Yesterday I went to replace my thermostat. Drained the coolant, and then unbolted the housing, and I found that the truck didn't have a thermostat installed at the moment.

So I installed the tstat (180), and then let the truck run for a bit after putting in some coolant. I'll fast forward to the part where I'm an idiot and only put in about 2 gallons of coolant... The truck was fine at first, and then got real hot real quick.

I only went for a short drive, but when I pulled back into my garage the overflow was boiling like crazy. I put the hood back down a bit and let the truck sit and cool. While that was happening I went to the store (not in the truck, of course) and bought four more gallons of coolant.

After it had cooled, I filled the radiator to the brim. This morning I went out to check on the truck, and the coolant level was where I'd left it. The radiator has a thermo cap on it, which read at ambient temperature (about 75-80ish) but the interior gauge (mechanical) was sitting at 230(ish).

I fired up the truck, let it warm up a bit and took it for a quick spin. The interior gauge rose as if it was normal but starting at the higher 230 temp. When I got back to the garage, parked the truck, and then let it idle for a few minutes, I hopped out to open the hood and check the reading on the thermo cap. It had just risen to 220 but was holding there... interior gauge was off the chart, of course.

There's nothing leaking, and I'll check the coolant level again once the truck cools off.

Where should I go from here though? What do I check next? What do you think is wrong with the gauge (the line doesn't appear kinked or anything)?

Thanks

 

A couple of thoughts.
If the gauge reads excessively hot but the engine runs fine (not boiling over) then don't fully trust the gauge just yet.
If the engine is boiling over, that is a problem regardless of what the gauge reads.

Since the interior gauge doesn't seem to match the temperature on the cap, you might have a bad sending unit, a bad gauge, or a mismatch between the two. Is the gauge original? Does it return to the bottom COLD when the power is shut off?
Since the gauge is showing a false reading, there is a problem with the wiring or with the sending unit, if it is the factory TEMP gauge. The Ford gauges work on resistance in the sending unit. If the wire is grounded, the gauge senses no resistance and will peg the gauge at the hottest. A short in the wire will do this, but an incorrect sending unit will send the wrong resistance, and the gauge will respong to the sending unit, not to the actual temperature.

Coolant: You should be using a 50/50 mix, not straight 100% ethylene glycol. 100% ethylene glycol will not cool effectively, and will actually make the engine overheat and boil over. (Until you figure this problem out, it might be worth it to use just plain water, with the cost of coolant being what it is.)

Radiator capacity: The original radiators were designed to have about 1" of space from the bottom of the fill to the top of the coolant, to allow for expansion. Modern systems use a coolant recovery cap, tube an tank, so don't overfill the radiator if original or of original design.

Based on what you have so far, there might be major engine work ahead. If you cannot stop it from boiling over, I recommend you have the coolant tested for exhaust gasses. Sometimes, if there is a cracked head or blown head gasket, the combustion gasses will push their way into the cooling system. This dramatically increases the pressure and forces the coolant out in a way that makes it look like it is boiling over, but isn't. The pressure just makes it look like a boil over situation.

Hope this helps.

 

Thanks, yeah I'm using the 50/50 blend. It's only ever boiled over the one time thankfully. It's not an original radiator, as the previous owner swapped in a larger one.

 

Is the thermostat installed with the coil toward the engine, and the pointed "cap" to the radiator?

 

Yes, I believe that's the one thing I managed to do correctly.

 

Are you running a 240, 300, or the 352?

 

It's a 352 (previous owner claims 390, so I'm going to check that very soon). I think it's possible that I didn't bleed the cooling system properly and it's sorted itself out at the moment (minus the gauge issue). Coolant level has remained stable and the truck seems to be running fine otherwise.

 

Yeah that sounds about right, best to keep cap off until the thermostat opens at least once before closing up system. Some of the newer cars have the closed systems that you have to bleed air off at a higher point than the radiator or they overheat.

 

Yeah,
I know when i was having a similar issue with my 67 mustang. I had to replace the thermostat and flush the system real good. I let the air work out and once i did that and put it back together all seemed fine. You never know about the gauges though. All though you could have a different temp at one point of the car to the other depending on where your sending unit is as well. Not sure you get a true temp of the engine from the top of your radiator

 

I am sure that is a relief!

Are you using the stock gauges, or are you using aftermarket ones?
A regular cab truck with the sweep type cluster will normally have factory FUEL and TEMP gauges.
A Custom Cab will have an additional OIL pressure gauge, plus an ammeter.
Except for the ammeter, these factory gauges use a voltage regulator behind the cluster to drop the voltage to ~6V or less, and the sending units are calibrated for this lowered voltage.
Aftermarket gauges usually use a full 12V, and the stock sending units generally will not work well with them.
Of course, it depends on the aftermarket manufacturer.

 

Make sure you install the correct diameter thermostat. 1965/67 F100/350 352's use a 2 1/2" diameter t/stat, while 1968/76 F100/350 350/390's use a 2 1/8" diameter t/stat.

The reason the t/stat was probably missing, is the previous owner couldn't find it.

NAPA has the 2 1/2" diameter t/stats, but are only listed in their paper parts catalogs, usually stashed under the counter.

So, in order to get one, you have to have the NAPA parts cluck to look in the paper catalogs.