94 YJ 4.0 overheating

[tygersquirt]

Hello all, me again.

I’m just finished up an engine rebuild of my 94 4.0. Took it for a test drive and noticed the engine temp was maxed out.

Here is what I have done;

Pulled thermostat, tested it in a pot of boiling water, and drilled air escape hole.

Water pump is new and rotating the proper direction.

Obviously flushed and used all new 50/50 coolant.

Burped the system with front end up on a log, no more air bubbles are coming out.

Top radiator hose gets hot, lower is warm. Both heater core hoses get hot. Heat blows hot.

This is the part that stumps me, after replacing the water temp sensor at the back of the block, no coolant leaked out of the head as if it were empty. But when I pulled the thermostat there was coolant in the head. All the videos I have seen, the coolant shoots out like a waterfall.

Now I’m trying to see what my next likely steps would be.

*Update* Just walked out and pulled the radiator cap and coolant spewed out of the temp sensor hole.

Thanks,
Tyler

 

[ytradio]

The first thing that I personally would do is use a manual temp gauge, maybe you already are. Then if the temp is still high you may need a new and larger radiator.

 

[tygersquirt]

The first thing that I personally would do is use a manual temp gauge, maybe you already are. Then if the temp is still high you may need a new and larger radiator.

That will be my next step. Thanks for the input.

Thought just crossed my mind, does the temp sensor have a ground wire? If so, mine definitely does not. But then again, starting the Jeep it will read cold, and gradually peg out after about 5 minutes of idle/driving.

 

[machoheadgames]

That will be my next step. Thanks for the input.

Thought just crossed my mind, does the temp sensor have a ground wire? If so, mine definitely does not. But then again, starting the Jeep it will read cold, and gradually peg out after about 5 minutes of idle/driving.

The temp for the gauge is only read off of resistance to ground. The wire from the gauge to the sender is a ground wire for the gauge. The sender varies the resistance to ground which fluctuates the gauge.

 

[Flyer58]

During the first hour of an engine break-in the temps will be higher but usually the radiator will cover the increase. Did you put Teflon tape on the temp probe or any other sealant on the threads to which will increase resistance? The probes need a good clean connection to the head.

 

[tygersquirt]

During the first hour of an engine break-in the temps will be higher but usually the radiator will cover the increase. Did you put Teflon tape on the temp probe or any other sealant on the threads to which will increase resistance? The probes need a good clean connection to the head.

No Teflon or sealant was used for the two temp sensors. I’ll give them a good clean and give it a try. I read online that the aftermarket sensors have a bad rep. I’ll hit the Jeep dealer in town today to see if I can get an OEM.

 

[deadpool1]

pulled the thermostat there was coolant in the head

the rad test told you all was good.
all thermostats made have a bleed hole or jiggle pin, hole if not stop buying china no name parts. (try USA STANT)

Top radiator hose gets hot, lower is warm

well you can not blame the RAD if it did that, 30 to50deg drop in temps end top end.
wrong stat, or put in backwards or not with bleed hole ( at the top, fully up hill top)
the engine must be full burped, of all air.! all 4.0L guys know it can be tricky, even service manuals warn of this and now to do it.
the gauge sender does not work in Air. ONLY IN Coolant !

the cars VIN shows 94 right? and VIN engine code shows. 4.0L right?> (as in no engine upgrade swaps in the past)?

use the correct senders (ECT and gauge) guage it has no ground wire only the sender shell is the grounded ! proof below.
the real ECT has 2 wires, one is ground.
old gauges and old sensors this old read wrong, (expect it) and use a IR thermal gun to find the real time. at RAD tubes and thermostat outlet and water pump inlet.
is the fan shroud missing.?
fan clutch bad?
one sensor is for the gauge only
the other is ECT (real name) for the PCM and EFI fueling.

my 95 dash gauge goes 100f to 260f, and 265f is boiling point of 50%antifreeze so where did your needle go exactly and when
does car have A/C?
YJ dash gauge sender is here.

the real ECT is below, for PCM

the real ECT does this ,(my graph) ohms 900 to 1000 ohms normal hot engine (key off hot, connector pulled to it) FSM pages show that.

 

[deadpool1]

there are 3 sensors to confuse 4.0L
ECT is 2 wire to PCM
IAT is 2 wire to PCM
and gauge coolant sensor is 1 wire. directly to the gauge temp.
do not mix them up, when testing.
the FSM free in sticky covers all 3 and now to test all 3. covered.
do not buy the wrong sensors. for any above if they will read wrong. vast types exist. SMP makes them and jeep has none now .
NOS is noS, what you find NOS is your luck.
the IAT has coolant, if pulled it is dry because it is the air intake plenum IAT air sensor. so leave it alone (2 wire)
the 1 wire is the gauge sensor/sender.

ect chart here.
ohms,test.
The ECT sensor is on the side of the thermostat housing, (not for the gauge at all it is for the PCM)
here is ect chart 1 from FSM actual
about 900 ohms hot, the IAT 100f is 5000 ohms air at 100f in the plenum (typ on hot engine)

now the gauge data from the same manual
I Do tests to find a problem and not buy the wrong parts and now 2 problems,. see?
as the FSM covers.
gauge tests 8E -47 (chapter 8e)
it tells you use real tools to see if engine heat matches the gauge , in the first tests./
the sender is on top rear of head. (gauge)
key on, pull the 2 sender wire, gauge dash goes to near zero.
if not gauge is bad (or the wire pulled is shorted out)


this is how to test the gauge. using resistor (in hand) 100 ohms would be mid scale)

also the FSM shows wire colors, and the 3 sensors are no the same at all (besides black ones seen )
getting DTC error code 22/? (overeheat) doing the DTC checkS?

the real gauge sensor is here, this year, only not on newer cars that went to 1 sensor for all . (ect)
it goes to the head water jackets. and must never be dry there. ever.
if is dry the system was not burped correctly, as is so very common on 4.0L
I use the heater hoses to burp mine, even me back in 1988 doing mine, remove the highest hose to heater pipe to pump and burb it here.(keyoff)
getting all air out is job 1, for cooling, you do not want air in the head jackets or the head can warp, etc.(coolant passages all must be air free)

 

[deadpool1]

use deep wall socket, and not break the plastic 1 pin connector.
okay?

 

[deadpool1]

now my real ECT 2 wire sensor (and iAT) both./
I made this xls excel plot, using log plotting trick, to show a jeep log sensor as a linear line. (log over log) << fun to do.
cool no?
so what to do with this,well, if engine seems hot (220f is not so) keyoff, pull ECT conn at the thermostat base. and DMM meter on homes
probe the 2 pins, see ohms hot. and below is the answer.

this log sensor seen on log graphic, plot is cool ,the black line is perfect log. no sensor is perfect but is close. about 1000 ohms hot
the coolant boils at 265f, water boils (pure) at 212F so never run just water,(at STP). the data on the left is jeeps,1995 YJ
or with $15 IR thermal gun point it to thermostat housing and to water pump huge input hose end metal of pump. and use IR gun to metal only is best.

the READ tube are metal and all can be read with the Gun. end to end. (ask how, this finds clogged RADs in 10min work)
on 1996 and up we simply OBD2 PID scan the PCM and it flat tells me the ECT temperature readings and can be even plotted with the too; cold to hot.
live data posted, to see overheating is real but sadly your car, needs DRB-II scan tools (long gone forever) and $$$$$$$$$
so on pre OBD2 cars, called OBD1 we learn to use a DMM to find bad sensors.
if that is too hard then guessing is the ploy for most.

on newer cars the ECT is more accurate and not the same part, so engine saves fuel and makes less smog. in1996,jeep no longer shows ECT data. dang

 

[tygersquirt]

Sorry for the late response, been super busy at work but have the next four days off.

well you can not blame the RAD if it did that, 30 to50deg drop in temps end top end.

I wasn't trying to say it was an issue, I was simply stating facts to help solve my problem.

wrong stat, or put in backwards or not with bleed hole ( at the top, fully up hill top)

I ensured it was a 195 T stat, verified it was working with the boiling water method, as well as bleed hole at the 12'

the cars VIN shows 94 right? and VIN engine code shows. 4.0L right?> (as in no engine upgrade swaps in the past)?

Jeep is 94' vin, motor is also 94 vin. The block was swapped as I put a hole through the side, but the donor block is a 94. Head is also 94 from original Jeep.

is the fan shroud missing.?

Fan shroud was installed for initial test drive, just took it out for easier access when changing T stat and serpentine belt. Plan to put it back in today.

fan clutch bad?

Fan clutch is perfecto, brand new and spins with motor.

my 95 dash gauge goes 100f to 260f, and 265f is boiling point of 50%antifreeze so where did your needle go exactly and when
does car have A/C?

My 94 gauge also is 100f to 260f. On start up, the engine temp reads 100, and after roughly 2-3 minutes of idling it will slowly make it's way past the 160f mark. The Jeep does have the factory AC and pulley installed. Verified the pulley is still in good condition before rebuild.

with $15 IR thermal gun point it to thermostat housing and to water pump huge input hose end metal of pump. and use IR gun to metal only is best.

My old man had an IR gun, couldn't find it so I just picked one up today and will test the Jeep when I get back home.

I plan to flush the radiator, burp it, and temp gun it. I have already hit every electrical contact with cleaner and a wire brush as well. Hoping I can get this squared away today.

Thank you for the write up, if burping the Jeep does not work again, I will hit the connectors with a multimeter and see if they are pushing the correct ohms.

 

[tygersquirt]

Update; after driving 5 miles and idling for 20 minutes, temps with IR Gun are as follows; (all temps taken multiple times and within a 5F difference each time)

Thermostat housing: ~158F
Top Radiator Hose: ~130F
Heater Core Hose to TStat: ~150F
Heater Core Hose to Water Pump: ~160F
Back of head near temp gauge (1 wire): ~215F
Bottom Radiator Hose: ~100F

 

[tygersquirt]

Update; after driving 5 miles and idling for 20 minutes, temps with IR Gun are as follows; (all temps taken multiple times and within a 5F difference each time)

Thermostat housing: ~158F
Top Radiator Hose: ~130F
Heater Core Hose to TStat: ~150F
Heater Core Hose to Water Pump: ~160F
Back of head near temp gauge (1 wire): ~215F
Bottom Radiator Hose: ~100F

Have not flushed the radiator yet. Doing this now and will update temps after burping.

 

[HRDROKN]

My 94 YJ had very much the same issue....

I had upgraded (or maybe actually a down grade) to a flowcool water pump and IIRC a colder hi-flow t-stat (not sure what temp), but anyway... took out for test drive and temp gauge started going way high. Stopped let it cool down, etc. Did all the tricks except drilling out t-stat larger (it already had a hole. Messed with it for two days! I was ready to put all stock parts back on and while sitting in my angled driveway, nose up, hood up, pondering what the f..... I heard a big gurgle from the motor. Problem gone! I'm convinced that even with my best attempts to make sure she was air free, I was failing. Hasn't been a problem since.

I want to change my thermostat back to stock'ish as I periodically throw a code 17, but a bit apprehensive to go thru that whole mess again!

 

[tygersquirt]

Another update;
I am not so sure my thermostats are actually opening up once installed. I’ve put in both of the ones I have, and while attempting to flush I am noticing few bubbles, but no more coolant is going into the radiator.

When running with no thermostat and plain water, temp gauge went no higher than 220F/225F

Thermostat housing: ~158F
Top Radiator Hose: ~130F
Heater Core Hose to TStat: ~150F
Heater Core Hose to Water Pump: ~160F
Back of head near temp gauge (1 wire): ~215F
Bottom Radiator Hose: ~100F

New Readings as follow;

Thermostat housing: ~160F
Top Radiator Hose: ~136F
Heater Core Hose to TStat: ~159F
Heater Core Hose to Water Pump: ~152F
Back of head near temp gauge (1 wire): ~208F
Bottom Radiator Hose: ~106F

 

[tygersquirt]

Another update;
I am not so sure my thermostats are actually opening up once installed. I’ve put in both of the ones I have, and while attempting to flush I am noticing few bubbles, but no more coolant is going into the radiator.

When running with no thermostat and plain water, temp gauge went no higher than 220F/225F


New Readings as follow;

Thermostat housing: ~160F
Top Radiator Hose: ~136F
Heater Core Hose to TStat: ~159F
Heater Core Hose to Water Pump: ~152F
Back of head near temp gauge (1 wire): ~208F
Bottom Radiator Hose: ~106F

This will be the third time burping. I’m letting it cool down at the moment and heading to the parts store to grab another TStat to try out, hoping that is my issue.

 

[tygersquirt]

Update;
New 195F Thermostat, burped, cooled, burped, cooled, burped. Still reading 260F+

Temp readings are as follows;

Thermostat housing: ~158F
Top Radiator Hose: ~130F
Heater Core Hose to TStat: ~150F
Heater Core Hose to Water Pump: ~160F
Back of head near temp gauge (1 wire): ~215F
Bottom Radiator Hose: ~100F

New Readings as follow;

Thermostat housing: ~160F
Top Radiator Hose: ~136F
Heater Core Hose to TStat: ~159F
Heater Core Hose to Water Pump: ~152F
Back of head near temp gauge (1 wire): ~208F
Bottom Radiator Hose: ~106F

Thermostat housing: ~180F
Top Radiator Hose: ~145F
Heater Core Hose to TStat: ~160F
Heater Core Hose to Water Pump: ~157F
Back of head near temp gauge (1 wire): ~215F
Bottom Radiator Hose: ~112F

 

[tygersquirt]

Well, considering the temp gun is reading correctly, the only other issue it could be is a faulty temp sending unit, or gauge in the dash. Will give it a look over tomorrow morning. I appreciate the help, everyone.

 

[ytradio]

No offense, but as I said at the beginning the fist thingI do in a situation like this is install a man. Temp gauge. I have an old Stewart Warner in dash that has been in a half dozen vehicles and an assortment of small local mount temp gauges that came from the instrumentation dept. where I worked. I never trust a factory electronic gauge.

 

[tygersquirt]

No offense, but as I said at the beginning the fist thingI do in a situation like this is install a man. Temp gauge. I have an old Stewart Warner in dash that has been in a half dozen vehicles and an assortment of small local mount temp gauges that came from the instrumentation dept. where I worked. I never trust a factory electronic gauge.

None taken, I was just covering all my bases to ensure no damage was being done to the motor. I appreciate the help!