1992 4.0 temp gauge goes up quickly


New Member
Oct 20, 2023
I’m sure this is a basic problem, but my temperature gauge when idling goes up quickly. As soon as I step on the gas, it drops to the midpoint where normally is. I flushed the radiator and it still is doing it. Any ideas and thanks in advance.
I’m sure this is a basic problem, but my temperature gauge when idling goes up quickly. As soon as I step on the gas, it drops to the midpoint where normally is. I flushed the radiator and it still is doing it. Any ideas and thanks in advance.
OP is your post just a post to ask others to guess for you, or actually do tests to find the cause. It's OK to guess,. it is your car and your cash.
there are only 2 ways GUE$$ing and testing.
but at least ask what you want a helper to do.
below is how to test.

huge lack of facts here not told, about cars history. or what all guages read (other)
92YJ-4.0l (just happened out of the blue or bought it like this,)????
YES, LOTS OF IDEAS. (A huge list in fact)
but first is 3 simple things, dash gauge surge.
  • RPM wrong at idle failing. 800 is minimum idle.
  • air in the loop
  • shroud missing
  • clutch fan bad is not correct RPM per the FSM book at idle.
  • bad thermostat is super common. and super easy to test and find it is bad.all are. not one ever lasted 30 year. ever.
  • RAD tubes clogged
  • RAD does not have 50% antifreeze it looks like orange sludge.
  • each of these causes have simple to test to discover them all. per below.
  • bugs packed in to front of radiator..
  • is A/C on at fail.?
  • coolant looks like rust
    running water for AF50% coolant.
  • pump impeller rusted to dust. RIP one pump
  • loop no burped of all air . oops. this list has no order.
  • bad head gasket.(or worse cracked head)
  • leaks ignored, (AF leaked out)
  • the fan clutch must not slip at idle the tests for it are fully told in the FREE FSM book in the sticky this forum YJ. facts.

(air in the coolant loop cause air to be at the top of head water jackets, and guess what? that temp sender can not work in air, so surges. (super common)
the sure is burp the loop.(learn now on 4.0L)
not sure how fast it does or when, only hot or does it cold or does it both or does it at warm up, (details matter)
next is the wire to the rear of head (gauge sensor wire) loose or rusty or bad? this is super simple common failure on all car rust (aka, corrosion of any kind)
on wire pins.
next is when the engine gauge acts odd, what DID THE DASH VOLTMETER DO, MOVE WITH IT?
IF YES ALTERNATOR PROBLEMS (ITS REGULATION) Is battery voltage going wild, 13 to 15vdc is normal.
Not told what is the temperature needle readings low and high, seen, as if fluxes.
there are 3 numbers there on the dial. 200 if normal even 220, 265 f is boil over. The cars fan stock is super powerful. if still stock is it?

next is the rear of 4L head gauge sensor clean?,. no sealant on it is allowed, it has brass threads and must be clean, no caulk no Teflon tape there or its ground fails. hard. or goes nuts as the engine shakes. a 1 pin sensor, and the shell is ground,. both must be clean to work.
rad cap bad?
if all this checks out,then get a IR thermal gun to see that the temperature surge is real and not just the silly cluster gauges and it old sensor 31 years old.
use the IR gun a =$15 tool and learn the truth if this, pointed at the metal of the thermostat housing,. this is the hot point.
not spam, harbor freight has the gun cheap and works, I have 2 of there's too.
180f is common,.temp.
the thermostat this old is no good they have a design life of 7 years, so when they fail they sure can fail in many ways, and surging is just one. of many are
dead, struck open, stock closed or super slow to warm up, or surges. seen them and fixed them with the gun showing the truth. easy it is.

this is the short list so far,. doing the tests means not having to replace all the parts.
can I assume the coolant is 50% Antifreeze now and 5 years old or less ?
does not look like orange slop.?

and it does not leak coolant to the ground or to the shotgun passenger seat floor foot wells.heater core bad)
fan clutch works\? (btw it is locked up at idle ,does not slip at idle)

serp belt not loose , making pulleys slip,oops, mines set to 100 lbs of force, per the FSM and special tool to do that.
fan shroud missing?
wrong radiator there./?( top cause engine swaps done wrong or?)
the IR gun will prove the radiator tubes are not clogged, ask how see comments below.. its super easy and fast test.(hot engine test)
is the coolant all rusty now, if yes, the pump impeller blades rust out and fail. (some are plastic and don't do that but cant see throught its housing)

data using the gun, best is only point to metal. parts. for best readings.
thermostat housing 180f (if stat is 180f) then
the water pump bottom hose, is suction side metal. gun shows will be 30 to 50 degr. cooler, (The RAD does that job, )

There is hot side and cold side of the rad, all tube per side the same. or one is clogged.<<< the most amazing test ever, saving tons of labor and work.

that was the short list, here is more
more evidence ,not seen or ignored.
got water in the oil pan
got oil drops or carbon bits floating in the coolant overflow tank??
got steam.(anywhere)
got steam cloud at tail pipe,. bad head gasket
engine overheats and engine power drops and pings. (knocks) and feel and hear gross engine misfire.

now you know there is a pile of possible causes and evidence of the gauge acting odd.

I never ever trust any dash gauge this old, I use real hand tools to see if the old dog dash gauge is telling me lies. this I do first.
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do this? if temps are suspected of truly being wild.
is RPM hot 800 normal, or below that, say 500, wrong.? see now all things matter, even the front of the RAD packed in bug guts.

here is the $15 tool. RPM must be correct and not too low, the IAC controls (PCM servo) the RPM on this engine and love to stick and needs to be cleaned. ( I use RPM hand tool to know true RPM, the dash gauge is not very accurate(low end). but 500 rpm is wrong when seen or lower.


this tool finds many faults easy.
a bad thermostat.
clogged up heater core
clogged (any ) RAD tubs.
late to the party me, I got my answer from SMP, yesterday ( I contacted them)
'question, what is the test data for the 1 wire gauge sensor (dedicated it is)
the answer is here
7 data points not just 3 as jeep did (FSM)

TS334 has the following resistance properties: SMP 1 wire sensor, data below. for this jeep.

29kΩ - 36kΩ at 32°F +/- 10% accuracy here.

18kΩ - 22kΩ at 50°F

9kΩ - 11kΩ at 77°F

4.9kΩ-5.7kΩ at 104°F

2.3kΩ-2.6kΩ at 140°F

860Ω - 970Ω at 194°F 915 ohms on average.

640Ω - 720Ω at 212°F

predict 180f as 1000 ohms at +/- 5% accuracy.
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