Cooling Systems

Damon Kuhn 3/10/2010

It’s about 186 degrees everywhere and I’m getting calls/noticing posts on cooling problems, so this is to take a stab at some helpful hints…..as always I am not responsible for my demented sense of humor or anybody failing to stop the engine to change the fan belt or otherwise modifying their physical or mental condition.

First off, you DO want a certain amount of heat in your engine. Technically, from a strictly book position, you want to run it as hot as possible without damage. Obviously there are limits and in particular, we generally figure the damage point to be 252F. That said, a good operating range is between 175-230 degrees F.

But you say, mine runs better when it’s dead cold….well maybe it does, however that would point to the engine being tuned improperly – typically the carb being too rich. NEVER EVER hammer on an engine until you have at least 150 degree temp. WHY? Well because the parts inside to include bearing clearances, piston to cylinder clearance and head gasket crush change as they warm up – if you go blasting thru the neighborhood at 75 degrees engine temp, you invite piston skirt problems, oil circulation issues and if there’s enough compression, blown head gaskets.

I like to see no more than 220-230F running temp. Preferably a little lower, because you have no real safety zone otherwise. Don’t expect you roller cam 14/71 blown big block to run at 165 degrees – the more power you make, the more heat there is to dissipate.

Now on to some specific hints: If it always ran fine and now all the sudden it’s too hot then before doing anything else, check the most likely culprits: RADIATOR CAP SHOT – take it off and observe the rubber gasket. If it looks like it’s not touching anything when it’s installed, or the rubber ring doesn’t exist, or it’s all cracked and nasty-replace it. Many times this is all it takes – if there is no pressure developing, the coolant will boil earlier than normal, turn to steam and not cool effectively. LOOSE HOSE CONNECTIONS – look for signs of coolant dribbling down the hoses or stains – again loss of pressure. LOOSE fan belts, typically will squeal, but not always – if it slips, the water pump is not spinning properly OTHER LEAKS = self explanatory //// Ok you say whaddaboutusguys that just built it and it noworky.

First off narrow down the problem – does it run hot (1) ONLY AT IDLE? (2) ONLY AT HIGHWAY SPEED? (3) ALL THE TIME? If its (1) ONLY AT IDLE, you more than likely have a fan problem, as that is basically the only thing moving the air across the radiator at that point. Logic here is once the car is moving the air flowing thru the grill is keeping you cool. If it’s a clutch fan, spin it by hand (WITH THE ENGINE OFF) and note how much it turns before it stops – more than a turn or so and it’s time to buy a new one.

Not that? Do you have a fan shroud and is the fan properly located in it? The shroud is designed to force the air to follow a path thru the radiator and cool air from the front across the radiator. Without one, you are likely just stirring up the hot air under hood and wasting a large part of the fans ability to cool. The fan should be located +/- about 1″ from centered in the rear opening. Too far back and you lose the effect, too far forward and you create negative vortexes and actually “stall” the airflow.

While on the fan subject, a high quality clutch fan is usually your best bet as it somewhat freewheels at high speed saving power and self destruction.

Another one of my favorites is the notion of taking off the mechanical fan in favor of an electric one. While there are cases (no space engine swaps) where this may be the only option, there is usually no good reason for this on the street in my opinion. Why would I say that? Well, you are giving yourself another reason to be dead on the side of the road – if the engine is running, a mechanical fan is turning – an electric one may give up at any time, leaving you with no low speed cooling. But, you say, I am saving HP! Not necessarily. Nothing is for free. In the case of a drag car with no alternator or a cut off, yea, but on a street car, the increased load is directly on the alternator which in turn takes more HP to spin, thereby negating most or all of your perceived gain. IF you must use that method, get a good high cfm fan. The little 69;95 jobs don’t move enough air to cool a lawnmower.

If it’s not the fan, did you install a fancy big water pump pulley to save HP? If so, take it off and try your stock one-many times the reduced speed caused by these pulleys not only slows the fan down but also slows the water pump speed so that normal water flow becomes something akin to a lazy snail moving thru the engine causing steam pockets etc.

Same thing goes for the big alternator pulley that causes your charge gauge to go negative at idle. Improper base timing – if you are not getting enough total timing, the engine will tend to run hot as much of the power is going right out the exhaust – the burn cycle initiates and therefore ends late – extreme cases can find the fuel mix still burning strong going out the exhaust, but at the very least you need more throttle for any given speed, causing the engine to work harder. If it’s not any of these, see section (3) IF it’s (2) ONLY AT SPEED, it sometimes gets more complicated.

Easy things first: Collapsing lower radiator hose – there should be a spring inside the lower hose to keep the water pump from squeezing it flat at speed – if your lower hose is the slightest bit mushy and there’s no spring, this is a good starting point – you can sometimes make a coil tightly around the outside of the hose with a coat hangar (or the inside if you are inclined) to test this – if it’s better or fixes it, buy a hose with the spring or buy the proper one and one of the flexible hoses to steal the spring out of.

No Thermostat – very common in the South and many times causes the coolant to flow too quickly at speed, not allowing time enough to cool in the radiator – you don’t have to buy one of the fancy 15 dollar restrictors – simply grab one of your junk thermostats or stand in front of the parts store and beg for somebody’s old one – snip the u shaped piece off and remove the guts-now install the ring in place of the thermostat. While on the subject, thermostats are generally a good idea especially in climates where there are a lot of days under 80F – they bring the engine to temp quicker. Improper max timing – if you are not getting enough total timing, the engine will tend to run hot as much of the power is going right out the exhaust – the burn cycle initiates and therefore ends late – extreme cases can find the fuel mix still burning strong going out the exhaust, but at the very least you need more throttle for any given speed, causing the engine to work harder. If all else is fine, and you have your vacuum advance disconnected, try rehooking it and see if your problem disappears.

If you built a 18:1 iron head motor go buy some decent fuel and set your timing where it belongs. If you need much less than 30-32 degrees on a Mopar to keep it from detonating, you need better fuel. Over advancing will cause issues also, but will generally result in audible knocking. Radiator too small – this will generally fall in number (3), but a radiator right on the edge may cause high speed only issues. On Automatic cars, too much stall speed – If your stall speed is much above your cruise RPM, the transmission will overheat and cause the radiator to do more work – this will also burn up your trans. Too small/smashed/plugged up exhaust – causes the engine to work much harder to maintain speed. Usually also results in poor performance, Dragging brakes – same effect as above. Fancy cardboard Superbird nose taped on the front of your Aspen blocking airflow (I actually saw this once)-point being, check for loose items that blow up and block the radiator at speed. Mixture too lean – check your plugs after a medium range cruise (try not to idle much at the end)- if they are white or burnt looking, jet up 2-4 sizes and see what happens.

Some added notes – this one courtesy Trannyman at Moparstyle.com: Nice article but would like to add another check for overheating while cruising. Those fancy solid blade fans at speed will sometimes impede the flow of air that would naturally come through the radiator. My buddy had a 79 Camaro that would run cool as long as you didn’t get on the freeway. It could idle for hours and not overheat, 5 minutes on the freeway at 60 mph it was boiling over. After hundreds of dollars thrown at it replacing water pumps, radiators, thermostats, hoses, etc, We finally took off the stupid flex fan that was on the car when he got it and going down the freeway it would now stay cool. We replaced it with a stock fan with a clutch and the problem was gone forever. Best we could figure is that the aftermarket fan was blocking the air flow at speeds and stalling the air around the radiator causing it to overheat. I have since noticed that some fan shrouds have vent slots in them with rubber or plastic flappers that will open up if more air is coming through the radiator than the fan will allow

Another from Maxwedge at moparstyle.com: A dealership line mechanic proved to me that, in some cars, the fan contributes a lot to the cooling at road speed and that there is no reliable test for a fan clutch…the FSM even states that a suspected fan clutch should “be replaced with a known good unit IF it’s (3) ALL THE TIME – is your thermostat opening? see if the upper and lower hoses are roughly the same temp-or leave the cap off when cold (DON’T YANK IT OFF WHEN HOT) and watch to see if it circulates when it warms up. IF not, replace the thermostat (don’t leave it out or throw it away – see number 2 for explanation) Is your radiator too small? If you are using the one out of Grampa’s Neon in your Hemi Roadrunner, it probably isn’t going to work. Is the water flowing too fast? If you took the thermostat out completely, this is possible-see (2) for possible fix Does the front of your radiator resemble a graveyard for various flying objects? If it’s full of bugs and mud, it won’t cool – spray out the fins and see what happens. Did you overbore the engine more than .040? Many times over boring .060 or bigger creates cooling problems due to inadequate heat transfer – those 3 extra cubes are gonna cost you big bucks for a massive cooling system. Overall Lean condition – see 1 and 2 for explanation Weak spark – if you have a weak spark, the engine will burn erratically and can overheat as a result. Clogged up radiator – if it came stock in your 1956 Belvedere it probably needs rodded out or recored.

Many other things can cause constant overheating such as high internal friction (inadequate clearances etc) but that will usually result in a poor running engine. Also if you are often mistaken for the mosquito fogger, your engine is laboring to burn the oil and probably won’t cool properly. I am sure I’ve missed some important one, but maybe this will solve some problems.