Hi Guys looking for help with my 92 f150 302.
Several months ago i tore into her to remove the timing cover to replace the crank seal, and ever since then, my coolant gauge will swing from the N to R and maybe a little past that sometimes, but its constantly swinging back and forth, and before it always stayed on or below the N.
So I tried a new tstat, same thing kept swinging.
So I bought a motorcraft 192* tstat, still the same thing.
So I put in a mech gauge, tapped the thermostat housing and installed sensor there. And it shows once the tstat opens it swings from 180* to 202*, it will settle at 180* for maybe a min then start back up to 202 then immediately fall back.
I put on efans last time i was home, something i already planned on doing and it didn't help at all.

The last two days Ive put on new gates lower and upper radiator hoses, new gates heater hoses and new heatercore, which it always worked when i turned on the heat(one thread mentioned it solved one guys problem)and yet still the same thing.
So searching the threads last night I read a few that had the same issue and a water pump change solved the issue.
So today, I installed a new gates water pump just waiting for the silicone to cure before i add coolant and wanted to know whats the best way to get all the air out of the system?
I may or may not be getting that part right.

 

Fill up the radiator with coolant. Leaving the radiator cap off start the motor, and let it idle until the t-stat opens. Top off the coolant periodically. Shut the motor of and let it cool down. Top of the coolant again when the engine is cold.

 

OK, ill give that a try.
What I have been doing is filling the radiator starting the truck and letting it run till i don't see any air coming out of the funnel. Its one of those no spill funnels that attach to the cap and allow you to fill while the engine is running without spilling fluid everywhere.
I been squeezing every hose while its running to purge as much air as possible and having the truck on a incline with the drivers side higher than the passenger side.

 

When I refilled my cooling system on any of my vehicles, I took off a heater hose that was the highest point next to the t-stat. Then poured coolant into the radiator till I see the coolant at the heater hose point. Then put the hose back on and most of the air should be gone. That way you should not "steam" a head with air stuck behind the t-stat.

Then do like the others have said and run engine without radiator cap and fill as needed.

I found that way works best to rid the air from most cooling systems on older vehicles.

I do like how the newer vehicles have a "burp" bottle that rids air quickly. I been kicking around installing one from a newer F-series truck/car that also has a low coolant sensor in it.

 

Also helps if you park the truck facing uphill or with the front up an a set of ramps.

 

I burp it with a funnel.

 

That's the funnel I have been using.

Okay, I filled the radiator with the motor cold, I took the small hose off the top of the throttle body and filled it till coolant came out of the tb and hose.
Started the motor and filled it with the funnel mentioned above till the tstat opened and shut her down and let it cool and added more then took her for a drive.
Right off the bat she seemed like it was holding steady till I got a phone call so i pulled over for about 5 min and drove again only to find it was still doing the same thing, but this time it was from 190* to 202*.
Not sure if it matters but a/c was on which turns my efans on.

Got pissed so i parked her, and got to thinking maybe i have a head gasket issue, so called oriellys to see if they had the tester for combustion gasses in coolant, they did so i drove there and most the way there she was holding steady.
Got to oriellys did my thing started the truck and drove home, but this time it went back to swinging from 188* to 202*, with ac on.

Called my local Napa told them my issue, guy on the phone said let her run for 45 min with the cap off and heater on high and watch for air.

Okay so that's what i did and sure enough a bunch of air came bubbling out, so I let her run for 50 mins and took her for a drive, and all be damned, she is still f&^king doing it.

Its getting better, but i want to know are these 5.0 that hard to burp the air out?? this is driving me crazy.

Oh btw coolant test showed negative for combustion gases in the system.

 

if you have no combustion gasses and it's still doing it, you either havent got all the air out, have an air leak somewhere into your cooling system, or maybe your gauge is wacky?

 

I using the factory gauge and i installed a mech gauge sometime back, both show the fluctuations.

I was thinking about an air leak, i don't see any coolant leaks, nothing in the oil or oil in the coolant, but im sure its still possible.
Whats a good way to check for air leaks in the cooling system, never had this much of a problem getting air out before.

I have a vacuum pump, i may see if i can pull a vacuum to check for leaks and or pull the left over air out??

 

I would pressure test your cooling system, see if it holds ~20-25 lb of pressure for a long period of time. If you have even the smallest leak it will allow air to enter gradually, causing the temperature swings you describe.

 

I agree, that's my next step.
I found about these on YouTube tonight, i think i will try it instead.
It tests the system and will fill the system in secs.

 

Those temp swings look normal. as long as it doesn't go over 210 you be OK.

I see the same thing with my E350 Bus with the 460. The coolant temp will swing from 190 to 205(with 192 t-stat) AFTER the engine and big radiator is up to temp. While warming up I have seen temp swings from 140 to 212 then will stabilize to above readings.

What was the temp reading during the 50 idle time?

 

Before I took the timing cover apart it always held steady.
It was doing the same thing but not as quick, the cooling fans kept kicking on knocking the temp down.

Here's a video i took today. I apologize for the audio i had the window down.

 

I think the thermostat for the electric fans is set too low. The electric fan shouldn't turn on until after the coolant thermostat has opened. The fans should turn on at around 205, and turn off around 195. This way the coolant temp doesn't fluctuate a whole lot, and allows the coolant thermostat to do its job.

The electric fan on my motorcycle doesn't turn on until it hits 212 degrees.

 

I didn't think about that, I will adjust it tomorrow to see what it does.