ok i have some serious issues with my 5.0s heating and cooling systems. i had tore down the motor when i added all my performance parts. after that it started to run really hot. the truck has no leaks at all. its filled up with coolant. yet the truck heats up really hot before it cools down. it heats up to the point of the red engine light coming on, then it cools its self all the way down. and this process constantly repeats. in that process my heat works then doesnt then its like not cold but not hot. i dont know what to do. when assembly of the engine was going on i did, break a sensor, i think coolant temperature sensor. its on a tube from the manifold and then a heater hose connects to it. i replaced that. could that have something to do with it..??

any help would be great!

 

Replacing the sending unit should make no difference.

You might have a pocket of air in the system, a sticking thermostat, or a weak water pump.

Whenever I open a cooling system I let the engine run with the radiator cap off until the thermostat opens and then it will usually take more water. You should also be able to see the coolant circulating when the stat opens.

I also make sure to fill the overflow tank because the system will sometimes pull more coolant in when it cools.

 

I had replaced the water pump aswell. I had also blew out the heater hoses to see if they were clogged. Is there a certain place where these systems are prone to air pockets

 

Are you sure you got the correct water pump? Assuming your 1990 uses a serpentine belt drive system you need a water pump designed for a "reverse rotation" operating environment.

 

I dont know... Is there any way to check reak quick

 

I don't know of any particular 5.0 air pocket issues, but its something I watch out for in any engine.

My 4.9 however is pretty much impossible to get completely full in one shot because the upper radiator hose is higher than the radiator cap. I suppose I could jack the front end up like you have to with some jags or park on a hill to fill it or something, but I just fill it like I described and then drive it a bit and let it cool and it takes care of itself.

The temp does come up higher than normal on that first drive, and pulls coolant in overnight as it cools. After that it is fine.

 

Temperature Fluctuations

A bad thermostat may open and close at random times throughout engine operation. When this occurs, an engine's temperature will fluctuate wildly as radiator coolant is prevented from flowing normally through an engine.


Read more: Symptoms of a Bad Thermostat in Cars | eHow.com Symptoms of a Bad Thermostat in Cars | eHow.com

 

I say air in the system. I had a car do the same thing.

 

Could be hot flashes from menopause

 

It really shouldn't be menopause, hell, she's only 23...

 

Haha well the water pump is the correct one, and I found a new thermostat in my garage so ill put that in and see if that helps

 

On a serious note, take a look at the new and old thermostat and see if it has a small hole, sometimes with a little plug/valve thing in it. If it don't that's your problem, that hole is there to let air/steam out from under the thermostat. That air won't heat the thermostat enough to open it. Then when it gets really hot or the water reaches it it opens quickly. So if it doesn't just grab a drill and small bit and drill one, that will do a lot to stablize your temp.

Replace your radiator cap while your at it, it holds the pressure. Keeping the pressure stable and up will do a lot to eleminate steam pockets.

There is a worse case though, pay attention to when it gets hot and how fast the pressure builds. A leaking head gasket or cracked head can blow compression gasses into the cooling system. If this is the case it will get hot really REALLY fast under load, and if you drive it right after cold start the system will pressurize before the water temp actually rises enough to do it.

 

Thanks for that, I will for sure take a close look at the thermostats and do what you said.

 

After making sure the thermostat has a hole;

An easy test is to know for sure if it's a head gasket or crack is to warm it up then make sure there is pressure in the system by squeezing the upper hose, it should be too hard to squeeze. Then CAREFULLY open the radiator cap and let the pressure out, close the cap and verify low system pressure by squeezing the upper hose, should be easy to squeeze. Go for a VERY quick but hard drive, stop, pop the hood and squeeze the hose again. If it's hard like it was after the initial warm up chances are a blown head gasket. Then CAREFULLY open the cap, if mostly steam comes out, then yup, blown gasket or cracked head.

 

Sometimes when a 302/5.0 engine is torn down and the heads are off, it's very easy to 'flip' the head gaskets, ie. front is back. There is a passage for coolant on only one end (I don't remember which) and if it is indeed flipped, you're not getting coolant flow. I've seen this happen dozens of times after a rebuild. Might be worth checking this.
Roger