need a new thermostat or water pump
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Does it overheat just as much at 5AM as at 3PM?
How is your fan clutch doing?
Can it sit and idle indefinitely without overheating?
If the fan clutch is not doing its job and the weather is really hot out then you will definitely overheat, even without pulling a load. Also if it is a standard-duty fan clutch and you rev more than around 2,000RPM engine speed while driving at highway speeds, it will freewheel. A severe-duty fan clutch will bring the fan to ~1,800RPM fan speed (or lower if the engine is spinning slower) no matter the engine speed. If you're buzzing along at 2,500 or 3,000RPM engine speed and the fan clutch is freewheeling for your aural comfort then the engine will run hot.
The cap doesn't control coolant flow through the system. You should be able to keep the engine cool with no cap at all if the coolant magically stayed in somehow. If the cap is allowing too much pressure to build, your engine would still run cool until a gasket/seam/hose/etc. gives out and dumps your coolant on the ground.
It might be that the radiator is plugged up a lot. With the way the cap looked, odds are good. I'd suggest: with engine cold, open radiator cap and look inside. Hopefully you are a couple of inches low on coolant (but not much more than that). If not, siphon some out or use a turkey baster to get it a couple of inches low so you can see the tubes. Put the excess coolant in the overflow tank or an empty coolant bottle. Start the engine and monitor the upper hose temperature by occasionally holding your hand on it while also looking into the radiator. When the engine gets warm enough, the thermostat will allow coolant to flow to and through the radiator. When it does, coolant coming out of the tubes should be just dribbling. If they are squirting like miniature garden hoses with miniature thumbs on them then odds are good the radiator is plugged. If there is no flow at all, then it is plugged in the top tubes.
How is your fan clutch doing?
Can it sit and idle indefinitely without overheating?
If the fan clutch is not doing its job and the weather is really hot out then you will definitely overheat, even without pulling a load. Also if it is a standard-duty fan clutch and you rev more than around 2,000RPM engine speed while driving at highway speeds, it will freewheel. A severe-duty fan clutch will bring the fan to ~1,800RPM fan speed (or lower if the engine is spinning slower) no matter the engine speed. If you're buzzing along at 2,500 or 3,000RPM engine speed and the fan clutch is freewheeling for your aural comfort then the engine will run hot.
The cap doesn't control coolant flow through the system. You should be able to keep the engine cool with no cap at all if the coolant magically stayed in somehow. If the cap is allowing too much pressure to build, your engine would still run cool until a gasket/seam/hose/etc. gives out and dumps your coolant on the ground.
It might be that the radiator is plugged up a lot. With the way the cap looked, odds are good. I'd suggest: with engine cold, open radiator cap and look inside. Hopefully you are a couple of inches low on coolant (but not much more than that). If not, siphon some out or use a turkey baster to get it a couple of inches low so you can see the tubes. Put the excess coolant in the overflow tank or an empty coolant bottle. Start the engine and monitor the upper hose temperature by occasionally holding your hand on it while also looking into the radiator. When the engine gets warm enough, the thermostat will allow coolant to flow to and through the radiator. When it does, coolant coming out of the tubes should be just dribbling. If they are squirting like miniature garden hoses with miniature thumbs on them then odds are good the radiator is plugged. If there is no flow at all, then it is plugged in the top tubes.
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I've never seen a traditional fan clutch last 29 years and still work. Especially on a truck. Especially on a 30/35/350/3500-series truck. Unless it was almost never driven and is still working on rolling that fifth digit on the odometer. Seems like replacing the fan clutch with a severe duty one (while you're there) would be a good idea.
Come to think of it, I don't think I've never seen a 30/35/350/3500-series truck that is more than five years old that has less than 10,000 miles on it.
It doesn't operate as a clutch, like when it is engaged the fan is locked to the shaft. If it did, your truck would sound like an Indy car whenever it revved above 2,500 or so, right up until a chunk of blade came shooting out your hood and impales your neighbor'* cat.
It operates more like a torque converter, like when it is engaged it does what it can through manipulating fluid (inside the clutch) to get the fan to spin up to 1,800-2,000RPM. That makes for smooth engagement and disengagement, less noise, less belt stress and wear, longer water-pump bearing life, and less life and property damage from chunks of fan flying off.
When you rev it, you'd have to rev it to 2000RPM+ and keep it there for a couple of seconds to hear if the fan is doing anything, and even then it would only do what it is supposed to if the coolant temperature is high enough.
Just had a thought: Long ago there was a cheap alternative to a thermostatically-controlled fan clutch. It was a non-thermostatically-controlled fan clutch. I think they operate as always-on (as in my second example above) regardless of temperature while saving you a few bucks at time of purchase. I've never used one because they are dumb. I don't know if they still sell these, I just know I haven't been asked if I want the thermostatically-controlled option or not in the last decade. You might have one that'* all worn out (and still dumb) in your truck. Maybe it was installed in 1994 when the factory fan clutch failed (or something) and it just looks old enough to be original.
Either way, it sounds like you need a fan clutch.
Come to think of it, I don't think I've never seen a 30/35/350/3500-series truck that is more than five years old that has less than 10,000 miles on it.
It doesn't operate as a clutch, like when it is engaged the fan is locked to the shaft. If it did, your truck would sound like an Indy car whenever it revved above 2,500 or so, right up until a chunk of blade came shooting out your hood and impales your neighbor'* cat.
It operates more like a torque converter, like when it is engaged it does what it can through manipulating fluid (inside the clutch) to get the fan to spin up to 1,800-2,000RPM. That makes for smooth engagement and disengagement, less noise, less belt stress and wear, longer water-pump bearing life, and less life and property damage from chunks of fan flying off.
When you rev it, you'd have to rev it to 2000RPM+ and keep it there for a couple of seconds to hear if the fan is doing anything, and even then it would only do what it is supposed to if the coolant temperature is high enough.
Just had a thought: Long ago there was a cheap alternative to a thermostatically-controlled fan clutch. It was a non-thermostatically-controlled fan clutch. I think they operate as always-on (as in my second example above) regardless of temperature while saving you a few bucks at time of purchase. I've never used one because they are dumb. I don't know if they still sell these, I just know I haven't been asked if I want the thermostatically-controlled option or not in the last decade. You might have one that'* all worn out (and still dumb) in your truck. Maybe it was installed in 1994 when the factory fan clutch failed (or something) and it just looks old enough to be original.
Either way, it sounds like you need a fan clutch.
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I've never seen a traditional fan clutch last 29 years and still work. Especially on a truck. Especially on a 30/35/350/3500-series truck. Unless it was almost never driven and is still working on rolling that fifth digit on the odometer. Seems like replacing the fan clutch with a severe duty one (while you're there) would be a good idea.
Come to think of it, I don't think I've never seen a 30/35/350/3500-series truck that is more than five years old that has less than 10,000 miles on it.
It doesn't operate as a clutch, like when it is engaged the fan is locked to the shaft. If it did, your truck would sound like an Indy car whenever it revved above 2,500 or so, right up until a chunk of blade came shooting out your hood and impales your neighbor'* cat.
It operates more like a torque converter, like when it is engaged it does what it can through manipulating fluid (inside the clutch) to get the fan to spin up to 1,800-2,000RPM. That makes for smooth engagement and disengagement, less noise, less belt stress and wear, longer water-pump bearing life, and less life and property damage from chunks of fan flying off.
When you rev it, you'd have to rev it to 2000RPM+ and keep it there for a couple of seconds to hear if the fan is doing anything, and even then it would only do what it is supposed to if the coolant temperature is high enough.
Just had a thought: Long ago there was a cheap alternative to a thermostatically-controlled fan clutch. It was a non-thermostatically-controlled fan clutch. I think they operate as always-on (as in my second example above) regardless of temperature while saving you a few bucks at time of purchase. I've never used one because they are dumb. I don't know if they still sell these, I just know I haven't been asked if I want the thermostatically-controlled option or not in the last decade. You might have one that'* all worn out (and still dumb) in your truck. Maybe it was installed in 1994 when the factory fan clutch failed (or something) and it just looks old enough to be original.
Either way, it sounds like you need a fan clutch.
Come to think of it, I don't think I've never seen a 30/35/350/3500-series truck that is more than five years old that has less than 10,000 miles on it.
It doesn't operate as a clutch, like when it is engaged the fan is locked to the shaft. If it did, your truck would sound like an Indy car whenever it revved above 2,500 or so, right up until a chunk of blade came shooting out your hood and impales your neighbor'* cat.
It operates more like a torque converter, like when it is engaged it does what it can through manipulating fluid (inside the clutch) to get the fan to spin up to 1,800-2,000RPM. That makes for smooth engagement and disengagement, less noise, less belt stress and wear, longer water-pump bearing life, and less life and property damage from chunks of fan flying off.
When you rev it, you'd have to rev it to 2000RPM+ and keep it there for a couple of seconds to hear if the fan is doing anything, and even then it would only do what it is supposed to if the coolant temperature is high enough.
Just had a thought: Long ago there was a cheap alternative to a thermostatically-controlled fan clutch. It was a non-thermostatically-controlled fan clutch. I think they operate as always-on (as in my second example above) regardless of temperature while saving you a few bucks at time of purchase. I've never used one because they are dumb. I don't know if they still sell these, I just know I haven't been asked if I want the thermostatically-controlled option or not in the last decade. You might have one that'* all worn out (and still dumb) in your truck. Maybe it was installed in 1994 when the factory fan clutch failed (or something) and it just looks old enough to be original.
Either way, it sounds like you need a fan clutch.
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It'* clock/counter based on staring at the car from the front as if it is about to run you over. Generally if it has V-belts it'* clockwise and if it is the smooth (non-grooved) side of a serpentine belt then it is counterclockwise. If it has a serpentine with both sides grooved then it probably doesn't have a fan clutch anyways, but at least if it does you can still tell by standing in front of the car as if it is about to run you over and deciding clockwise or counterclockwise.
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It'* clock/counter based on staring at the car from the front as if it is about to run you over. Generally if it has V-belts it'* clockwise and if it is the smooth (non-grooved) side of a serpentine belt then it is counterclockwise. If it has a serpentine with both sides grooved then it probably doesn't have a fan clutch anyways, but at least if it does you can still tell by standing in front of the car as if it is about to run you over and deciding clockwise or counterclockwise.
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Either way, if the fan/water pump spin the same way as the crankshaft then it is clockwise.
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