Temp guage going wild

[97RangerXLT]

Noticed this coming home from work. the temp gauge would peg itself on H and then immediately drop down to the middle range (where it's supposed to be) but then spike to full hot. this was especially noticeable at highway speeds, once i was going 30 or slower it seemed to be ok, but would still read hotter than normal.

First thing I did when I checked was look at coolant level. looks good, not low. The truck did not feel hot or smell like it was hot either.

So I am thinking that the temp sending unit is going bad. anyone want to confirm that diagnosis?

AJ

 

[RonD]

Yes, could be temp sender OR plugged heater core or by-pass

Follow the two heater hoses from the firewall to the engine
If they connect directly to intake and water pump then there is no by-pass so it is the heater core that is partially or fully blocked.

If you find a 4 hose by-pass then it may be blocked or broken.


The heater hoses need to pass coolant from intake to water pump when ever the engine is running, in my '94 the heater core is that connection, no by-pass.
When my core got clogged up temp gauge would swing high then back to "normal" almost randomly, although it did go higher if engine had a higher load.

I just by-passed the core until I replaced it, took hoses off firewall and connected them together.

If you have a 4 hose by-pass valve then select no Heat and see if temp gauge still swings high, if so by-pass valve could be the problem.
Select Full heat, and see if it swings high, also check if you are getting Full heat inside the cab, if it does swing high and heat is not HOT HOT then core is plugged

 

[chewy012]

I had my temp gauge go wonky on me. Heat worked and was hot hot. I'd flush the system and throw a new sender and sensor in. Probably $60 in total. Both mine where gnarley brown as was the coolant. 25 year old rig that I'd owned for a year, who knows the last time anything g was done, ya know?

Sent from my XT1254 using Tapatalk

 

[97RangerXLT]

Thanks Ron and Chewy.

Started it up and the needle pegged just as I left the neighborhood. Truck was not even warmed up yet. Once the truck warmed up I had good heat, so I am leaning towards the sending unit.

I will probably take Chewey's advice and replace both the sender and sensor. $25 for both on rock auto, and that is for the Motorcraft brand.

Might go ahead and replace the lower radiator hose and flush it too. It has been since July 2010 that I have done that, and the lower hose is original to the truck.

AJ

Sent from my Samsung Galaxy Note 4 while sitting in my underwear

 

[RonD]

Yes, needle going hot just after start up would take clogged core or by-pass off the table.