Overheating problem, can't figure it out - Chevrolet Forum - Chevy Enthusiasts Forums

Overheating problem, can't figure it out

Old  August 30th, 2006, 6:48 PM
Overheating problem, can't figure it out

Hi I am new to this forum but I have a 1991 Silverado with 5.7 350 with an overheating problem. Changed out the thermostat, has a newer water pump that went out about a year ago. Truck only over heats in the summer, and at idle with the air conditioning on? On the freeway with a/c full blast it doesn't ever get hot. I flushed the system and that didn't help. The truck doesn't have a fan clutch, if engine is running, fan is turning.(?) The only thing I can think of is the radiator now.

Any ideas?????[sm=helpout.gif]

Old  August 30th, 2006, 7:42 PM
RE: Overheating problem, can't figure it out

Radiator could be pluged inside and out, I would take it out and clean the fins if they look clean take it to be flushed. WHy no fan clutch? have you removed it or what?

Old  August 30th, 2006, 8:30 PM
RE: Overheating problem, can't figure it out

Thanks for the reply, the truck didn't have a fan clutch when I got it. This truck has been the most reliable vehicle I have ever owned. at 150k miles piston ring went out, so I put a brand new motor in it. The truck would get hot just like this with the old motor too?????

I am wondering, it doesn't overheat on the freeway with a/c on, but only at stop lights. The motore seems to idle down and I'm wondering if it's not revved up enough to push the water through??? Could that be caused by a plugged radiator? The radiator is the only part of the system other than the heater core that hasn't been replaced or cleaned out in the passed couple of years....

Old  August 30th, 2006, 9:34 PM
RE: Overheating problem, can't figure it out

I would flush the radiator and go the the junk yard and get the right fan set up for it. The heater core would not be a problem, the only thing it would do is not heat the inside of your rig, if the heat works and it dont leak the core is fine.

Old  August 30th, 2006, 11:48 PM
RE: Overheating problem, can't figure it out

Just because the fan spins with the motor on doesn't mean that it doesn't have a clutch. If the front of the fan has heat sinks, it probably has a clutch. When the motor is off and cool, try to rotate the fan by hand. You should be able to rotate it. It will spin with the engine on due to friction.

You can also look up in the online parts stores and determine if there is a fan clutch for your truck.

I'm not sure how to test if it is engaging at the right temperature. If the clutch doesn't engage, the fan won't spin at full speed and air flow will be low.


Old  August 31st, 2006, 11:19 AM
RE: Overheating problem, can't figure it out

There is no fan clutch on the truck. It is bolted directly to the water pump. I know it sounds unlikely, and you can't move the fan with the engine off. It's really weird. Last night the radiator started leaking on the driver's side high. Hose looks good, pretty sure the leak is out of the radiator. So I guess I need a new radiator anyway. But it still feels like the engine is idling too low with the a/c on. I don't get this.

PS- If the fan is automatically spinning, putting a fan clutch on it to try and fix this is a waste of time right?

Old  August 31st, 2006, 7:31 PM
  • Join Date: Jul 2006
    Posts: 504
RE: Overheating problem, can't figure it out

If your hoses are not very new it wouldn't hurt to put new ones on it while you have the coolant drained anyway. Then you know they are good too. If they are fairly new I wouldn't worry about them though. I would also try to see if they had a fan clutch or if that was an option. I will take it that the fan came off the old engine too?

Old  August 31st, 2006, 9:55 PM
RE: Overheating problem, can't figure it out

I will let you guys know what happens with the new radiator installed. I'm hoping this fixes the problem but I don't know. It's not like the radiator was leaking before when it was overheating?? Regardless, it's leaking now so I need another one unfortunately.

Yes the fan set up was the same on the other engine. I actually had a shop put the new engine in the truck. They didn't say anthing about it not having a fan clutch.

Old  September 1st, 2006, 8:47 AM
RE: Overheating problem, can't figure it out

Is the fan a rigid blade or do you have a flex fan? If its a flex just leave it on there, if its rigid find something else because it will make you lose MPG and is not good on the water pump.

Old  September 2nd, 2006, 12:30 AM
  • Join Date: Sep 2006
    Posts: 2
RE: Overheating problem, can't figure it out

Jesse, I had lots of overheating problems in my V8 converted S-10 Blazer. The fact that you have cooling at highway speeds when the engine is loaded means your radiator is OK. The difference between highway and stoplight is airflow through the radiator. If the basics are in order, like the suction hose isn't collapsed, you have plenty of coolant, thermostat isn't stuck, etc. then you need to look at airflow.

There might be a lot of junk between the condensor and radiator, or one or both might be plugged up with junk.

But I'm going to bet your fan situation is messed up. That truck came with a clutch fan but now it has something else. Is it a flex fan? That's a fan with thin metal blades. No big hub like a clutch fan has, just a flat thin looking fan that bolts to the water pump. The blades are thin so they flatten out at higher RPMs. The clutch fan is bigger and heavier with a large hub that moves the fan forward, away from the engine.

Both fans are aggressive so they pull lots of air at idle, and both ease up at higher RPMs, so they don't turn your engine compartment into a wind tunnel, and wear out your belts and water pump from too much load.

The clutch fan is set up with the fan about 2/3 within the fan shroud and 1/3 out. The fan shroud is important. It's easier for air to circulate from the back of the fan (high pressure) to the front of the fan (low pressure) than it is to pull air through both the condensor and the radiator. The fan shroud blocks that circulation so that air has to come through the radiator and condensor.

If you have a flex fan in there it's probably not positioned correctly relative to the shroud. The best solution is to get a clutch fan, GM would have saved money if possible by using a flex fan if it was suitable. I have a flex fan in my V8 S-10, but only because there's no room for a clutch fan. It took a fair amount of work to make shrouds and baffles to get it all working. Make sure all your shrouds are in place, if not go get some at the scrapyard.

good luck,


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