See if you need more coolant? Does it hold pressure? If not that is your answer. You have an internal water leak if you cannot find the drip. It may show as just a blue/green track down or around the bottom of the radiator core or elsewhere.

 

coolant level is fine. it holds pressure. haven't found any water leaks at all. had heat all week until today, when i really needed it. here in florida it doesn't get cold to often but when it does you need heat.

the inlet heater core hose gets hottttt. the outlet is barely even warm. I am gonna flush out the heater core again this afternoon and see if that helps.

dan b

 

There is a thread video showing a master tech pulling the dash forward enough to replace the heater core and blend door actuator mod. Watch all 3 of his videos for good advise.

http://api.viglink.com/api/click?for...13524031871412

 

Flushed my heater core until I was getting a good stream of clear water through it. Then flushed it some more.

Now i have wonderful heat. had to turn it down this morning it was so hot. Guess i didn't flush it enough when I changed the heater hoses.


dan b

 

Some crud was still in the engine and tubes? It doesn't take very much to mess it up. Warrrrrrmmmmm !

 

I think it is your heater core. I have the same problem. I have flushed the core twice. First time we just flushed first throught the return hose. Believe that it is important to get the trash to come out from the direction it come in. I got heat but only if the fan was left on the slow speed. On high speed it would cool down. If I raced the engine up around 3000rpm heat would come back. I suspect this was because the water was moving throgh the core quick enough that it did not cool off as fast before returning to the engine. The heat faded a little over time but did not go completly away. Fushed again to tonite (3rd time). This time I flushed it and used air to push as much out as possible. Then poured radiator flush into core and boiled water to pour in with it. Did this cause the flush called for putting it in the radiator adding water then running engine till it gets hot. Let it set for an hour then flushed clean again. always starting with the out hose first and alternating back and forth. I know it did not get totally flushed as I could see it lose flow when I flushed it on the intake side. I could see it slow down as if it was plugging back up. However each time I have flushed the flow has gotten better. This time the flow was very strong. I now have heat with the fan on full speed. It might not be 100% of what it was but I do have heat on all fan speeds. It gets hotter on the lowers speeds. I suspect it may fade again, and I may need to flush and clean again. But I would rather do this than pull the dash to replace the core. Had a mechanic tell me that I would never get all the trash out and that was why they always just replaced them. He also said that to get the trash out of the cooling system you have to pull the pluggs on the engine block and flush. I won't do that unless it keeps plugging up or I have more time to do that. I will give an update on the progress if the heat fades again. Hope this is some help to some one.

 

You should pull your water pump and check the fins. Mine was doing the same thing after back flushing the core. Pump had only one weak thin fin left. Truck never over heated even pulling enclosed loaded car hauler.

 

try flushing heater core with water and air hose at the same time...my wifes sable did that and afterwards it runs like a champ.

 

my truck is doing this. i just changed teh heater core, blend door and thermostat considering my old one had somehow warped. could it be the water pump? ill pull it tomorrow and check the fins.

 

Ok so I now have heat while driving but not while sitting still so I now believe my water pump is out

 

i recently had a problem with my 1997 f150 with a 4.6 blowing cold air then overheating then dropping back to normal temp and blowing warm air again. i installed new oem thermostat and same problem. i put in bars headgasket fix and flushed the heater core both ways with both water and compressed air. this worked for a couple days and then the same problem happened again. i parked the truck nose uphill without the cap on and ran it for about a half hour. heat worked awsome all the rest of the day until the next day and then the same problem. so i burped all the air out again and it's been fine ever since, that was about a month or 2 ago.

 

This definitly the heater core pluging up. I have had heat and as the truck warms in the cab it gets confortable and is able to work the fan on higher speeds and stay hot. I have checked everything and it always goes back to flushing the core to make it work better. What ever is pluging the core is to heavy to flush out. I know this as the back flush is strong and the forward flush starts strong and weakens.I plan to devise a way to hook a shop vac to intake side and see if it might suck the trash out. If that does not work I will keep soaking with cleaner and flushing as it does get better. It just be a slow process to get it cleaner. Bottom line is this may be a spring project to pull dash and core to flush it upside down or replace it. I did find that I had a small leak at the mounting bracket for the oil filter,that is also the intake connection for coolent into the engine. This was also the source of an oil leak' Guy talked to at the auto store said that they havebeen selling a few of those seals. when I further progress with this I will post more.I always say that machines don't have brains you cannot let then beat you, they cannot out think you.

 

If U reach the point of clog and flushing doesn't solve the problem the clog has built up a deposit on the core tubes to the point it is impossible for water to get thru. This buildup is like a calcium deposit chemically welded to the tubes. One hose hot the other cool/cooler is the give away. If U are sure your doors are working then a new core is the best option. U can keep messing with it with temporary results of heat and then cold or just replace the darn thing.