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Gets too hot in traffic

14K views 68 replies 18 participants last post by  Blueraj 
#1 ·
My 69 ss 396/375 gets too hot in traffic, ok when driving. Correct radiator, fan, thermostat, etc. All new. Has vintage air but I've not been using it lately. Another problem. Maybe I can add an electric fan if it will fit? Taking the thermostat out may be an option but I'm not sure about that? The car seems to be getting too hot but does not overheat. Gauge goes into the red until I start driving away from lights and stops. Gauge seems correct per a temp gun. Any ideas?
 
#27 ·
BBC's came with panels between the radiator support and inner fenders
Are these the "D" shaped panels that block off air behind and just to theoutside of the healights?.. they have a rubber seal edge to them.
and have a 1 inch thick foam gasket across the top of the radiator support
Im sure I used to have a rubber seal along there???? dont now.
I always considered these items to have something to do with stopping rain , dirt etc getting into the engine compartment, rather than increase pressure for better air flow...
Trouble is If these are for air flow they would only have effect while the car is moving...these issues are idle.

Rad cap may need to go to 18 lb to prevent boiling when shut down after a drive.
From memory cooling system pressure spec is about 22lbs, an 18 lb cap get close to spec, and with extra pressure the cores want to curve the raditor (over presuize a raditor on a bench and watch it curl).. which places loads on soldered joints, Failure over time....With coolant levels at correct marks on the side of the radiator, over flow is not an issue...check both cold and hot...fill to cold level...get to running temp, CAREFULLY remove cap, and adjust hot level if needed.
 
#28 ·
Ok today I bought a new thermostat, they told me per their computer the stock thermostat was a 195. I thought 185? IDK. I also noticed that I can here hissing from my cap after I turn off the car. Seems wrong so I bought a new cap too. If neither part helps I will back flush.
 
#29 ·
they told me per their computer the stock thermostat was a 195. I thought 185? IDK
Damn someone has a parts book/ computer data base that is correct...
We had a discussion on this a while back, and from memory it was Everett above who came up with the correct info....180 / 185 / 195 wil all work fine... I run a 140, but thats because of my fuel type and the tollerances when built the engine to do so...I have even run a 120....that is how good the stock cooling system is....it will run the engine under load/ towing conditions in summer and not go above 130 with a 120 thermostat.
 
#30 ·
Maybe I missed it if someone mentioned pulley diameter, but my original 375/396 water pump pulley seems smaller in diameter than yours. That makes a huge difference. (figured it out while building my ski boat) Anyway, if you can't tell if yours is larger or the same from these pictures, say the word and I'll measure mine tomorrow.
Engine Auto part Vehicle Car Automotive engine part

Vehicle Engine Car Auto part Motor vehicle
 
#34 ·
Actually it's just the opposite - the faster the water goes thru the radiator ( up to the point of physical damage), the more heat is dissipated. Here is some good cooling info from folks in the business of cooling:
This has been argued, discussed and therorised about...
Then comes the real world practical reality....a raditor with few or no blocked cores and no thermostat restriction will run a car hot....the hot water goes from one side of the raditor too fast, therefore the temp on the 'cold ' side is warmer...warmer coolant into the engine doesnt pichk up as much heat...also because it is moving faster , hotter in the inlet side, again even less time in the raditor and even hotter on the colder side.
All substantuated with thermeters, and even covered in detail by the old school guys of the time like Smokey Yunick.
There is therory and what happens in the real world
And there are now a lot of us who have chassed our tails in cicles and found , like the Smokey Yunicks found the GM cooling system in our 1st gen era NEED a thermostat, not just to contol temps but restrict flows....
 
#37 ·
How does a radiator work at all? Long before Smokey there was this guy called Newton. If the engine produces more power, say going from and idle to highway speed, how does it dissipate the increased heat? Could his problem be too much air flow, maybe the air is going through the radiator too fast to pick up any heat....

Why does a thermostat open when the engine warms up? A: to reduce the flow so the water stays in the radiator longer? B: to increase the flow so the radiator will get hotter and it can dissipate more heat by using the laws of physics?
 
#35 ·
New radiator cap and 195 degree thermostat, no help. The car runs hot in traffic with no thermostat or new thermostat. Back flushed the radiator until the brown turned clear with the engine running and heat on. Re-serviced, exactly the same problem. My next step will be to remove the radiator and take it to a shop and have them work it. Only other thing I can think of is the water pump which seems to work to me.
 
#36 ·
New thermostat...that mean u assume it is working corectly? Its not uncommon to have a new thermostat , even out of the box to be faulty...always "know" check in a jug and a thermometer.
U cant "back flush" with the engine of... back flush is reverse flow.
Checked behind the stop **** yet?... what is in there is best indicator of the internal condition of the cores...

Quich check of water pump... ie the rear occassion of a spun impeller in the shaft splines....with cap off, rev up, watch inside the radiator (correct water height) and the coolant will hit the opposite side of the header tank in a very big way....
Note here a impeler that is sliping plus a radiator with blocked cores will do the same...raditor in good condition the water flow will be weak.
A quick check for blocked cores... leace car over night to cool right down.. (or after a full flush fresh water) idle engine on fast idle....and feel how the raditor heats up...blocked cores... usually lower in the raditor...stay colder longer
 
#39 ·
I have another tid bit on fan shrouds, while they are essential for low speed cooling by creating a suction that pulls air through the radiator as well as channeling that air to help remove surface temperature off the engine surface they are also essential to high speed cooling. At high speed, without a low pressure area behind the radiator, the front of a car acts more like an air dam and, just pushes through the air. When the engine cycles to the temp where the clutch fan engages a low pressure is created behind the radiator via the proper location of the fan in the shroud and helps all that high speed air to funnel through the grill, through the radiator and so on. Without the shroud, the fan by itself is largely inefficient for the task.

I know the op has the proper shroud, I just thought this was good info for the topic of efficient cooling.
 
#40 ·
Why does a thermostat open when the engine warms up? A: to reduce the flow so the water stays in the radiator longer? B: to increase the flow so the radiator will get hotter and it can dissipate more heat by using the laws of physics?
Why does a GM...( well most) thermostats have a restrictive opening? when fully open...would not take much to make a thermostat that has far less restriction right?
As ?I said above... there is therory, and what happens in real life...and when monitored , to which I repeat... one finds because that because a given volume of water passes thru the same raditor at a far shorter time at a constant, given air flow, it exits at a higher temp (basic Newton Physics).. If there is a higher temp of coolant entering the engine, it will...(bastic Newton physics) exit at a higher temp...
The difference with our 1 gen raditors and modern radiators is ours are way over built, no significant internal restriction .. modern factory raditors with modern building budgets are made to the min requirements of cooling.
 
#47 ·
You failed to answer the basic question - why does the thermostat open when the engine gets warmer?

One thing you need to remember, it's called a radiator not a water cooler. The object of the system is to radiate heat, not cool the coolant to a certain temperature. A radiator is most efficient at transferring heat when it is as uniformly hot as you can make it, as demonstrated by Newton. There is no 'time' in the cooling equation, because the system is a closed one.
 
#41 ·
BTW, I can't put my hand on the front of the radiator because the a/c condenser is in the way so I can't tell if some areas of the radiator are cooler that others. I wonder if it's my console gauge? I doubt it bought it was brought up as a possibility to me. It seems to work normal but after running a long time in traffic it will climb into the red pegged area but the car is not smoking or making overheating noises but it seems really hot. I guess I could order a new $50 gauge. I have a new temp sender I'll put in later this week because it was cheap.
 
#42 ·
Does anyone know if I can hook my multimeter into my fuel gauge wiring at the console and read what the temp is by converting ohms to temp. Like so many ohms = a certain temp? Same with my fuel gauge because that reads E when ten gallons left. Haha, another problem.
 
#43 ·
I wonder if it's my console gauge?
From you descriptions of behavour, in particular using the word "climb" doesnt indicate issues with gauge or sensor..
In saying that it is good to know...more happier driving... exactly what temp the gauge reads at what postion the needle is in...
'calibrating' is simple...
remove the sensor....clip a ground to the body, connect the gauge wire and hang in a cold electric jug of water (gauge wire not in the water)
Now turn the jug on and with a thermometer u know the temp of the water, and compare with what the gauge reads...
I adjusted my gauge to read at the RED band (Red=stop rght?) temp of 235 degs...
Sorted that temp and temps above 212 F using oil rather than water..
235 is also the idoit red light temp....

I also run idiot lights for water temp and oil pressure...
One doesnt spend a lot of time watching gauges, espec on a road trip....and track cars usually run idiot lights for similar reasons.
A red light come up on the dash gets ones attention immediatly
The oil pressure sensor hole behind the dizzy, has a T into the block....one end has the idoit light switch, the other the gauge sensor
I use the switch for the water temp in the head and gauge in the valley cover by the thermostat.
 
#46 ·
What is an electric jug of water and what kind of thermometer do you put in the water?
Electric jug... it is a kitchen tool... a container with an electric elewment in the bottom for boiling water to make a cup of tea or coffee.
A thermometer...we are talking temps engine cold around 40/50 deg to 212 (boiling piont of water at atmosperic pressure) a glass mercury or even an accruate cooking thermometer that measures in that range.
 
#48 ·
The other day I was driving the car in traffic and it got hot again and it started to wind out the gears and shift hard. It suddenly was real low on trans fluid. I think the trans fluid overheated and vented out of the trans because there is no leak. Does the TH400 do this?
 
#49 ·
No. If it blew oil out of the vent it would be incinerated, or at least overfilled, not low on fluid.:)
 
#51 ·
I found the vacuum line that runs from the intake manifold to the vacuum modulator completely melted so that no air could pass through. I guess that's why the shifting was so bad. Not so sure about the trans fluid being low. I got under the car and could not see where the fluid leaked from but when driving it in this condition it would wind out the gears and smoke from the hyd fluid dripping on the headers occasionally but I can't figure out where its coming out.
 
#52 ·
but I can't figure out where its coming out.
So u have eliminated the dip tube, fittings , gasket etc.
To check this u need a hoist.
Someone in the car up on the hoist, load the trans a little, ahd go thru the gears.
look in an area at the se of where the cooler inlines adapter screws into the side of the trans....
Those fittings screw into a taped thread in the casting which is very poorly suported in that casting pattern. Over tighten those fittings or tighten up the pipe fittings an litle over enthusartically and can cause a hair line crack, which may only leak when the trans has heated up.
If it is, there is no real chioce other than order a new set of bushes. bearing for the trans, dissmantle, grind out weld up the crack, add a little weld to gusset the area better, re tap the thread, and put back together.
 
#56 ·
Todd, any chance you could send a couple plane tickets so I can come over and take a test ride?
 
#58 ·
u guys in the states have a far better chance of driving these car than us here in NZ.
I have driven a few but none that is basically 'stock'....
We have to get a warrant of fitness cert every 6 months to be legal or have insurance on the road...Clubs , word gets around those certifiyers who take their job seriosly and use them....It is very interesting to hear... the honest good and ugly opinion of these guys who have driven them..all classics Dodge what ever....small country NZ.
Given the age . modern techonlogy put aside
 
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