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Old 11-29-2017, 11:10 PM   #1
borjawil
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Default Overheating issue and bouncing gauge

94 4.0l 200k

Gauge bounces. Will go up to a certain point around the m or a on normal, hold for a few seconds then go back down and repeat.

Replaced head gasket and intake gasket due to cracked head. Temp gauge did this before head failure. Temp sensor replaced.

Tried bleeding air from coolant by sitting on an incline, taking top off rad, plugging hose to reservoir, and running engine. Bubbles come out. Coolant level dips and rises about an inch or so.

Tried to bleed for about 15-20 mins. When shutting off engine coolant speed out of rad, so I put cap on while still running the shut off. Drove vehicle about 50 or so miles on highway. Heat not, then started to cool down to warm, then cold and my gauge started rising too high. CheckChecmed reservoir temp and coolant was cold. So my rad is likely not sucking in coolant.

I know the nipple tube thing at the reservoir is loose and leaks. Could this be my issue? Sucking in air?
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Old 11-30-2017, 11:55 AM   #2
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Your heater core is plugged up, easy to change on 1994, 2 hose clamps and 4 screws

Had same issue on my 1994 4.0l

Heater core is the cooling system by-pass, no valves on the hoses so heater core needs coolant flowing thru it all the time or temp gauge(and engine temp) will go up and down.


If in doubt it is the heater core, just by-pass it by remove hoses at firewall and connecting them together, if temp gauge settles down then you will know it is the heater core causing the up and down

Last edited by RonD; 11-30-2017 at 11:58 AM.
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Old 12-01-2017, 01:40 PM   #3
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Correction on my original post. I connected the hose from the reservoir to the tab below the nipple. Im dumb, it was dark. Will rebleed and see if it improves. If not ill bypass the core as instructed. I flushed the system prior to first head gasket. Everything flowed fine but I'm sure I could have dislodged something and clogged the core.
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Old 12-04-2017, 10:26 AM   #4
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Removed hoses at firewall to core, connected. Before doing this gauge didn't move. Now gauge moves but only to the "N" in normal. Ideas? Spring part of the thermostat faces away from the block correct?
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Old 12-04-2017, 11:09 AM   #5
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No, the spring part is the temperature sensitive side, it needs to be on the engine/block side

Thermostat has a wax inside the cylinder you can see thru the spring, as the wax is heated up by engine coolant is gets soft and valve starts to open, it is accurate +/-3degF
So it needs to be on the HOT side of the thermostat housing, which would be the block side

It usually only fits in that orientation.

There will also be a hole in the thermostat plate, often it will have a loose steel pin in it, that's the "jiggle valve", yes, that is what it is called, lol.
Thermostat should be placed with this hole at the 12:00 position, it is there to let air out when refilling the system.
Not a big deal, but when installing a new thermostat you might as well understand what the hole is for and put it in the correct position

Last edited by RonD; 12-04-2017 at 11:18 AM.
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Old 12-05-2017, 11:29 AM   #6
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Ok just so Im clear. Looking at the thermostat from the side. One side is wider/longer. There is a spring on it. The other side comes to a shallow point/cone. Cone faces the front of the vehicle or thermostat housing. Spring goes into the block. Small holes with jiggle valve goes up or 12 o'clock.

Will install this way and flush my system and see where Im at. thank you.
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Old 12-05-2017, 02:50 PM   #7
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UPDATE:

used blue devil in the coolant system. drove around for about 10min let it idle before that for about ten. Filled up system small funnel on top of rad, coolant reservoir closed and hose for that connected. Ive wiggled the heater hoses and rad hose on pass side to top of rad.

With cap off, funnel filled, let it idle. No gauge reading. About 10min in, lots of fluid comes out of funnel. Before this some fluid would overflow. like a slow small trickle. When fluid comes out after ten min of idle there are bubbles and churning/steam (its cold in michigan). I try to refill as fast as I can during/after this. Fluid being expelled settles down in about 30secs. Goes back to what the slow trickle. After another 5mins approx it does it again.

couple questions. When burping the system (truck is on ramps to raise front) I've been told to run till it warms up (thermostat opens?) then shut off. Should a lot of overflow come out of the top of the rad when shut off or am I supposed to put the cap back on? Would leaving the cap at the half turn (its locked on) be ok to do and drive around the block to let air out? I don't know if that is what that half position is for on the cap.
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