Have a 94 f150 300 i6, 5 speed manual. Here's my problem: noticed my temperature getting warmer than normal (in between "A" and "L" on "NORMAL"), already knew water pump was going bad, so I decided to replace water pump, thermostat and fan clutch, also flushed radiator (there was a lot of sludge). After this, the problem worsened, it actually started overheating. Figured I needed to bleed the air out of the system so I did, several times. So today I did a warm compression test, coolant pressure test and replaced radiator cap and hoses. The compression test read: 150, 150, 150, 148, 150, 150. The coolant pressure test read 13 psi and pressure was not dropping. I also noticed, when I took the upper radiator house off, the coolant was dirty brown, but the coolant in the radiator was green and fresh. Could there be blockage in the block? Maybe the radiator? I had the water coming out crystal clear but didn't flush the block. The radiator is less than 2 years old. Any thoughts/ ideas are greatly appreciated, sorry this is so long.

 

I would suspect the new thermostat the first thing. You said you bled the air out of the system several times. I always had a beast of time getting my coolant filled after draining my radiator on my '88 with the 300 l6, had to let the engine get HOT and then the thermostat would open and fill the block, until I learned to disconnect one of the heater hoses.

If you have the air bled out of the system, again I would suspect the new thermostat.

 

I may try bleeding it again because the thermostat is definitely opening as it should, upper radiator hose has pressure and is hot after running truck. Also I forgot to mention I drove the truck 30 minutes before it started overheating. And I mean drove it, very aggressively. If I drove it regularly it probably would've taken longer.

 

Also if I let it idle it will stay cool all day long, just when I start driving it overheats. I let it warm up completely before test driving it.

 

Do you have fan shroud?

Remove the thermostat, and see if the overheating persists.

There should be a drain plug near the starter. Drain the block, then flush with the plug out. The driver side plug on my 460 is still clogged after flushing it about a dozen times over the last 2.5 years.

 

No on the fan shroud, but it's been doing fine for 2 years since I got it running. I didn't know about those drain plugs I'm gonna have to give that a shot. I need my truck back, it's a lot harder to haul stuff on a motorcycle.

 

I would still suspect the thermostat. It may be opening some but not all the way. It shouldn't take as much coolant flow at idle as driving it. If you mess with the thermostat I would consider filling the block with coolant to just below the thermostat before installing the thermostat. That should pretty well eliminate any air pockets. I second the part about getting a shroud.

 

Ok thermostat will be checked today after work, also gonna run some coolant flush. I'll check oreily for the shroud as well.