Truck in question is a 94 B4000. When I was test driving the truck, I noticed the coolant was full of a bunch of Barr's Leaks, so I negotiated accordingly. I also noticed that the heat was really weak and only blew luke warm air, so I figured the heater core was probably coated with a healthy dose of coppery sludge. I also noted that the temp gauge only goes up about 1/4 of the way even on the hottest day so I thought maybe stuck thermostat.
Skip forward a couple months when my intake started leaking. I was in the middle of moving so it went to a shop for the repair and I requested a coolant flush and a new thermostat. Not sure what temp it is, but it was just pulled off the parts shelf from the VIN so I assume it is "correct." It still operates exactly the same; low temp gauge, poor heat. I didn't expect a miracle with the flush and stat, but I was hopeful.
I might understand that the temp sensor is also covered with barrs leaks and would be slow to register, but on a longer trip the heat should eventually soak through it and give an accurate reading. Last month I drove 180 miles on the highway and during a fuel stop and oil check I noticed things didn't feel very hot under the hood. The radiator was just warm. I loosened the radiator cap and it didn't scald me with steam, it just burped a little bit of coolant. I would say the temperature of it was about like drinkable coffee, but certainly not really hot. Ambient temps in the 50s.
-Fan clutch operating normally; stays engaged when the viscous fluid is really cold (the first 1/2 mile or so) and then spins freely and passes the newspaper test. It has never engaged for high heat, even on 90 degree days with the A/C on.
-verified proper heat blend door operation
Normally I would just jump straight to replacing the heater core or trying to flush it out with solvent, but the whole thing has me puzzled. I don't think it is necessarily a heater core problem since the rest of the symptoms all point toward actual low coolant temps.... and I'm not talking like "oops we put a 160 stat in," I'm talking like 120 degrees at best. I can lay my hands on the intake or valve cover and it just feels a little warm.
Theory 1: the whole coolant system is so gunked up with barrs leaks that it doesn't transfer enough heat to the coolant... but if it were that bad I would have such an inefficient cooling system that it would overheat/ping. I would also think that the oil would be taking the majority of the heat, but when I checked the oil I was able to wipe it off with my fingers and it was not that hot.
Theory 2: both the old stat and the new stat are defective in the same exact way.
Theory 3: some other brilliant solution that you will tell me below.
Thoughts?
Im willing to bet they didnt flush all the goop out, and the new thermostat is clogged/stuck already.
Prof Brap has offered a very possible option. Without you seeing firsthand what it looks like, it might be difficult.
My old k20 came without a thermostat and ran at about 120 until I put one in
Yeah if it was Theory 1 you'd have searing hot oil, pinging, and probably other more serious problems. The new thermostat being stuck open for some reason is about the only theory that makes sense.
The not being able to see it makes it hard. Does it have coolant in it, along with what color is it right now.
I started noticing about 1 in 3 new thermostats as bad about 5 years ago. I don't know if maybe they aren't using beeswax anymore or what, but I test them all now with a candy thermometer and a pot of water on the stove or grill before I put them in now.
There have also been a few rounds of part number consolidation lately too and it's possible that the one that is cross referenced to your truck now has an oversized built in bypass. You might want to look at a few different part numbers and see if you can spot something obvious.
I agree the thermostat sounds fishy. I assume it's pretty easy on that truck to pop it out and take look & test it. Having one run UNDER temp does seem strange by nature.
If you don't have time right now to pull it - you could block part of the radiator with cardboard and see if it runs a little warmer. If it's some OTHER problem that could be a really bad idea of course, but your description of the underhood temps makes me think you'd be ok.
Professor_Brap said:
The not being able to see it makes it hard. Does it have coolant in it, along with what color is it right now.
Coolant is at the proper level. The original coolant was decidedly brown from the green mixed with the copper anti-leak. New coolant after the flush is decidedly bright green with a hint of brown in it. There is no way to effectively flush that junk out, so it will forever be a little brown. The tech that did the work noticed some gunky copper in the coolant passages when the intake was off, but none of it looked rusty and crusty.
Professor_Brap said:
Im willing to bet they didnt flush all the goop out, and the new thermostat is clogged/stuck already.
A possible option, but it would have stuck instantly. It's possible, but in the 7 months I've owned this truck, it has never made it up to full temperature; either before or after the flush and stat.
And short of a solvent hot tanking, you'll never get all the goop out. That stuff is always going to be there, just in smaller and smaller concentrations with each flushing.
The best way to get the gunk out is purple power, got most of it out of a few cars with that gunk in them.
I would say a bad tstat got put back in.
A couple other possbilities:
- The shop never actually changed the tstat at all
- The original tstat was installed backwards/upside-down and the replacement tstat was also installed the same way
As a point of reference I will mention that my 93 B2200 has always run cool. Like 1/3 of the way up the gauge. I've owned her for...damn, 7 years now.
Doing a citric acid flush will also remove that stuff, though it will definitely expose any weak points in your cooling system seals.
TIL what Bar's leak is. Listening intently even though I'm not driving the car that can't stay warm right now. 
Cover the radiator and see if things warm up. At least that will tell you if the temp. gauge is working and if the heater is putting out heat.
You are driving a Ford product. Trusting what the temp gauge on the dash says is like trusting a hooker you bought in downtown New Orleans when she says you don't need a condom.
Drive it with a scanner to confirm actual temp. Then, find someone with a Hecat and get him to flush your heater core.
More of a b2000/b2200 problem, but they're really hard to bleed all the air out of the cooling system. Crappy heat and a temp gauge reading low are usually a sure sign there's an air lock in the cooling system on those. Might be worth investigating...
In reply to EastCoastMojo :
How's the heater? See my post above.
I just put the 2.2 in my b2000 and I'm having the opposite problem, can't keep the damn thing cool enough... Off to investigate water pump function and thermostat operation, but it's looking more like the radiator's internally clogged.....
In reply to daeman :
Heat works but has never been spectacular. I'll add this to my list of things to check out once it warms up around here. 
Weird, never heating up seems to indicate too much flow rather than not enough.
No temperature control from the thermostat, if one is there.
In reply to EastCoastMojo :
Jack up the front end or park it up a nice steep hill and run it with the cap of for about 20 minutes or so. It's because the heater inlet and outlet are more or less the high point of the cooling system.
Mine seems to be properly bled, and keep in mind that this problem has existed for 18,000 miles; both with the original coolant and after the flush. Both the supply and return for the heater core are below the radiator cap, so any air should be pushed through and end up getting pushed out the cap to the reservoir.
You live too far north. It's a Florida truck.
My 00 explorer runs really really cold and its the same engine. The plus side is on a 110 degree day, in city traffic with the AC on for hours at a time it never overheats.
The downside is when it was -20 years back after a 2 hour drive and sitting in traffic torque pro said my coolant temp was 58 degrees
Sounds like a design problem more than a malfunction problem. I would verofy temps with a scanner like stated above but everything you say about it being cool under the hood makes it sound like it is in fact not getting up to temp.
Back in the old days guys would put a piece of cardboard in front of half of the radiator to get the temps higher through the winter.
