Coolant temp bouncing

[borjawil]

Coolant temp gauge bouncing

94 4.0L 220K

New temp sensor, new coolant, system was flushed, head and intake gaskets replaced. Problem was there before intake/head repair.

My temp gauge will move up hit about the middle of the gauge then slowly move back down to a little past the "n" on "normal". Then come back up after 30secs-1min. It'll repeat this cycle constantly. I have attempted to purge the air from the coolant, not sure it is finished. I crimped the connector for the temp sensor to make it tighter but that hasn't helped. Ive also checked my thermostat for proper orientation. Any help is appreciated.

 

[alwaysFlOoReD]

'91 4.0l/5-spd. New water pump and thermostat. Flushed system several times. Mine does the same in winter months. The cycle slows considerably in the summer. I've given up and just run it.

 

[RonD]

It could be someone used too much tape on the threads of the temp Sender, the threads are the Ground for the temp circuit, so bottom few threads need to be left bare, it needs a good ground connection to intake.

Next would be Heater core is getting clogged up.

The heater core is the water pump by-pass, you can tell if there is no cab heat control valve on the heater hose, so heater core has coolant flowing thru it all the time.
When core starts to clog up temp gauge will go up and down.

You may also notice heat in cab is not as HOT as it used to be, still OK but not blow you out of the cab HOT, 190degF coolant

Or by-pass heater core by pulling hoses off core and connecting them together, then see if temp gauge settles down

Heater core on these years is easy to get out to check or replace, just the 2 hose clamps and 4 screws in cab

Heater core is very light with coolant drain, if clogging up all the coolant can't drain out so it will be heavy and you can hear coolant trapped inside if you shake it.


Overflow tank cooling systems are self-purging, you just need to get most of the air out, loosen heater hose at fire wall and fill system until coolant start to come out there, then reconnect, any remaining air will make its way to the top of rad over a couple of warm up/cool down cycles, and be purged into overflow tank where it bubbles up to the top, and is gone.

After a couple of warm up/cool down cycles open rad cap after engine is full cooled overnight, it should be full to the top, if not then you have a problem with overflow tank(bottom has debris in it) or a leak in the system.
As an engine cools down coolant is pulled back in from the overflow tank to keep the system topped up