slant (Ace Hardware). Don't worry too much about V-8 vs straight six. I read that GM used the same sensor in multiple engines. That they are all push-rod engines w/ cast iron blocks is probably more important.
 
Old thread, but I finally resolved the overheating issue.

I gave up and brought it into a local shop, they checked it with an infrared gun and found that it was running at 198 degrees - normal. But when they would shut it down, and this is the same thing I kept experiencing, it would spew. So they tried 3 separate radiator caps and had the same problem with all of them. They thought that the mouth of the radiator was warped and not sealing properly. They sent me on my way, replacing the water pump and alternator belts, and tightening them - said they were loose. So that might've been part of my problem.

I took the radiator out, back to the shop that built it for me, and the guy pressure tested it. 16 psi with my cap. Perfect, though he recommended I go with a 14 psi cap. He said I was probably overfilling it (even though I was only filling it to top of the tubes), told me to put less water in it, install an overflow jug on it and call it done.

So that's what I did - I installed a 32 oz gatorade bottle with a 3/8" hole in the cap for the overflow hose, plus a tiny air hole, wired it to the grill, and no more mess. How about that.

Plus, in all that effort, I also got my dash gauges working again and I can actually see the temperature now. :p

I learned a lot in troubleshooting this problem, but I also learned that a lot of problems have very simple, non-technical solutions. And I'm a very happy driver who now takes his 4 yo to school and makes a 50 mile roundtrip drive to work every day. Only 16-17mpg, but the uniqueness factor is worth it. :)

Thanks for everyone's help on this one. Sorry the solution wasn't more glamorous.
 
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