kdemler

I'm new to posting to the site but have used it extensively in the past. I'm having a frustrating time with my heater in my '98 F150 (4.6/2WD). I replaced the heater core a year ago and had heat for a time but never hot. About a month ago the heater started blowing cold.

Step one: I reverse flushed the heater core with about 70 psi of air through the return line; blew out somewhat rust colored coolant, nothing too bad. Heater core seemed fine with no blockage. (Note: I did this with my son's S-10 and it worked great). No leaks detected and flow was good. Pretty sure I got all the air out of the system afterwords. No heat

Step two: checked the blend door and found small crack; replaced it and verified the actuator was working while the plenum was open. Upon reassembly, still blowing cold after 10-15 min of driving (normally warm by then).

Step three: replaced thermostat with new 195 degree tstat (incidently, the old tstat tested good) and ran water through the radiator to ensure it wasn't clogged; no issues. Re-filled with 60/40 mix and drove to operating temperature (according to dash temp gauge); blew warm (never hot) initially but turned very cold after about 10 min. Possibly indicates poor coolant circulation.

Step four: replaced water pump assuming blades were worn/disintegrated. Pump was fine but replaced anyway with new pump; heater blew colder than ever before. Assuming I might have some air in the lines, did my best to bleed the coolant system and ran for about 10 min at 2500 rpm. Awesome amount of heat so took it for a test drive. Had great heat for about 2 miles then slowly turned cold. Odd thing is if I turn the heater control (not temp controller/potentiometer) to 'Off' then back to 'Floor' it gets hot for a couple of seconds, then returns to cold. Double checked the blend door actuator and can hear it running when temp controller is moved from hot to cold.

Addtional information: The top hose is hot as well as both hoses to/from the heater core (110 degrees F when placing a stick thermometer against the hoses). The bottom radiator hose is cold all the time and the water in the resevoir is about 80 degrees F. Would love to check the radiator temp but Ford didn't give me a radiator cap to access it...engineers.

The outside temp during this period has been between 40-45 degrees (unseasonably warm for Ohio this time of year).

I removed the new tstat to ensure I didn't get a bad one. When tested, opend at 190-200 range. I'm not sure what else to check, replace or hit with a hammer. Any insight will be greatly appreciated and may land you on my Christmas mailing list. Thanks

KDemler

bill123

I read about a blend door sticking it mixes a/c air and involves removing the dash halfway from the passenger side. it was on another forum it said the a/c is on except floor heat setting. cant recall what forum for ford trucks

1997F150XLT

I am having the exact same problem. I have done evrything that you have listed in you post. I have even placed black fiberboard in front of the radiator, only about 1" on each side of the radiator allowing air through it.
Is there a process for "venting" the cooling system? What about running a 210. ° thermostat? I'm at my wits end with this heat issue. I am out of ideas for this problem and can't think of anything else that I can do. The only other thing that I can think is that air is trapped somewhere in the system but that usually causes over heqting problems. I guess my next step is to take off the manual fan and install an electric fan that I can control. Any other ideas? Anyone fixed this issue after they have had no heat?

kdemler

Well I think I have identified my issue but haven't tested the theory yet. I believe the radiator is partially clogged. Even though I stuck a hose down the top inlet and ran water through the radiator with good flow, I never measured how much water the radiator would hold.

What led me to this hypothesis: I removed the thermostat and filled the system with straight water. The system only held approx. 2.5 gal of water but I didn't really think about that at the time as I thought the engine could have held some as well. Next, I ran the truck at 2500 RPM for about 10 min and had flow through the system (indicated by movement of water in the resevoir and water temp getting to about 150 F). The upper hose had a great deal of pressure and was "bloated" most of the time (indicating water was being pushed through but had resistance in the radiator). Heater was hot. Not really thinking along these lines yet though, I drained the system and filled with coolant (60/40 mix) thinking I had corrected the issue. However, with the increase in density of the coolant (vs. water), the flow was nowhere near what it was with the water and I got heat at 2500 RPM but it went away at idle.

The bloating of the upper hose indicated too much pressure which indicated blockage. Additionally, I wasn't getting any temp increase at the lower radiator hose indicating there was little or nothing returning to the engine. Finally, I didn't get any temperature increase in the resevoir also indicating no flow through the lower hose.

I plan on removing the radiator this weekend and checking for capacity; that should let me know if my theory is valid.

I will repost the results when I'm done. Good news is the radiator only runs about $200 at Advance Auto for a new dual core (recommended) and $180 for single core. Good thing is, they are extremely easy to replace.

kdemler

Thanks for the reply. I have checked the blend door several times and the fact that I get heat at high RPM (and it goes from hot to cold when I adjust the temp ****)indicates the blend door is operating correctly.

1997F150XLT

Awesome I never thought to check the hose pressure. I hope this solves your problem and mine too. Look forward to the result of this radiator swap.

stilbo

Same problem... Flushed system (removed radiator hoses and thermostat) didn't measure flow rate through radiator but with 3/4" garden hose at 65psi I'd say that there could not be any blockage.
When I changed the thermostat I installed it exactly the way it came out.... incorrectly. The previous owner had placed the o-ring around the outside edge of the thermostat housing instead of on top of the thermostat itself. While burping air with the radiator hose I heard the thermostat clicking up and down in the housing, checked the factory manual and realized that the o-ring needed to be repositioned.
Okay... NOW I have an infrared laser pointer temperature gun that accurately reads +/- 0.5 degrees F and when I check the thermostat housing after a run down the road and back the hottest the housing reads is 167 degrees. My heater output air temperature is 121 degrees.
With a 195 degree thermostat I would think that I'd get closer to 195 degrees....
I too thought about replacing the pump and thank you for saving me that labor...
I may be getting fooled by the temp gauge in that it reads 1/4 up rather than 1/2... My Explorer, Mustang and even my '70 Ranchero all attain a 1/2 way reading in the same amount of driving from cold.... So... An easy thing for me to change out will be the temp sending unit...
But in your case it sounds as though you are far from 167 degrees.
I have the factory service manuals but there is no mention of cold operating temps in the cooling section or troubleshooting section. I will be looking at the rest of the cooling system hoses to see if there's a possibility of bypassing and I may buy a new thermostat housing to check against my stock one to see if there has been any errosion over the last 205,000 miles due to the the o-ring being positioned incorrectly.
I know it sounds simple but is your thermostat o-ring positioned correctly?
One thing that really sucks about this condition is that the ECU sees a cold engine and runs the fuel mixture rich. And with $3.50 a gallon fuel, that sux.

1997F150XLT

SO you are stating that the thermostat is being sucked open and never really closes to bring up the temp?

1997F150XLT

Any luck yet?

stilbo

Not so much "sucked" but pushed. If the o-ring and housing flange isn't holding the thermostat down and seated into the intake manifold it will (mine did) 'float' when forced up by the coolant stream. If the thermostat 'floats' the gap around the edge becomes a 1/8" thermostat bypass. That is if our systems are not reverse flow systems which I believe they are not.

Have you taken a temperature reading at the top of the thermostat housing? Amazon.com has the laser pointed temp guns for $18-$89 and I opted for a $27 one and it is remarkably accurate for being low priced and Chinese.

If you are getting a housing temp anywhere near what I'm getting I'd say that your heater core has a restriction.

Without going back and reading this entire thread.... have you determined that you have 100% flow through your heater core by disconnecting the in/out lines and flowing water through the core? I made up hose barbs to garden hose adapters years ago for heater core flushes.

I DID replace the gauge sending unit with a Motorcraft unit from Ford and at $12 it was a cheap way to determine if the old one was faulty but I still only get 35% up the temp zone on my gauge. So it is apparently in agreement with my 167 degree operating temp at the thermostat housing.

Maybe I just have an overly efficient cooling system? But..... I always thought that the thermostat determined the final, high operating temperature.

I've not swapped the housing yet nor have I changed the current 195 thermostat out with a Motorcraft unit but that will probably be my next attempt to elevate the operating temp. The current thermostat was purchased from Advance Auto when I flushed the system and they did not have the "BEST" quality Robert Shaw thermostat in stock so maybe, just maybe a 195 Motorcraft thermostat will make a difference.....

I'm also going to check the coolant for hydrocarbons but I don't have any dropping coolant level issues or surging coolant on start up nor would I think that a head gasket leak would be evidenced by a consistant 167 degree operating temp. I've seen overheating symptoms with a blown head gasket but not consistant low temps.

P.S....... I searched for thermostats with a higher temp than 195 and there are none. Probably a good thing. I remember running a '99 4.6 Expedition from Phoenix to Michigan towing an MGB on a trailer in July and we flushed and changed coolant and installed a new 195 thermostat and with the a/c blasting at ambient temps of 85 - 115 the gauge never went past the 50% mark..... So a properly operating 4.6 is apparently fine with a 195 stat at a very wide ambient operating temperature and that makes sense as Ford engineers no doubt designed the 4.6 to operate at very wide ambient operating temperatures with the 195 stat...

Let me know if you determine : 1. Housing temp 2. 100% heater core flow. 3. Thermostat seated properly in manifold.

Good thing it's been a mild winter... Or are you north of International Falls MN? Hope not..........