Hey guys,

I have a 1999 F-150 4.2 v6.

Recently it's been overheating after about 10-15 minutes of normal driving. I have tried to diagnose this thing for over two months and I'm at a dead end. Even after it reaches operating temp, and even when it's overheating my heater is not working - it is blowing air, but it is not hot.

1.) First thing I replaced was the thermostat - no dice. I double and triple checked to make sure it was put in correctly as well.

2.) I had the entire thing pressure tested. There are no pressure leaks anywhere in the system. The radiator holds pressure, the block holds pressure, everything. There is no coolant in the oil, and there are no coolant leaks from the outside or inside of the truck. I even replaced the radiator/overflow cap, just in case it was cavitating. Nothing.

3.) The coolant is flowing, but I had the radiator flow tested anyway - it was only flowing about 13gpm, I think its supposed to be 20gpm - I replaced the radiator anyway.

4.) I've heard that refirmb'd waterpumps cause problems with parts being slightly out of spec so I replaced the waterpump. Still nothing.

5.) I thought the head gasket was blown, but there are absolutely no signs, that I can tell, that it's blown.


Does anyone have any suggestions I can try? I really feel like I've reached a dead end.

 

Flush out your heater core. Actually flush out your entire system on piece at a time. You can drain it out and catch the coolant since it sounds like its all new. Then use a hose and some air pressure (if you have it) to flush out the block, the heater core and the radiator- I think your issue is mostly in the heater core. Then when you fill it up, leave it run with the cap off to make sure all the air is out of the system.

 

I've flushed the system a few times - however I haven't since let it run with the cap off after I last filled it up with coolant, I bet there could still be air in the system. I'll give this a shot, thanks!

 

Mechanical or electric fan? Fan clutch bad? Snap on used to have a die you pulled water vapors into to ket u know if you have a blown headgasket/cracked head.

Richard

 

Yeah napa has it too it's called a block tester, I will see if I can find the part number. It tests for HC vapors in the coolant

 

Sounds either like the thermostat is not at the right temp rating or there is a restriction in heater core or somewhere

 

Good point, it should be a 195 degree thermostat. It would be stamped on the thermostat housing if you are unsure of what you bought or it could have even been boxed wrong...

 

Sorry it took me so long to get back to you guys - I've been working non stop the past 3 weeks.

It is a 195 degree thermostat - and I've replaced it twice.

It has a mechanical fan, and I've confirmed that it is working.

I'll try and find this - I'm really hoping that's not it!

I know it's possible I have a restriction in the heater core - but I know that it's not clogged completely - would it still overheat with just a restriction in the heater core?

 

I'd have look at the hose routing, but heater cores usually are not a main component of the cooling system. Coolant flow used to be controlled to it for cabin heat. I honestly have not payed any attention to my hoses. What about those fan questions? Mind telling me how u confirmed that? If at operating temperature, shut off the engine, spin the fan by hand. If it spins more than a complete round, you really need a new fan clutch. The fan will spin pretty good by hand if the temp is cool. The thermocouple in the front of the clutch controls the freewheel of the fan. If you still feel it's the heater core. Bypass it. See what happens.

Richard

 

Ok after eliminating seemingly everything possible now, including the fan, I *think* that my gauge is not reading properly.

When my truck started to "overheat" I would pull over and shut it off and wait 15-20 minutes for it too cool. However, recently, I started it right back up after it "overheated" and the temperature was at normal again. I've repeated this a few times now. There is literally no way an engine could cool off that quickly. It seems that the heater not working could possibly be another issue in and of itself. SO I bought a new coolant temperature sending unit - not to be confused with coolant temp sensor that sends the temp to the PCM. However, I cannot for the life of me find the thing.

I've found at least 3 different areas where it "should" be but I'm still struggling to find it. I found one place where it could be, which I have pictured below, but its just a brass bolt. I don't think that's it unless its been moved by a previous owner.

Anyone have any pictures or anything that can give me a clear indication as to where this thing is?







Thanks again for all the help!