2002 F150 5.4L, Heater only luke warm...Here's what has been done so far and the problem with the heat...last year heat was only luke warm, had heater core and coolant system flush done along with a new thermostat, got by last winter but never had really good heat..this year never thought about it until cold weather, again luke warm heat. took truck to a different mechanic, he did a heater core flush, it helped but still wasn't really hot heat like it should be. we cut access behind glove box and the HVAC box to check the blend door, it is working properly. took the truck back to the same Mechanic and had him do the following. Heater core flush & back flush, replace water pump and Thermostat again. had real good heat when we picked up the truck and had good heat on the 25 mile drive to home. after it set overnight, drove into town 25 miles had luke warm heat again. shut truck off to go inside a friends house for 15 mins. drove truck again and this time the heat was warmer (not too bad) drove around town for 20 or 30 mins. went into walmart for 10 mins noticed even warmer heat on the drive back home but not really hot. I'm trying to figure out Why the heat is warmer after the truck sits for a short time. and also noticed if I shut the blower fan off awhile I get warmer heat when first turning it back on. like the heat is building up while the blower fan is off. the mechanic said a few globs of black junk was seen when he flushed the core..what now? have a new heater core installed? any suggestions. also I will recheck my coolant level again tomorrow before starting the truck.

 

Sounds like a new heater core is needed. Not allowing sufficient coolant flow.

Side note. When the fan is off, it will allow the heater core to warm up faster because there’s no air blowing through it. That’s why some people leave the heater off on a cold morning until the engine warms.

 

Use a scan tool, see if the engine is running at the proper temperature.Are both heater hoses very warm?? You may have air in the system from the work done on the cooling system.

 

Deleted message

 

Do you mean that's why some people leave the blower fan off until the engine warms up?

 

I asked the Mechanic if he used the bleeder valve to get air out of the system, he said he put the cooing system under pressure and this should remove any air. had the same thing going on before he worked on it, reason we had the water pump replaced is because we thought maybe not getting good enough coolant flow. even though both heater hoses feel hot( they feel hotter on the bottom side, not as hot on the top) it still seems like its the heater core causing the problem. that's how it was before he worked on it, we haven't checked the heater hoses since the last repair. Thanks

 

It's my Son's truck, he didn't start or drive the truck today. I tried to check the temps of the heater hoses before this last repair was done. using a laser thermometer I had 150 at inlet hose and 140 at outlet hose, a lot of people say that's not hot enough but I think it is because I just reading the surface of the hoses not the actual coolant temp...I also have several people saying even though there's only 10 degrees difference going in and out of the heater core, it could still have sediment in the bottom and coolant is just running over the top of the cores so both hoses are hot but only warm heat. any advise on this? Thanks

 

both inlet and outlet hoses feel hot on the bottom and just warm on the top. I've had a few people say it could still be the heater core even though both hoses feel hot, because the coolant is running over the top of the cores inside due to sediment in the bottom, causing insufficient flow. I wish i knew an expert that could diagnose this thing properly. Thanks

 

Should be hotter than that. What is the temperature at the upper hose near the thermostat? A scan tool will give you a better indication of the engine's temperature if you have access to one.
The engine's temperature will increase some for a short time after the engine is shut off. That's because the flow of coolant stops.

 

This is my Son's truck and we are about 40 miles apart, I will see if he can use his scan tool to check the engine temperature............Since the engine temperature increases for a short time after it's shut off, do you think the heater core temperature would also increase during that time? also the mechanic removed the flow restrictor from the inlet heater hose. I wonder if that could affect the heat from the heater although this low heat problem existed before he removed the restrictor. Thanks