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74 K5 Running Hot... At a dead end

BIGRED74K20

BIGRED74K20

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The truck is a 1974 k5 blazer 350, sm465/203, factory AC. When I brought this home I never drove it to see if it ran on the hot side. Shortly after I converted it from manual to automatic (TH400/205) and swapped in a D60/14bolt. I kept the 350 in the truck with the AC and the only thing I did was remove all of the California emission crap and put headers on it. Reused the 3 core radiator that was in there for the transmission cooler.

About 3 weeks ago as I started to finish everything up on the truck, I flushed the cooling system, drained and filled it (60/40) water coolant ratio, installed new hoses and a 195 thermostat. Started the truck with heater on, added coolant as it needed and made sure all the air was out of the system. As it was idling the truck got up to 200, 210, 215 then I shut it off. Let it cool a little bit (took awhile) then took it for a short drive around the neighborhood, went a 1/4 mile and by the time I got back it was pushing 225.

Next morning I removed the thermostat, started it up and it immediately ran cooler (180) I then took it down the highway and it got back up to 210/215. So cruising down the highway did not lower the temperature whatsoever, only stays idle.

After this I checked and made sure that the lower hose wasn't kinked, thermostat was good and that the coolant was flowing through the radiator tubes (flowing pretty good) Next morning I started it and let it idle and sat at 200 degrees.

Next I pulled the water pump and it looked to be good/new condition. Replaced the water pump with a HD pump and reassembled everything (left stat out) Ran the truck and it didn't make a difference. Ran a infrared thermometer all along the radiator while it was running and it read a consistent 110-120 top to bottom with no cool spots.

Tried a universal temp gauge just to rule out the factory gauge and very little variation in temperature...:dunno:

Any ideas or leads would be greatly appreciated at this point:1zhelp:

Thanks in Advance!
 
Where are you pulling your temp readings at?
Is the shroud in place and in good condition?
Is the fan about 1/2 in/out of the shroud?
Does it have Vortec heads?
Does the thermo open when the engine approaches 195? Can you feel the hot coolant flow from the thermo housing through the upper hose about the temp it should be opening? (I would try a 180 next)
 
76zimmer said:
Where are you pulling your temp readings at? Cylinder head
Is the shroud in place and in good condition? Yes, and its all there.
Is the fan about 1/2 in/out of the shroud? Yup
Does it have Vortec heads? Nope
Does the thermo open when the engine approaches 195? When the thermostat was in, it didn't seem to circulate and maybe thats why it wasn't opening? Can you feel the hot coolant flow from the thermo housing through the upper hose about the temp it should be opening? (I would try a 180 next)
Click to expand...
No because I don't have the thermostat in the truck.
 
you need a restriction in the cooling return to radiator, if no thermo or restrictor in the thermo housing, coolant could pass through the radiator too fast to effectively cool it.
 
Stock V belts & water pump ?..sometimes a serpentine belt & pulleys get swapped on an older engine and the water pump is a reverse rotation one compared to a v belt one..

If its not that I would say the radiator or block has become plugged or restricted enough to not provide enough coolant flow or remove enough heat from the coolant..the lower hose should have a spring inside it too,though that usually only causes overheating at highway speeds or higher rpms,but it could get sucked shut any time too..especially if the lining inside has come loose..
 
Like Zim said, you need a restriction in the thermostat housing to slow things down compared to wide open.

And I would agree with checking the radiator closely to see if it has plugged fins, or plugged tubes, or corrosion build up in the tubes causing "insulation " and lack of heat transfer.
A 3 row should cool it well if all is good.
 
Your testing of the radiator efficiency, you didn't see a decrease in temp from driver to passenger side?

I would agree with checking your upper hose for heat (how about the drivers side tank), and make sure both the upper and lower hoses are pressurized. If the radiator cap isn't building pressure, the lower hose can start to collapse.

It overheats with and without one. The problem isn't the thermostat (or lack of) IMO.

Normally by looking you can't tell a radiators condition internally, but if your IR temp readings seem good, it's somewhat harder to blame it. They do go bad over time as coolant anti-corrosion packages deteriorate when people don't change or recharge the coolant. Did you by any chance watch the radiator coolant temp as the engine warmed up vs. at temp only? With no thermostat the radiator inlet temp should track engine coolant temp closely.

Also happens once in awhile that people get the wrong water pump (for serpentine, which is counter-rotating) but you've had two, pretty unlikely both were mistakenly the wrong flow direction.

Lastly, if you pulled EGR and added headers, without tuning the carb, it's going to run lean. Worse under cruise conditions, but nonetheless won't help here. Can't imagine it's the root cause, but it may be a factor.
 
76zimmer said:
you need a restriction in the cooling return to radiator, if no thermo or restrictor in the thermo housing, coolant could pass through the radiator too fast to effectively cool it.
Click to expand...

The only reason its not in there is because how hot it got with the thermostat installed in the truck. Im going to pick up a 180 and install it to see what happens. Thanks
 
Last edited:
Kain said:
are you running a clutch fan? is it working right?
Click to expand...

Yup. Whens its cold and you start it up the fan kicks on in 5 seconds or so. When its up at 200 degrees, I stuck a rolled up magazine inside the fan to see if it would stop or slow down and it just kicked the magazine out of the way. Thanks
 
diesel4me said:
Stock V belts & water pump ?..sometimes a serpentine belt & pulleys get swapped on an older engine and the water pump is a reverse rotation one compared to a v belt one..

If its not that I would say the radiator or block has become plugged or restricted enough to not provide enough coolant flow or remove enough heat from the coolant..the lower hose should have a spring inside it too,though that usually only causes overheating at highway speeds or higher rpms,but it could get sucked shut any time too..especially if the lining inside has come loose..
Click to expand...

The new pump I installed is the correct standard rotation pump, not one that has reverse rotation. When I pulled the pump I made my own spring for the lower radiator hose out of s.s filler rod just so I could rule the collapsing out as well. Thanks
 
6872xtc said:
Like Zim said, you need a restriction in the thermostat housing to slow things down compared to wide open.

And I would agree with checking the radiator closely to see if it has plugged fins, or plugged tubes, or corrosion build up in the tubes causing "insulation " and lack of heat transfer.
A 3 row should cool it well if all is good.
Click to expand...

After I changed the pump and filled the system back up, I looked inside the radiator and I could see the upper portion of the tubes and there wasn't any corrosion just a light coating of rust? But the coolant was pouring out of each tube, no trickling or anything like that. Thanks
 
dyeager535 said:
Your testing of the radiator efficiency, you didn't see a decrease in temp from driver to passenger side? Maybe 10 degrees from the passenger side to the driver side.

I would agree with checking your upper hose for heat (how about the drivers side tank), and make sure both the upper and lower hoses are pressurized. If the radiator cap isn't building pressure, the lower hose can start to collapse. The upper hose has heat but I will need to double check to see if it is pressurized.

It overheats with and without one. The problem isn't the thermostat (or lack of) IMO.

Normally by looking you can't tell a radiators condition internally, but if your IR temp readings seem good, it's somewhat harder to blame it. They do go bad over time as coolant anti-corrosion packages deteriorate when people don't change or recharge the coolant. Did you by any chance watch the radiator coolant temp as the engine warmed up vs. at temp only? With no thermostat the radiator inlet temp should track engine coolant temp closely. I have not performed this. I will definitely try that out today.

Also happens once in awhile that people get the wrong water pump (for serpentine, which is counter-rotating) but you've had two, pretty unlikely both were mistakenly the wrong flow direction. Good here, made sure of that when I ordered this last one.

Lastly, if you pulled EGR and added headers, without tuning the carb, it's going to run lean. Worse under cruise conditions, but nonetheless won't help here. Can't imagine it's the root cause, but it may be a factor.
Click to expand...
So this is the only thing I haven't addressed yet. Ive done some research regarding the difference between carburetors out of a vehicle that came with a Automatic trans vs a Manual trans and from what I have found is that the Carbs that came with a Automatic transmission will have an overall richer calibration and apparently this is to compensate for the fluid losses in the transmission.

I have read that a carb designed for a automatic can be installed on a manual but will experience a slight loss in fuel economy. And a carb designed for a manual transmission should not be installed on a vehicle with an automatic because of the possibility of running lean..?
 
nvrenuf said:
Timing? Lean?
Click to expand...

Ive messed with my timing and have adjusted it from 8-13 and it made no difference in the temperature but I am currently running my timing at 11 degrees advanced. I haven't pulled the plugs in a week or so but Im going to do that again today. The truck fires up fine, idles good and runs pretty well on the highway but I have a feeling that it could be lean.
 
I do have another Jet quadrajet carb for an Olds but I am unsure if the fuel inlet will clear the thermostat housing. I might try this swap since I used to have this on my K20 with an automatic just to see if it makes a difference.
 
I'd be less likely to worry about the manual/auto causing as much as headers and loss of EGR. I pulled EGR off a 305, no other mods from factory, and could instantly tell there was a change in how it ran at cruise. I've heard it called a "lean surge", and I certainly felt it. Flat ground, light throttle, ~35MPH or so, it was noticeable.

SBC's have terrible exhaust flow as-stock, the reason headers are worth ~25HP on an SBC is because they are evacuating the cylinders far better than the manifolds ever did, and that evacuation results in more effective cylinder filling...since the fuel of the AFR equation is governed by the carb, if it's not richened up, the AFR will be out of whack...too much air, not enough fuel.

Note that the Quadrajet is going to be all over the map in terms of fueling...too many variables. Engine displacement, camshaft, compression, axle gear ratio, AC or no, manual or auto, EGR/emissions, all factored into the fueling calibration from the factory, and all handled in a variety of manner within the carb and it's tune. Almost impossible to bolt something on and it be "correct". How far off it is? Anyone's guess I suppose.

But again, that seems to be an unlikely cause for a problem as severe as yours. If anything, you are just leaving extra HP on the table.

You are running a 15+PSI radiator cap? This is a pretty interesting read if you haven't read up on cooling systems before: http://stewartcomponents.com/index.php?route=information/information&information_id=14

The thermostat governs the MINIMUM engine temperature. No matter what temp thermostat you install, changing the temp rating will never solve an overheating issue.
 
I had the same problem with mine...I did everything the Garage told me to check including the fan clutch. It felt and acted fine...but that is what it ended up being. It was working just enough to pass all the tests but when I put the new clutch in that solved the problem
 
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