So it was a toasty day today, 96 degrees, and I was driving around doing errands in town, all stop and go typical city traffic. Came back home, and I noticed I heard a boiling sound. So I popped open the hood to check it out and there were very light wiffs of steam coming from the overflow hose (I don't run an overflow tank, and the water level in the radiator is just above the bottom of the top tank.)

Checked temperature, and it was well below 212, in the high 180's. But, I still was hearing something bubbling, which I assume is water boiling, in the upper radiator hose. If I touched the hose I could feel it as well. At this time I also revved the car in the driveway and noticed that the bottom radiator hose did not collapse, and water did rush into the top tank so the pump is doing its job.

So my question is, what is it that I'm hearing/feeling, if its not water boiling?

Later on in the evening, as ambient fell maybe by 5-10 degrees, I backed off the timing by 5 degrees (from 10deg btdc to 5). I did this because I thought I heard the slant backfire as I was driving in town. I took another drive, this time up the local hills at highway speeds. After about 15-20 miles, the temp was 155. I'm not sure what did it, was it the drop in timing, decrease in ambient, or increased airflow because of higher speeds. Perhaps a combination of each.

My cooling setup: A/C radiator (but no A/C setup), Kragen water pump (GMB brand), 16lb radiator cap, and if I recall right a 195deg Stant Super thermostat. For coolant, I run RO water, ~25% antifreeze, and a bottle of redline water wetter. Also drilled a small hole into the outer edge of thermostat as suggested by someone on here a while back.

I'm wondering about the level in the tank. You said it's at the bottom of the top tank. Why not fill it up? Also the hole in the thermostat is a probably not needed. Most thermostats I have seen have a bypass built iinto the housing or a small hole. Sounds to me like its just pumping air which I would think may make your temperature eratic depending on where the sensor is. Fill the tank and go from there.

Sensor is in the stock location, directly across from the housing on the other side of the head.

Pumping air? The bubbly/boiling sound was happening with the engine off.

I fill it to the bottom of the top tank because thats what someone on here mentioned doing if you don't run an overflow tank, so it will boil in the top tank instead of spewing out on the road. Will filling up the top tank really make a significant temp change when driving in the city in hot weather?

if he filled it up it would just get hot and when he stopped it would blow almost all of what he added to the ground... Pierre is this a car pre-reserve tank, or your just broke or missing and not running one? if so is the radiator newer style with a small tank on top? If this is the case I would see about getting a reserve tank... I added one to my 67 and my temp used to jump around all over the place, now its steady..

About the boiling, have you checked to make sure you have a 50/50 mix, anti-freeze also increases it's boiling point of the coolant..

the first thing I would do is replace your radiator Cap. The Cap should hold 16 PSI before it opens and pressure will also increase your boiling point, just like how a pressure cooker cooks fast, it allows the water to be hotter that 212F before it boils, sense it's under pressure. I think many people over look replacing these, you should replace them every time you change out coolant. The open vent you described also makes me think that might be the cause...

GL

_________________
Mike C
http://cuda.iclownz.com
67 - Barracuda Coupe - 225 - Automatic

It is an A/C radiator (bigger then standard), full size tanks.

I used to have an overflow tank on there but it was too big and never fit right, so I never reinstalled it after swapping engines.

I'm running about 25% antifreeze. The redline water wetter bottle recommended 15-20% antifreeze if I recall correctly for street driving. I'm not sure how the thermodynamics of it all works, but I know that although antifreeze may increase boiling point, you will run hotter with a water/antifreeze mixture then straight water. So adding more antifreeze to my radiator may not be the fix....

I could try a new cap. You bring up a good point about this though, the cap may indeed be faulty if it was letting wiffs of smoke out the vent while the water in the top tank wasn't boiling.

With a thicker mix, your correct it won't cool quite as well, the reason for this is because water is a better heat transfer medium. With that said if you start boiling, the Vapor gas in the cooling system will really kill how effective your coolant is going to be.

Where I live a 50/50 mix is almost required, but I don't think I don't think I would ever run less than 35-40% even in a mild climate. I was going camping a few years back with my sister, it was a nice 102 day, her neon started to over heat about 60 miles out of town after about 4 stops I went and bought a gallon of prestone at a gas station, and drained enough to get it all in the car. We didn't have to make a single stop after that. After that I had a new respect for the boil over protection anti-freeze has.

About the larger tanks, pre-reserve tank cars had a HUGE top tank, its purpose was hold on the water when it was heated and expanded, so you didn't loose any, which is now the job of the reserve tank.
My 67 None A/C car has a narrow width wise rad, but the tank is larger in volume than my 77 w/ AC rad.

Anyone know the exact year they switched to Reserve tanks? I believe it was 70, but might be 72.

_________________
Mike C
http://cuda.iclownz.com
67 - Barracuda Coupe - 225 - Automatic

my 70 HAD the large top rad tank
I swapped in a V8 A/C volare rad and added a reserve tank.
it worked great last year but this year is struggling since I raised the compression. I just added redline water wetter, now to see if it really helps.

_________________

If you were hearing this noise after turning the car off, the first thing I would do is check the themostat. If it is sticking, it is keeping the water from reaching the radiator and it could boiling in the block.
I live down here in Fla and do not use anti-freeze in any of my cars as it takes the place of water, which is the "coolant". I have found that adding more anti-freeze makes one run hotter than running it with 90% water, a little anti-freeze and Red Line water wetter.
My slant runs at 195 on the interstate turning 2200 rpms on a 94 degree day.

I don't think its boiling in the block becuase the temp at the sensor was 195. Maybe the top hose still had some water in it when I turned the car off and thats what I heard boiling? I'm not sure, just guessing....

I have the 4 blade fan, the original one on there. Perhaps it doesn't move enough air at low (idle) speeds and that combined with the hot ambient....

The head is the hottest part of the engine, and the last thing to see coolant pass it. As the coolent flows through it, it pulls heat out of the head, but not enough heat to make the water boil before it passes through. When you shut the engine off, the water stops moving,but the head is still at something like 400 degrees. Granted, the combustion is over and thus no longer creating heat, ,but the head holds more heat than the water sitting in it can absorb without boiling over. Boiling after shut down is very common. The question is,"is it boiling while you are idling? If not, a catch can, or reserve tank will solve your "problem".

tahnks re jones I was reading all the posts here and was shocked no one had thought to change the thermostat I am sure this is your problem. This hapened to my truck last summer.

I was poking around on the web and it seems common for some cars to overheat during low speed operation because the mechanical fans just don't move enough air.

Seems that the common solution is an electric fan. Lots of cases online about people using the fan from a 3.8L V6 taurus engine adapted to their cars. I was bored, so I went junkyard trolling for one of these. After some fiddling, I have it installed in my car now, just need to wire it up and wait for a hot day to see how it will work. Sure seems to move plenty of air though!

I used to have a 68 Valiant with a 225 that would get hot if you were sitting in traffic or in slow traffic. The longer you were in traffic the hotter it would get. It didn't matter if you had the AC on or not. I live in Spring TX so we have temperatures that are very similar to yours. What I ended up doing was putting a later model Valiant/Dart AC radiator in it with a shroud, a 5 or 6 blade fan and a 195 degree thermostat and an overflow bottle from a later model Valiant and a 50/50 mix of anti freeze. Remember the anti freeze acts as a water pump lubricant and a rust inhibitor. The 195 degree thermostat allows the water to stay in the radiator longer to cool down more. Oh yeah and you will need a good 16lb cap for a closed system. After I did that the car could idle all day long with the AC running in 100 degree heat with the temp guage not even moving. You couldn't make it get hot. Even the AC worked better than it had before with the car not moving. More air was being pulled across the condensor and the radiator. In the winter time the heater worked faster and better even. This was because there was no air in the coolant system. Personally I would not run a car without a shroud and an overflow bottle even if it didn't have AC. The boiling you are hearing is normal. The head has a lot of heat in it after the engine is shut off with no place to go so the water boils.

_________________
Former bent 6 owner
Mark

Just thought I'd update because of the recent talk about fans. I wired up the fan with a hayden controller. I chose the model with a sensor that had wires coming out of it, not a capillary tube because I didn't want to deal with the possibility of the tube kinking. I also used a seperate solenoid type relay that handles more current then the one the controller came with.

The sensor was supposed to go between the radiator fins, just underneath the top radiator hose. I tried this, but the fan only stayed on for 10 seconds or so and went back off, even with the temperature screw adjusted to max. What was happening was the fan was creating such air flow (even though the sensor was a few inches away from the fan) that the sensor was being cooled off before the coolant was. So I clipped the sensor to the top radiator tank, on the side opposite the fan, and it seems to work well there.

I was driving around town in 95+degF weather, and the temp at the stock sensor location in the head was 188. Overall I'm satisfied with this upgrade. No more deadly fan blades in engine bay, and yes I did notice an improvment in power. Not huge, but quite noticeable.