Truck overheats at highway speed but not in town

[rreed]

FWIW it would be more like 66.6/33.3 as I took a clean 5 gal. bucket, dumped in two gal. of Prestone, one gal. of distilled water. Poured that into another clean water jug to pour in the radiator (try filling your rad w/ an open 5 gal. bucket, I dare you). While topping it off I don't recall if I used water, pretty sure I just poured straight out of another leftover Prestone jug..

Well I'm confused as to why 70/30 (which is more what mine probably is) is bad; it says on the jug what 50/50 would do for you, 70/30 it indicates a higher boil/lower freeze. I've always seen/heard 50/50 or 70/30 all my life, I just figured it meant more antifreeze=better esp. after reading the jug..

I will say I've always ran 50/50 in everything else w/o issue but since this is supposed to be a tow vehicle (if it'll ever run for more than a day or two at a time) I went w/ the stronger protection of 70/30 the jug indicated.

I will try to get my hands on a shroud. So the airdam, does it connect to the stock bumper? Mine is not but there is a good lip of the 2x4 c-channel I could attach to. I'd like to see some good pics of them and how they attach. I need to check the fan clutch, as mentioned I'm not convinced the new Hayden is working any better than the factory original one. I've never heard the fan roar and even at over heating I can spin it pretty easy.

I greatly appreciate everyone's help so far, thanks all!

[GWChris]

Well, I'm not sure the issue is the 70/30 (or whatever) mix, but here is the difference:

While 70/30 may have a higher boiling point, that is not the same as having a higher heat capacity. In other words, yes it boils at a higher temp, but it cannot transport heat from the engine to the radiator as effectively. If it cannot trasfer heat as effectively, then that would leave your engine hotter and the radiator cooler, all other things being equal.

[Hammer]

Water is better at transfering heat then antifreeze.
More antifreeze in the mixture will reduce the heat transfer.
I know GWChris just posted this, but I wanted to put into very simple terms.

Now, something to think about. At speed, a number of vehicles have a HARDER time moving air past the radiator then at slow speeds.

There are a number of reasons why this happens. But if your other components are all in working order, then here are a few things you can think on.
The air pressure in your engine bay will actually build up the faster you go.
This is because while air comes in through the radiator area, it has to go OUT under the vehicle.
BUT, under the vehicle is a high pressure area because of all the air moving under the vehicle.
This is where the stock fan, shroud, etc comes into play.
Having a shroud around the fan will FORCE the air from the fan to be pulled from in front of the radiator. And it will then build up enough air pressure in the engine bay until it can expel the air out (typically under the vehicle).

Want an easy test? Take your hood off and drive it at freeway speeds.
The air will have a much easier time escaping from the engine bay, and a LOT more air will flow past the radiator.

Get a shroud.
Make absolutely SURE that the fan clutch is actually working! It seems a lot of cheaper ones barely work even when NEW!
Change your mix to 50/50.
Add some water wetter. Cheap to buy, Redline, Royal Purple, etc. are available on the counter at a lot of parts stores, and they DO work. I wouldn't do this part of it until you have solved the other issues.

Timing. Just because your base timing is fine doesn't mean your overall timing is ok, and is definitely doesn't mean that timing under load, especially the lighter load while cruising, is anywhere near ideal.

[Don S]

..
rreed;

... Even small Air Dams can reduce air pressure under a vehicle and aid cooling. Wingnuts make for quick removal for off roading. If I make another one it will be 6 inches X 5 feet. This one is bent and battered from years of service but still does it's job. Most new vehicles are designed to reduce air from getting underneath or they have air dams installed at the factory.



Have a good one while you can still laugh about it.. Don S..
There was a time when some people got rich buy providing a better product at a lower price.
Now they get rich by ripping us off and paying off the politicians. Can you spell DEREGULATION

[letank]

air dam

My 74 does not have a air dam... i used to have a shroud.... which was removed in Moab.... no overheating.... even at 65mph..... even in 90 degrees weather.... or in the desert .....

Of course as said.... water is a better heat exchanger... and i am running 30% coolant and 70% water, which is good to about 15 degrees....

[steven79]

try 70% water not antifreeze, he posted> "Dumped it and filled w/ fresh 70 antifreeze/30 distilled water" and a 195 degree t-stat and Don S is dead on on his stuff too.

[steven79]

The prestone company and all others tell you more is better(for them) It is so they can sell more product, I run 15 to 20% low tox in mine in the somer and 40% if it looks like it will get real cold.
Mine runs hot at 55+ too but i know it is running lean at that speed from a small carb but i got it were it only gets to 210 to 220 max on real hot days.

[Chevelleguy]

What did the coolant look like when you bought the truck? If it was rusty, I would change the water pump. Over the last 20 years, I have worked on three vehicles that had mystery overheating problems. All of them had bad rust from running no antifreeze. The cause of the overheat? Water pump impeller was rusted away causing a low flow condition.

[Kenall]

The air pressure in your engine bay will actually build up the faster you go.
This is because while air comes in through the radiator area, it has to go OUT under the vehicle.
BUT, under the vehicle is a high pressure area because of all the air moving under the vehicle.
This is where the stock fan, shroud, etc comes into play..

Is this why some FSJ shrouds are open at the bottom?

[shimniok]

I can't find any 'proof' online right now but I think that having a 50-50 mix is going to offer better heat transfer as I think water does a better job of transferring / absorbing heat than anti-freeze.

I'd also check carb mix and timing -- things that could increase combustion temps, in other words, dumping more heat into the water jackets than the system is able to dissipate?

A shroud is probably a real good idea to have, but not sure how much it helps at the higher speeds. I would think at hwy speeds there is enough pressure in front of the vehicle to force air through. I seriously doubt that air flow is lower at hwy speeds unless something is wrong. Otherwise flex fans and rpm based fan clutches would not exist.

Maybe water pump is suspect? Are there any performance problems that might cause overheating?

Michael

[Blake]

I too am having problems with my 401 bored 60 over, 37's, 4:10's and new BJ's aluminum rad.

As of this second, I have the factory grill removed and a single electric puller fan. 195 stat. Today, the temp was about 85 outside and it ran at the 195 stat just fine around town.

It was overheating while 'wheeling at 12K feet this past weekend. That was with the front grill, front taurus pusher, rear puller.

I'm planning to go with a GOOD mechanical fan clutch, homemade shroud, oil filter heat sink, rear tranny cooler for the water and normal tranny cooler for the tranny oil.

I'll post my results as I find them.

I think it IS possible too cool a 401 bored 60 over, but EVERYTHING will have to be PERFECT to do so.

[backroadin']

You said that even when it's overheating, you can still spin the fan easily. I believe a good working clutch fan would not do this. When the engine is hot, it should be really hard to turn the fan - i.e. the clutch is engaged.
I would see if you can scrounge up a solid fan (no clutch, just straight fan) and put that on and see if it helps. At least then you can rule the clutch fan out for good.
Also notice you said you had an edelbrock carb? That post earlier about the wrong jet sizes causing an overheating problem is kinda interesting - maybe double check that angle too?
I would change the coolant mixture first before anything, however.

IMHO, although shrouds and such obviously help in cooling, if you've got a problem with the system, then they'll only help to mask it further.

Good luck

[shimniok]

I donno, I kind of feel like one should restore the system back to the oem state and go from there. Without proper airflow to me that isn't unmasking the problem, it's creating or contributing to the problem.

For the 401, I would expect the aluminum radiator to "mask" (or fix) a lot of problems. I have a pal with a 401 that is (I think?) 0.060 over or at least 0.030 and with higher CR. He can't reliably keep it cool. The altitude may have mucked with the mixture enough to increase combustion temps and *maybe* that is part of the problem, I dunno. It is a little scary that an alum rad isn't enough to deal with a bored 401.

Michael

[GWChris]

I wouldn't think a 401 would ever get bored.

OK, that's it, I'm going to bed.

[ChuckWag]

Overheating at highway speeds

We had this exact thing happen on two separate occasions...different things fixed it each time, for opposite reasons.

The first time, we found the thermostat stuck wide open. When that happens, at highway speeds the coolant flows TO FAST through the engine, and does not stay in there long enough to perform as a effective heat exchanger. Higher RPM=more heat, with Less heat exchange= poor cooling.

The second time, we found the water pump impeller was spinning loose on the shaft, so produced the same effect for the opposite reason...coolant flowing too slow, and getting too hot.

I agree with the others that say LESS antifreeze is better in hot conditions, because it takes very little to avoid boil-over. The only reason (besides capitalistic greed) that the manufacturers say to go higher percentage of antifreeze, is to lower the freezing temp...it takes more to do that than it does to prevent overheating.

Hope this helps,

ChuckWag