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overheating at high speeds

k20

k20

3/4 ton status
Joined
Sep 9, 2001
Posts
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Location
Mineral Springs, NC
Hey all, have a slight prob. My truck starts overheating if I run it at high (75, 80,.....*wink-wink*) speeds. I have a big block 4 core radiator, new fan clutch, new hoses, new t stat, etc etc. Temp gauge doesnt move if i idle in 100 deg weather in traffic all day long. But if i get it up to 80ish(yeh yeh swampers aint rated for that) it starts creepin up, but the second i drop down to 65 or so it cools right off. I only have about 3/4 of a fan shroud, I can get another for abt 30 bucks shipped, just wondering if thats the prob. Thanks guys.
 
at high speed a fan shroud isnt needed
did you pop a head gasket recently?
I had that problem due to combustion gasses reacting to coolant, or for what ever reason, the water pump was toasted
in my case the heater core died first then the overheating issue at speed
 
That is a possibility considering I only have the stock tbi on the bastard.............BTW, i just walked back in from lookin at it again, dont have a pressure tester so cant do that, there is no oily residue on the side/back/front of block or heads like where antifeeze dries, and there are no bubbles i can tell in the radiator.
 
A friend had an oldsmobile that only overheated at high speeds,we changed the water pump,radiator,cap,and thermostat,the problem persisted--on day his father asked him if it had a spring inside the lower hose--we looked--it didnt--went to the parts store and got a new upper and lower hose,the lower one came with a spring in it!.We installed it,and it never did it again!--his dad had the same problem on an old ford pickup years before,he had his mechanic replace all the belts and hoses before a long trip,10 miles up the highway it boiled over--the mechanic picked him up in the wrecker,and towed him back to the shop--his old hoses were still in the rubbish barrel,he fished them out and found the lower one still had the spring in it--evidently the new one had none,they expected the mechanic to know this and re-install it in the new hose! /forums/images/graemlins/doah.gif--they put the spring in,and it was fine.I guess the lower hose can get sucked shut at high speeds without a spring,cutting the flow to the engine. You may also want to be sure your water pump is the correct one--they have reverse rotation ones on serpentine belts VS regular rotation for a V belt,but both will bolt on. /forums/images/graemlins/crazy.gif
 
Yeh I have the right pump, but now that you talk about the spring i dont remmeber if I had that......and with that high flow aluminum pump.......Im sure its putting more suction on it that the stock one
 
My b.b. used to do that what I had to do was remove the 1 heater hose from the radiator and put it to the block no more hot only 185 down the road. Is 1 of your hoses hooked to the rad. ?
 
You need a better radiator. The only reason I can think of as to why a vehicle would overheat under high speed is the lack of necessary heat exchange occurring in the radiator.

You may look into a THICK two core aluminum radiator. At high speeds you're already going to have good airflow and at that RPM your water pump is turning sufficiently to provide maximum flow.

Under high speed your engine is under heavier loads than you'll ever see in traffic, and thus the need for heat exchange is higher, and despite adaquate airflow and flow of coolant, you're simply not shedding enough heat in the radiator to keep the engine cool.

This is pretty common with any vehicle that has extensive engine modification like yours does.

A stock type 4 core is designed to cool an engine 10% smaller than yours with quite a bit less compression that burns far less fuel than you're capable of burning.
 
I was with you on radiator at first, but someone else melted their pistons by running a stock TBI on a "hot" engine. As MJ mentioned, running lean is a possibility.

Insufficient cooling is a possibility, but with the light load on the engine cruising and the amount of airflow, compared to idling, you would THINK that overheating would be exhibited at idle as well.
 
Yeh I have one heater hose going from the radiator to the top firewall port, and then from the bottom port on the firewall to the manifold.

But Tim, the thing I dont understand is why would it cool back down immediately when I drop 5mph. It overheats at 75, but then drop to 70 and it drops right back down. I cruised to school and back at 70 today and no overheating, gauge never moved, but the plot does thicken, more on that in a minute. If im runnin 75 and it starts, it will continue even if I duck in behind a semi (thereby reducing engine load aka drafting) it will keep creeping up. But the moment I drop 5mph, it begins to cool. Im beginning to think that the high speed of the water pump at that speed is closing off the bottom rad hose. Im wanting better ideas before I go to the trouble/expense of draining all that coolant.

Now, as to why the plot thickens. Today on the way back from school, I got a check engine light. It was code 32, which is EGR system, I did a test of starting it, push up on diaphragm, and the idle didnt change. Isnt it supposed to? What does this signify, and could it be related? Thanks again guys.
 
[ QUOTE ]
The only reason I can think of as to why a vehicle would overheat under high speed is the lack of necessary heat exchange occurring in the radiator.

[/ QUOTE ]

Yes, but the question is why? Insufficent thermal transfer or too much heat being generated thus exceeding the capacity of the radiator?

Can you verify the fan clutch is actually working? Generally you can hear the clutch come in with the increase of the fan noise, but with big tires... /forums/images/graemlins/dunno.gif

Fan clutchs ARE important for proper cooling at highway speeds, contrary to popular belief. I was involve years ago with a overheating condition @ highway speeds while working at the dealership. Real similar to this, stock V-8 RWD Buick. I installed new radiator, cleaned condensor checked for leaking head gaskets, checked carb for lean condition etc. Just a bad fan clutch, didn't make any money on that one but was one heck of a learning experience.

With vehicle sitting still run motor 1500-2000rpm until it warms up. Around 200-210ish you should hear the fan come in and coolant temp should drop down and fan dissengage. If you have a 160* stat or something you may need to partially block the radiator with cardboard to get the temp up. If the clutch checks out my next step would be a full shroud.
 
Someone else had a similar problem. I wonder if it isn't a *locked* (still a failure) fan clutch that causes overheating on the freeway. The fans aren't supposed to be locked up while cruising as long as they aren't seeing excessive temps.

Could always pull the fan and see what happens. You do NOT need a fan with an adequate cooling system while cruising and no load. (towing, lugging engine uphill, etc) Tested it myself. Idle with no fan=instant overheat (if up to operating temp) but anything over 25MPH fan wasn't missed at all.
 
This has already been mentioned, but I will give my personal experience.

I had the same exact problem, and I replaced the radiator. The problem went away.

I did have just a plain old 350 with carb and none of that emissions crap, so I don't how all that would affect the problem. I don't even think I had a fan clutch; just a straight flexfan.
 
[ QUOTE ]
But Tim, the thing I dont understand is why would it cool back down immediately when I drop 5mph. It overheats at 75, but then drop to 70 and it drops right back down.

[/ QUOTE ]

Becuase you're reducing engine load.

Check your ignition timing and watch your O2 sensor outputs with a scanner, but I'm still putting my money on insufficent heat exchange in the radiator.

Too much motor, not enough radiator.
 
I replaced everything you did including putting in a 4 core rad omce I moved the heater hose all was good just get yourself a fitting for the engine and a plug for the rad. With the hose hooked to the rad it is constantly pumping hot coolant from the engine through the heater core and into the radiator.
 
I dont think the fan clutch is the problem it is fairly new, and w/ the truck off i can spin it by hand, but its not so easy that it will keep spinning when i take my hand off. The clutch that was on the 454 setupd before was locked up, to sping it by hand u were spinning the pulley under the belt lol. Also there aint no roar at speed, like there was w/ the locked up one.
 
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