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Temperature gauge erratic

1995 f150 5.8 263k

Hey guys! My temp gauge started doing something weird: started up normal and went mid until thermostat opened and then decreased and was steady on the “O” like it usually does but while driving it would randomly go super low and then back to “O” repeatedly. So I’m thinking either the temp sending unit, wiring or cluster. I was going to just replace the temp sending unit (and ring out the wiring while I have it out) but looking on rockauto, there is different colored motorcrafts ones: red or green. Does the color matter, if so, any idea which one I need?

Thanks!
 
Although I’m still curious which color sensor I would need, I don’t think that’s my issue anymore… now my gauge is reading low all the time (not even to “N”) and never gets to operating temperature. My heater appears to work like usual, but when I started the truck from cold with the radiator cap off, coolant was instantly spitting out. So now I think the thermostat is stuck open. Anything else is could be?
 

dustybumpers

don't play well w others
Coolant instantly spitting out of a closed system is vapor lock. Better pressure test. You may find your miss. My bet, a head gasket is going south.
What happens if you let it sit and idle to get the vapor lock out?
Hopefully I can check back in, if not, pm heavy doc, get him to text me
 

heavydoc

Instigator
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Coolant instantly spitting out of a closed system is vapor lock. Better pressure test. You may find your miss. My bet, a head gasket is going south.
What happens if you let it sit and idle to get the vapor lock out?
Hopefully I can check back in, if not, pm heavy doc, get him to text me
I was thinkin the same thing about a head gasket. Put a balloon over the radiator fill neck with the cap off. If it starts filling the balloon you prolly have exhaust entering the cooling system
 
I’ll do the balloon check tomorrow and grab the compression tester from my buddy. Blown head gasket doesn’t sound fun.

Thanks for the help guys!
 
The balloon did NOT fill up. Did I need to do anything else with the overflow tube or anything for this test to work? Otherwise, good news right??

I’ll still run the compression test when I get a chance.
 

heavydoc

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The balloon did NOT fill up. Did I need to do anything else with the overflow tube or anything for this test to work? Otherwise, good news right??

I’ll still run the compression test when I get a chance.
Overflow will need to be pinched off if it is open
 
I did the test again plugging the overflow outlet. The balloon slightly fills with air, but not enough to stand up on its own.

I will post the compression test results later this week.

Thanks for the help!
 

heavydoc

Instigator
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I did the test again plugging the overflow outlet. The balloon slightly fills with air, but not enough to stand up on its own.

I will post the compression test results later this week.

Thanks for the help!
How long did you run the test? It really shouldn't have filled at all.
 

heavydoc

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Napa sells and exhaust gas detection kit, there are 2 different ones, 1 for gas and 1 for diesel. They look the same but use different fluid.
 

dustybumpers

don't play well w others
My exhaust tester is a dollar bill.
idle the truck/car.
Walk back and hold the dollar bill by the edge, take it near the pipe. It should flap and try to blow it away. If it flaps and sucks it in and flaps you have a miss on a cylinder.
 

dustybumpers

don't play well w others
I only ran it for a few minutes until I felt the coolant start to warm up. I'll grab one of those exhaust testers also, thanks!
You have to run until the thermostat opens. If you run on warm up you are not running the whole system, only through heater core, and top end {and in your case 5.8 throttle body} until it opens. once it opens the heads and block become involved
 

heavydoc

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My exhaust tester is a dollar bill.
idle the truck/car.
Walk back and hold the dollar bill by the edge, take it near the pipe. It should flap and try to blow it away. If it flaps and sucks it in and flaps you have a miss on a cylinder.
I was referring to the one that tests the exhaust gasses in the coolant
 
My exhaust tester is a dollar bill.
idle the truck/car.
Walk back and hold the dollar bill by the edge, take it near the pipe. It should flap and try to blow it away. If it flaps and sucks it in and flaps you have a miss on a cylinder.
I forgot about the dollar test, Good idea! I'll try that tomorrow.

I didn't realize the thermostat had to be open for the heads to get involved, thank you for clarifying that.
 
I started the compression test but only made it through 6 until I lost light (and my patience after plug #2). I know you’re supposed to do them all at once to be consistent, but it didn’t happen. Also I am questioning a few things: I can’t get my engine to operating temp (due to the origin of this thread), I left the other 7 spark plugs in and tested one at a time, and although I took the air filter out to ensure the engine got a good gulp of air for the testing, i realized on the 4th plug I forgot to go WOT (%#%*!) so I continued the rest without WOT to be consistent. Do any of these items nullify my testing, or can I trust my relative results?
5: 130 psi

6: 145 psi

7: 145 psi

8: 145 psi

1: 135 psi

2: 148 psi
 

heavydoc

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When I do a compression test, I only crank the engine over about 4 revolutions per cylinder. The engine never starts so I don't do WOT
 
Finally finished it up and I redid #5 because it was the first one and I didn’t really know what I was doing:
5: 148 psi

6: 145 psi

7: 145 psi

8: 145 psi

1: 135 psi

2: 148 psi

3: 145 psi

4: 147 psi

I think it’s pretty good! From what I’ve looked up, it’s within spec and within 10% of each other. What do you guys think?
 

Fellro

Moderator
Staff member
They are pretty balanced except that 135, which while in the 10% margin barely, raises concerns to me. A leakdown test would be the next check.
 
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