I have a 1994 f150, xlt, 4x4 with a 351w. The heater core has been replaced, 4 different thermostats, water pump replaced, cylinder draw down test, and cylinder pressure test. Still no heat. At an idle, the truck comes up to operating temp and there is heat. Take off down the road and the temp gauge drops down and I lose heat. The temp gauge bounces back and forth between the “N” in normal to about the middle. Still no heat in cab. Suspect a slight head gasket leak, but no white smoke from exhaust. Any ideas on what to try before doing head gaskets?

 

are you continuously filling, the radiator, did heater core have leak,what was readings on cylinders when you pressure tested them?
cylinder pressure test should look like this
Part 1 -How to Test Engine Compression (4.9L, 5.0L, 5.8L)

 

I would check the blend door. It's the door that opens and closes when you switch from vent to heat. At idle you have heat even though the door is stuck open because very little outside air is coming in since you aren't moving but when you drive then you loose heat because of all of the outside air.

 

also check your white vacuum line under hood, it gets so brittle it will literally disintegrate in front of your eyes, next to passenger side hood hinge

 

The blend door actuates as it should. So I believe the vacuum line is fine. I was told the blend door is working properly. And the results of the cylinder test were fine. I had a licensed repair shop do that. The heater core was replaced due to no heat originally. I was told it was plugged. I have the old one I am going to test. I have not been having to add coolant.

 

The only other thought I have is to ask if they flushed the entire coolant system when they found the heater core plugged? Perhaps the new one is plugged again....

 

Air in the cooling system? Has it been "burped" ?

 

The repair shop thinks I could have a cracked block. They believe there is air getting introduced into the system. I would think if there was a cracked block, when the system builds pressure, it would leak out not draw air in. The truck is being taken for a 2nd opinion. Any other ideas on what could be the issue? It acts like there is an air bubble but it won’t work out or burp

 

I had a very similar issue on a 95 F-150 with the 5.0 that my grandson wanted to buy and finally purchased. The maintenance records from the garage that did the repairs to it showed that the water pump was replaced and the next one was for a "No Heat" problem. The repair shops final response was that they kept getting air in the heater core, possible head gasket or cracked block. This was all within 400 miles. We wound up getting the truck cheap and tore into it. Found the impeller on the rebuilt water pump was spinning on the shaft causing no circulation to the heater core. The motor would overheat a little at idle sitting still but then cool a little. The heater hoses never warmed up like they should. Not sure of your sequence of events but it is a possible cause. Good luck

 

did they give you the right water pump, reverse rotation is ours, did they route the belt in correct direction, should be pic of routing on radiator support under hood

 

Are the line to and from the heater core hot? Is it known that coolant is actually flowing through the heater core?

 

Ok so I have blocked off 3/4 of the radiator, which now keeps the temp up where it should be but the heat is not great, one heater hose is hot, so I am gonna backflush the heater core.
But I am wondering I have a short hose coming out of thermostat housing that goes to the water pump, I don’t know what that’s for? I am thinking of replacing the lines from the rubber heat core hoses to where they go to the motor. Any thoughts?

 

If the return hose from the heater core is not hot, then you will not get heat in cab. Blocking the rad shouldn't be necessary, I can get enough heat in mine at -40, so the system is capable of making heat, if it is working properly. The short hose from the water pump to the waterneck is for the return. There may be a metal elbow on the intake manifold that the return hose from the heater connects to. Here is a pic of a simpler engine bay that may show things better.

 

it helps keep water moving through the motor when thermostat is closed, and the water pump from cavitation, do you know what thermostat they put back in it, hopefully a 195*

 

Before the rad was blocked off, the truck wouldn't heat up. At an idle it would come up to op temp, bring the rpm's up and the temp would drop to almost cold. And I would lose heat. Blocked half the rad and it still did it. So I blocked 3/4 of it and temp stayed pretty constant. I believe this was a southern truck and was used for pulling a big trailer. The rad in it is really big, I believe a 4 core, along with what looks to be 2 different coolers in front. One at least I believe is a tranny cooler, not sure the other. What is the head scratcher is that if there was no flow through the block, why wouldn't the motor overheat? The fan is clutched, I thought maybe it was a direct drive plus the bigger radiator just wouldn't let the motor make heat.
The problem has 2 independent repair shops baffled, one being a ford dealership. I am hoping someone here may have run into this problem.
I am going to back flush the heater core again and remove the heater blower and verify the blend door actually operates.
It did seem last night when I really got after the truck hard, it started making better heat. Which didn't make any sense.