I’ve replaced the thermostat on my '99 Explorer and it didn’t help. If the air temperature is below 50 degrees (F), I just can’t get the needle into the normal range. I don’t believe that it’s a sensor problem because the air coming from the heater is just mildly warm, not like you would expect after running for 30 minutes. The car has always taken a while to warm up but this year it seems much worse than in the past.
Ideas or suggestions?
Sounds like low coolant but if you replaced the stat I assume that you filled the system back up with the correct mix of water and anti-freeze? Remember if the car was cold when you did the stat change, you may have only filled either the block or the radiator (depending on the layout) with coolant.
Is the reservoir full when hot?
The new thermostat might be faulty. The symptoms are certainly typical of a stuck-open thermostat. If the thermostat replacement didn’t change anything, though, this is quite unlikely.
The radiator fan clutch might be locked up. This would cause excessive airflow through the radiator. Often with this problem there’s a “roar like a jet engine” noise from the engine compartment. With the engine off, you should be able to turn the fan, feeling some degree of resistance (it should not spin free). If it doesn’t turn with little or moderate effort it’s faulty. On most vehicles I would expect the engine to heat up normally until the thermostat opened, then cool down too much. However, your mention of it always being slow to warm up suggests it might have a bypass that always sends some coolant through the radiator, which could keep it from warming up even with a good thermostat. I would also expect this to be more noticeable when the outside air is cooler.
There might be an air pocket in the cooling system. This typically results in the engine running too hot but little or no heat from the heater, as there’s not enough liquid coolant to keep the engine cool nor to flow through the heater core. If the temperature sending unit (which feeds the dash gauge) is immersed in air rather than liquid, it will read low.
If it’s an air pocket like Gary T mentions, you may need to crack open any bleeders in the system to release the air. For my vehicle, I need to jack up the front end a bit, crack the bleeder, and fill the system until a steady stream of coolant is released, then I close it up and I’m good. Not sure if this is what you need to do with your Explorer, but worth looking into. Check for more then one bleeder in the system too, and if more then one, bleed the lowest one first, then the top one.
I will also go with an air pocket.
Why would my car run cold?
Say What? 
If there are air pockets blocking circulation of cooland the engine should overheat in short order and potentially sieze. May be a cold running engine.
The air pocket is probably in the heater core…so it’s not that the engine is running cold, just that the heater is ineffective.
A blockage in the heater core is another possability. Was the old T-stat intact when removed? Never worked on an Exploder, but the previous generation Bronco-II/Ranger it evolved from has among the worlds easiest to access heater core, and less than 50 clams for a new one, so I’d say this probably doable by anyone who owns a few tools.
This is not necessarily true. Air pockets can exist in the heater core or part of the engine preventing full circulation of the coolant. In mild climates the engine will not O/heat, but the heater does not work worth a damn.
There is one other possibility. did you replace the t/stat with one of the correct temperature rating? If the car came with a 195F t/stat and your new one is a 160F, the temp gauge will not read in the normal zone.
Thanks for the replies so far.
I was going to add the same comment but go on to say that sometimes we here in Florida don’t put one in at all. It is rare occasion that one is needed to heat the air quickly so occasionally we leave it out all together. If the car was purchased used this may also be the case.
As for bleeding the heater system, I haven’t seen bleed fittings on fords, the advise in the service manual is to disconnect one heater hose from the heater core. This is a pain, because the hoses get vulcanized to the fittings, and are a nightmare to get loose without damaging the hose.
So, free of charge, here is Kevbo’s better way: Go to the local generic autoparts store (AutoZone, Checker, Kragans, depending on your location) and buy a Prestone cooling system backflush kit. It should be only a few dollars, certainly less than $10.
Install this kit on whichever heater hose has the highest high point, and install the fitting at the highest spot on the hose.
By loosening the cap on the backflush fitting, you now have a simple way to bleed the air from the high spot in the cooling system, which is all the Cologne blocks should need.
And using the backflush kit as intended, and changing the coolant wouldn’t hurt anything.
Distilled water is a good idea when refilling the cooling system, especially if you live in an area with hard water.
Kevbo,
I installed one of those Prestone things on a car I owned several years ago, assuming you mean the thing that you hook a garden hose up to and then blow gunk out of the radiator neck. I didn’t know that they still were made and I although it seemed to work well, I never felt really good about cutting into the heater hose. I may need to look into getting another one.
As a side note, since Kevbo mentioned hard water. My water is extremely hard, approximately 40 grains of hardness (it’s practically crunchy). Because of this, we have a water softener. Since softeners work by exchanging calcium with sodium, will water with this level of salt damage my radiator?
I’ve had this problem and my thermostat was stuck open.
I had a very similar problem with my 96 Explorer, low gauge reading and tepid heater. I think it was the temperature sending unit that went. I don’t have the service record anymore, so I can’t tell you exactly what it was, but I distinctly recall a part being faulty, and that part NOT being the thermostat I expected it to be.
I had a 99 Ford Ranger with that problem and it was the temperature sending unit, same as with Cheesesteak’s Explorer.
I strongly advise against this no matter what climate one is in. The purpose of the thermostat is not to get the engine hot enough to get heat out of the heater. The purpose is to have the engine run hot enough to work efficiently (heat from the heater is just a handy side benefit). Even pre-computer engines will have shortened longevity from the cumulative effects of less complete combustion, but computer-era engines - in other words, the overwhelming majority of cars on the road today - can suffer more. The computer sees the engine as never being fully warmed up, and its fuel-control strategy never gets to where it should be. There are even a few designs where the absence of a thermostat disrupts the coolant flow and can cause overheating. The cost of a thermostat is much less than the cost of wasted fuel and premature engine wear caused by the absence of one.
A faulty temp sending unit (or faulty temp gauge) can certainly cause the gauge to read low, but it cannot cause the engine to run cool nor affect the degree of heat from the heater. It is not physically possible for a heat problem to be caused or solved by a sending unit.
Now it is possible that the OP has two different problems, e.g. a faulty sending unit and restricted heater core. If the thermostat, coolant level (i.e. no air pockets), and fan clutch are in good order, this would be the route to explore.
Measuring the actual engine temperature would be very helpful. If it’s where it should be and the gauge is reading low, the next step is to replace the sending unit. If the gauge still reads low, it’s almost certainly a faulty gauge.
If the engine temp is proper and the heat isn’t hot enough, there’s either restricted coolant flow through the heater from a partially plugged heater core ora malfunctioning heater control valve (if it has one), or a malfunction in the heat blend door operation (if it has that instead of a control valve).
Gary, don’t some Fords have a circuit that prevents the heater fan from being turned on until the engine is warm? I remember reading about it in conjunction with the old Tauruses…
Yes, some climate control systems delay blower operation until a certain temperature is reached. I was being a bit simplistic in my pronouncement too avoid being too long-winded (ha!).
However, as far as I know, those systems rely on the coolant temperature sensor (to the computer) and not the temperature sending unit (to the gauge). Now, some systems (not a '99 Explorer) have only the former and use it for both engine management and gauge functions, but even so I know of no effect this would have on the temperature of air from the heater.