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Thermostat replacement success- leads to questions


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So I successfully replaced my thermostat which the correct type-which is of course rare and expensive (Joy! Lol). My engine appears to be much happier temperature wise. Butttttt it did raise some other questions. I just recently started using my dual tanks as I was told they didn’t function when I bought the truck but either they do 1) and the gas gauge just doesn’t read correctly on the rear tank or else 2) when I switch to the rear tank perhaps somehow it continues to pump fuel from the front tank but the gauge reads differently? I’m not sure which is more likely. Anyway I noticed when I switch between tanks the temperature gage would actually change settings which seems strange as it shouldnt be related to what tank the fuel is coming from but it does lead me to question the accuracy of the temperature gage. Any ideas as what might be happening? Also there appears to be several vacuum connections to the external housing on the thermostat elbow and I can’t seem to find out what those are. Where they connect into the housing when I had it out they look pretty corroded and I’m questioning how will they function but I can’t figure out what on earth they are-there probably for an eimage725.jpg.cb9dab85ae0911f45cd9b22f92fff5fe.jpgmissions system since they would activate when the temperature of the engine is higher I’m guessing? I’m going to take a picture. They Are the blue and white things in the picture.
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First, congrat's on the thermostat change. :nabble_smiley_good:

As for the gas gauge, there are two separate issues here:

1. The reason that flipping the switch to the other tank changes the temp gauge is that the fuel, temp, and oil pressure gauges are fed by the instrument cluster voltage regulator, ICVR. And if one tank is relatively full and the other relatively empty that will make a big change in the current draw through the ICVR, which changes the voltage to the other gauges. That's not unusual but there's a way around it if you want to go there.

2. It is possible it continues to pump from the front tank when you flip the switch. You should have the 3-port valve, and those can certainly fail and not switch from one tank to the other. You can check this by pulling the fuel line going to the front tank and trying to drive the truck with the switch flipped to the rear tank.

On the vacuum connections, I really can't tell anything from your little picture. I've blown it up a bit below, but that's getting pretty fuzzy. In any event, there were some extra vacuum devices on the 81's, and I suspect the one you are asking about is the temperature-switched valve. I don't remember exactly what it did, but you do not need it if you aren't running the full emissions package.

If you had a picture of where those lines go we could probably figure out exactly what you have. But, before posting more pics please go here and read up on how to post them so we can actually see them well: Bullnose Forum/Forum FAQ's.

image725.jpg.93fac2745ea17b3feaf55f403716de84.jpg

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One thermal vacuum valve probably controls EGR, and the other, over temperature high idle by supplying full manifold vacuum to the advance can, instead of through the restrictor.

On the 3 port fuel valve 1 thing you can do with a helper is get under the truck and hold you r hand on the valve. Then have the user with key on flip the switch between the 2 tanks.

When this happens you should feel & maybe hear the valve switch between the tanks.

1 step more would be to remove all hoses and put a long hose on the outlet.

With helper blow in the long hose and feel what port the air comes out then switch tanks and see if the air now comes out the other port.

If the air does not switch ports valve is no good.

I just did this to my 81 F100, when feeling the valve it did not "click".

I then pulled it and bench tested and could barely feel the "click" and the air test failed so I replaced the valve.

BTW I did not know if this worked or not as the duel tanks came from my parts truck but 1 tanks was empty so thinking it did not, it should now.

As for the temp gauge Gary could be right but I would also check all the grounds mainly the body to frame and frame to battery, may even check battery to motor when at it.

On the vacuum lines do you have a sticker on the radiator support showing the vacuum lines?

Dave ----

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On the 3 port fuel valve 1 thing you can do with a helper is get under the truck and hold you r hand on the valve. Then have the user with key on flip the switch between the 2 tanks.

When this happens you should feel & maybe hear the valve switch between the tanks.

1 step more would be to remove all hoses and put a long hose on the outlet.

With helper blow in the long hose and feel what port the air comes out then switch tanks and see if the air now comes out the other port.

If the air does not switch ports valve is no good.

I just did this to my 81 F100, when feeling the valve it did not "click".

I then pulled it and bench tested and could barely feel the "click" and the air test failed so I replaced the valve.

BTW I did not know if this worked or not as the duel tanks came from my parts truck but 1 tanks was empty so thinking it did not, it should now.

As for the temp gauge Gary could be right but I would also check all the grounds mainly the body to frame and frame to battery, may even check battery to motor when at it.

On the vacuum lines do you have a sticker on the radiator support showing the vacuum lines?

Dave ----

Sorry for the delays in responding. I’ll try and post some pics soon. As to the tank issue I’m thinking the easiest thing will be to fill a gas can and then drive until I run outa gas then switch tanks and see what happens. If nothing I’ll still have gas in the can to get me home!

Also I will check my wiring this weekend. My horn started working for a bit and is now out again which suggests a wiring issue. I also have all my emissions stuff as it’s just too much for me mentally to handle removing it all right now.

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  • 2 weeks later...

Sorry for the delays in responding. I’ll try and post some pics soon. As to the tank issue I’m thinking the easiest thing will be to fill a gas can and then drive until I run outa gas then switch tanks and see what happens. If nothing I’ll still have gas in the can to get me home!

Also I will check my wiring this weekend. My horn started working for a bit and is now out again which suggests a wiring issue. I also have all my emissions stuff as it’s just too much for me mentally to handle removing it all right now.

So I finally ran outa gas! (can't believe I am saying that like it is a good thing!) it died just as i pulled into home depot. I found out my switch/rear tank doesnt work as i had hoped :nabble_smiley_cry:. Pulled out my gas can to refill and it took a while to restart - i finally loosened the gas cap in case it was preventing air movement/creating suction in the tank as the pump tried to remove volume from the tank. Seems unlikely that a 40 year old system is that air tight but within 6 seconds after that it roared back to life.

The up side is that since replacing my thermostat, i found a beautiful (used) section of flexible hosing for the air cleaner at the junk yard, just sitting on the ground under a truck! Someone looking for parts musta chucked it. Since installing both these items it seems like not only is my engine staying cooler but i am getting better gas mileage (i think). I don't have to stop nearly as often for gas (or at least that's how it feels).

On a side note, my roommate borrowed my truck and got multiple compliments - including from an older guy (former dealer) who could appreciate how higher end this truck was in "it's day". So i feel like my hard work is paying off! Now if only i can be there to hear the compliments for myself :nabble_smiley_happy:.

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So I finally ran outa gas! (can't believe I am saying that like it is a good thing!) it died just as i pulled into home depot. I found out my switch/rear tank doesnt work as i had hoped :nabble_smiley_cry:. Pulled out my gas can to refill and it took a while to restart - i finally loosened the gas cap in case it was preventing air movement/creating suction in the tank as the pump tried to remove volume from the tank. Seems unlikely that a 40 year old system is that air tight but within 6 seconds after that it roared back to life.

The up side is that since replacing my thermostat, i found a beautiful (used) section of flexible hosing for the air cleaner at the junk yard, just sitting on the ground under a truck! Someone looking for parts musta chucked it. Since installing both these items it seems like not only is my engine staying cooler but i am getting better gas mileage (i think). I don't have to stop nearly as often for gas (or at least that's how it feels).

On a side note, my roommate borrowed my truck and got multiple compliments - including from an older guy (former dealer) who could appreciate how higher end this truck was in "it's day". So i feel like my hard work is paying off! Now if only i can be there to hear the compliments for myself :nabble_smiley_happy:.

Congrat's! Now you know the valve doesn't work, what are you going to do about it? Many, myself included, have just run on one tank.

As for the MPG, yes when you get everything dialed in, including the cold air hose to the air cleaner, which was a good find, it all adds up.

:nabble_smiley_good:

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Congrat's! Now you know the valve doesn't work, what are you going to do about it? Many, myself included, have just run on one tank.

As for the MPG, yes when you get everything dialed in, including the cold air hose to the air cleaner, which was a good find, it all adds up.

:nabble_smiley_good:

That’s great! Good to find that piece(s) of the puzzle.

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Congrat's! Now you know the valve doesn't work, what are you going to do about it? Many, myself included, have just run on one tank.

As for the MPG, yes when you get everything dialed in, including the cold air hose to the air cleaner, which was a good find, it all adds up.

:nabble_smiley_good:

HA! I was that kid in grade school that didn't write/draw or deface any notebooks, "trapper keepers", lockers, etc. I'm going for full blown originality. :nabble_anim_jump:. Although i will say my new-to-me old radio just ate one of my 8 track tapes :/.

The avatar i just found is my goal. Its almost like the original owner just pointed to the catalog and said "i want that one".

The tanks may have to wait for a bit tho - i'm getting awful excited to try and rub/polish my paint to see how much better it can look!

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HA! I was that kid in grade school that didn't write/draw or deface any notebooks, "trapper keepers", lockers, etc. I'm going for full blown originality. :nabble_anim_jump:. Although i will say my new-to-me old radio just ate one of my 8 track tapes :/.

The avatar i just found is my goal. Its almost like the original owner just pointed to the catalog and said "i want that one".

The tanks may have to wait for a bit tho - i'm getting awful excited to try and rub/polish my paint to see how much better it can look!

...and in that vein i will say i found the same wheels as in the avatar this past weekend. Trying to find someone near Portland to blast and color match them to that white paint tho for less than a fortune has been difficult.

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