I have a 4.0 that I just replaced the thermostat on, its now December and for the last week or so I've had an interesting issue. First let me tell you what I've done to the truck to help better identify the issue. Other than running full synthetic high milage 10w30 oil I haven't done anything to improve performance.

I have a new thermostat less than 4 months old, new fan clutch thats rated for severe duty, and an all aluminum 2 core radiator, new water pump and new radiator hoses. Basically everything in the cooling system is new, the only thing I haven't replaced is the heater core but it doesn't leak so I'm not worried about it. Other than that its stock, the thermostat is rated for OEM temp at 195° F.

Now to the meat of the issue, normally my temp gauge sits between the O and R of Normal on the gauge, I figure this is because severe duty fan with aluminum radiator. Its done that since I rebuilt the engine back in August and did all that other stuff. It sat between O and R in 100+ degree weather no problem at all. Now its winter, and for the last week its been a little cooler. Between 30 and 50 degrees. Last week it started fluctuating between R and the end if normal towards the hot side, not rapidly but noticeably quick if I sat at a certain RPM for a bit. I should also note that my 5 speed manual transmission has been threatening to grenade itself and does not have a functional 5th gear. Im working on that issue. Right now I have 4 gears.

At highway speeds cruising between 65 and 80mph I'm sitting between 3 and 4 thousand rom, I try to limit my time on the freeway and mostly do 3500 rpm max. I noticed last week that it was going up past my comfort zone (middle of normal) and approaching Chernobyl levels of heat. My oil pressure gauge was also doing some odd stuff, its usually on the high side but it crept down towards middle of normal too. In the following week I've noticed a very similar trend, at least with my temperature. Even at 2k rpm through town my gauge will fluctuate. I suspected head gasket problems but there's no coolant in my oil and no oil in my coolant, nor have I lost any coolant whatsoever.

I noticed it climbs up even at idle and when I hold it at 1500rpm for a second it cools back down. However at idle it does not go over the A in normal.

Since I haven't lost any coolant could this mean that I have a faulty thermostat or am I just being dumb and thats what its supposed to do? The radiator cools off pretty quickly when I shut the truck off, could it be that the thermostat is just opening a wee bit and closing when the temp drops forcing the engine to get pretty warm before it opens again or am I just being dumb? It didn't do this in the summer AT ALL so thats my only explanation. My initial fear was that I cracked my heads but since there's no contamination or loss of fluid on either side I doubt thats the issue.

TL;DR Did I **** it up or did it **** itself up?

 

N O R M A L doesn't help because gauges were different over the years
Use 1/4, 1/2, 3/4, or 1/3s

After warm up temp gauge should be at just under 1/2, about 190deg, 1/2 is about 205deg, above 1/2 when climbing a grade or pulling a load is OK as well, 3/4 means pull over and shut down engine or you WILL crack a head

After warm up if temp gauge randomly goes up and down while driving or stopped then heater core is partially clogged up
The heater core is the water pump bypass, so needs good flow, always, whether you are using it or not

1994 was the last year heater core was EASY to change, lol
2 hose clamps(in engine bay) and 4 screws(in the cab), core just drops out and new one is installed, put a towel down under it, the old one will have fluid inside, and will be heavy


If temp goes up ONLY when stopped then the new fan clutch is bad

 

Alright, those are measurements J can work with. Usually it sits between 1/4 and 1/3 when at its "warm temp" after the mods. In the last week or so its been hitting 1/2 or above while driving, usually no more than 2/3 though. My thought is that with the beefy cooling I gave it the thermostat can't figure its life out since if I let off the throttle or go to a lower RPM it cools back down very fast to its "normal" 1/4 to 1/3, then maybe goes back to 1/2 or a little under.

I know my heater core isn't clogged, I flushed it before I installed my rebuilt donor motor and it blows hot air all the time. The cold/hot slider aparently doesn't work, its either hot or slightly less hot.

Should I maybe try a thermostat rated for 180 instead?

 

The thermostat doesn't care about whats been added outside of the engine, like radiators or fans

Its a pretty simple device, the coolant/water on the engine side of the thermostat circulates thru the heads, block, water pump and heater hoses while thermostat is closed
As the coolant warms up the WAX inside a sealed chamber melts and expands in volume pushing open the thermostat's valve to allow coolant to flow thru upper radiator hose and that causes the cooler coolant to be forced out of radiator thru the lower rad hose and into the engine/water pump

The Test for stuck open thermostat is to start COLD engine, and feel the heater hoses and upper rad hose, both are cold of course
Wait 2 or 3 minutes and heater hoses should be getting warm, upper rad hose should still be COLD, thermostat is CLOSED, if upper rad hose is also warming up then thermostat is not fully closed, not working like it should

Boats use a WHOLE lake as the radiator for the engine, and thermostats work just fine to keep these engines at operating temp, so size of radiator doesn't matter, unless its too small, lol, and the fan doesn't matter unless its broken

After 8min of run time a cold engine should be a full operating temp, so above 1/3 and below 1/2, 190deg
If yours is not above 1/3 then do the test for open thermostat, you could have a bad one

Also test the new fan clutch
Before starting COLD engine(after sitting 6-8hours), open the hood and spin the fan, should be hard to spin, this is called a Cold Lock
Start engine, you should HEAR the fan sucking air because its cold locked
But that sound should go away after 5-10 seconds
Shut off engine and spin the fan again, should be easy to spin, its unLocked, free spinning now, if not the fan clutch is bad, its staying locked all the time or is locking and unlocking randomly which causes temp swings up and down

When filling or draining cooling systems always pull off 1 heater hose at the firewall, this allows air in and OUT of the engine side of the thermostat, so allows better draining AND allows air OUT when refilling, which is very important because the heater hoses is where air can be TRAPPED when refilling

The dash gauges are electrical so subject to voltage swings
If gauges seem to change more that usual then it would be a good idea to monitor the system voltage
You can get inexpensive digital volt meters that plug into cigar lighter plugin, some even have USB charger ports
These can be very helpful to see real time voltage when things act up

Key on voltage should be 12.3v to 12.8volt, 12.3v is 5/6 year old battery, 12.8v is a new battery
Engine running should be 14.3v to 14.8v just after start up, this is a "fast recharge cycle"
Then after 5 to 10min it should drop down to under 14volt, 13.5v to 13.8v
Over 14volt long term will "cook" a 12v car battery

Up and down voltages would of course cause gauges to change, and means voltage regulator in the alternator is failing or one of the fields in the alternator is failing


The slider cable for temp can be seen thru glove box opening
Fold down glove box all the way
Look at the top left of opening and you will see the cable for temp control
It operates the Blend Door, this is a door/flap inside the air box that directs air from the fan either thru the heater core(HOT) or around the heater core(COLD) or anywhere in between
You can see the arm the cable end is attached to, that's what moves the door, you should also see the adjustment, which is just a screw/clamp that holds the cable sheath in place, like adjusting brakes on a bicycle

 

"Key on voltage should be 12.3v to 12.8volt, 12.3v is 5/6 year old battery, 12.8v is a new battery
Engine running should be 14.3v to 14.8v just after start up, this is a "fast recharge cycle"
Then after 5 to 10min it should drop down to under 14volt, 13.5v to 13.8v
Over 14volt long term will "cook" a 12v car battery"
Huh I don't think mine ever goes back to the 13 volt range,, it almost always stits at 2/3 or 3/4 voltage and only goes to 1/2 if I have the blower fan on high. I'll have to get a little digital reader for sure. It sit at 1/3 voltage if the key is on but the trucks not running.

As for clutch fan, I'll check it out I dont think its cold locking anymore. It's got a warranty tho ugh so I'll have them give me a new one. Im gonna be oissed if both my fan clutch and thermostat are already dickered, but I'd rather know than not. Thanks for the wisdom!

 

To revive a pertinent thread for me....

Just did a head gasket/valve cover gasket/thermostat/water pump on a '93 3.0 v6 (I am original owner). Had a 180 tstat and put in correct 192 tstat. I get the odd fluctuations like the orignal poster, between left of the N and the center of the R. Never ran so high, was historically just the right half of the N. Kept heater open and fan on during today's test.

It's the fluctuations that have me stumped. I will check voltage as per Ron and replace the cap (because it's cheap 13# or 16# with A/C for OEM?). I don't think it's the heater core as I just did a 20 minute drive with the heater on and nearly melted, it works fine. No gurgling in the heater core as when I detect air there.

If it's air, is the gauge reading low or high as the air bubble passes?

I thought to discount the fan clutch as it seems to turn fine with some resistance either cold or warm, and I didn't think it was of consequence during my 20min drive (freeway).

Naturally I am nervous as I don't want a overheat, headgasket/head machining isn't cheap. I will continue to try and burp air. When I replaced the tstat I did it on ramps, funnel filled, ran for 15 minutes while squeezing upper hose for the initial burp, even had a trick to sort of fill the upper hose before I connected to the upper radiator (involving a c-clamp, small blocks of wood, a plugged upper radiator entrance, and funnel partially filled, all to get as much fluid in and least amount of air).

The old 180 tstat did not have a flipper valve(!). I tried a high flow 180 tstat with a flipper valve and just had wider fluctuations, only at a lower temp overall. The OEM 192 tstat has a flipper valve and the fluctuations are a little more narrow, just higher.

 

Random temp fluctuations just below 1/2 and then to above 1/2 that doesn't have anything to do with stopping or cruising along means your Heater core is getting clogged up
The heater core is the water pump by pass so needs the same flow thru it all the time
You can try reversing the heater hoses at the fire wall, might clean it out a bit, you should do this on ANY vehicle every 2 years or so, to help heater core last longer, called back flushing
Heater cores have no assigned IN or OUT

Good news is 1993 Ranger heater cores are cheap and very easy to change
2 hose clamps in engine bay, and 4 screws in cab

Google: changing heater core in 1993 ford ranger

There is a short video
Put a towel down when pulling out the old core, coolant will spill out

1995 and up you have to pull out the whole dash, lol

 

Ron,

Roger that, I think it'll be easy to flush first, and yes it's easy to replace. I think I'm resolved to that being the problem, thx!

 

Well, to continue...

Heater core was flushed thoroughly and other than a little tan crud coming out it ran clear, flushed it both ways with pulses and various flow rates from the garden hose. Judging by the volume coming out and the heat it is putting out I thought I had this licked. Turned out I still have the fluctuations. I don't have the 'gurgles' in the heater core lines when I have air in them so I'm fairly certain the air is out. Who knows, I could be wrong-

Moving on to the fan clutch, it seemed a slight chance like it might be locking up erratically , so for 50 bucks I changed it out, with no results also.

Checked the operating voltage at 14.6 fairly steady.

I guess I'll re-visit the heater core, sure seems to put out the heat in spades. Both hoses hot to touch. I'll put an infrared thermometer on things to try and troubleshoot some more.

 

Since heater is working can you FEEL the temp of heat air go up with the gauge, it should if temp gauge is correct
Warmed up engine is 185-195degF, below 1/2 on gauge, 1/2 way is 205degF, 3/4 on gauge is 230degF, so should be way HOTTER air if coolant is reaching that hot

 

Ron,

So I checked temps with a digital thermometer and IR thermometer, the gauge and the heat coming out of the panel vent do correspond.
After a 20 minute drive here are the vitals...
Radiator hose upper 147F
Radiator hose lower 87F
Heater core hose in (starboard) 141F
Heater core hose out (port) 132F
Dash vent at 20 minutes 136F to 146F, 138F to 142F mostly stable, moves with dash temperature gauge.

So I'm guessing I'm left with either air still in the system, or some other variable causing the fluctuation. I can't hear the 'gurgle' in the heater core hoses when I do have air in the system, tried bleeding on incline several times. New radiator cap, new thermostat (tried several), new fan clutch, new water pump, recent head gasket/head machining, recent timing chain cover gasket/new chain & gears), 212K miles. Probably not hurting anything but it sure drives me nuts watching that dang gauge....

 

Upper and lower radiator hoses are way off

At most you should see a 25degF drop at most, least is 15degF
You are showing a 60deg drop
Retest that lower hose

Engine should be running at 185degF upper and 165degF lower, after full warm up, assuming 190-195degF thermostat

195degF to 215degF is OK for upper hose under high loads
As said lower hose should be 20degF cooler than upper

Even if temp readings are all off by 40deg, so 187degF and goes down to 177degF
I wouldn't worry to about a 10deg fluctuation at this time
Could be thermostat is the problem, new or not

Engine Thermostats usually have a 15 to 20deg operating range, so if they are rated as 190degF they will start opening at 185-190deg, and won't be fully open until 205-210deg
So 10deg fluctuation is not normal but could happen

 

Re-tested the upper and lower radiator hoses, 150F upper and 106F lower, still a lot of variance there. Using a small IR meter holding it about .5" from the hose, moving it around looking for the hottest spot by the connection. Heater core hoses still in the 135-145 range. I changed the radiator some 20 years ago so I don't know if it's some heavy duty unit or not. Of course none of this flucutation started until the recent timing cover gasket change and head gasket change.

With the weather finally warming up here I noted a spike in the temp gauge from warm idle after driving for say 20 minutes hard, 1/3 to fairly over 1/2 briefly, then back down to 1/3. Sure gets my attention.

Full disclosure, I've tried new 180, 180 High Flow, 190, 192, 195 thermostats (Balkamp, Stant, Motorad) all with the same resulting fluctuations so I'm kinda thinking it's something else going on.

New O2 sensors 3 years ago. New Idle control valve 2 years ago. I do get condensation out the exhaust about 3 minutes after start-up and then it takes about 5-10 minutes of warm up for it to go away. Just having done a head gasket naturally I'm watching that closely along with coolant levels.

If I had to guess I'd say my tailpipe looks a little more sooty than I remember, I will pull some plugs to check for rich running. My mileage is still 17-21 or so, so not really a change there. '93 3.0L V6 manual 5 spd, A/C. No check engine light and it functions on start-up (not burned out).

I will continue to try and bleed air, I have a steep incline area I can use.

 

The Overflow Tank system is self purging of air after just on drive cycle, i.e. one full warm up and full cool down = 1 drive cycle

If, after driving a few days, and you take the rad cap off and rad is full to the top there is no air left in the system, if there is air then there is a problem with the system

Heater hoses and upper rad hose should be 185+ degF
140degF is way too cool to get full combustion, which is the cause of the soot and rich running

Heavy duty cooling system parts can't "over cool" the engine, the thermostat is there to maintain the 185-195deg temp in upper engine/heads for full combustion
In winter months the radiator may not even be used, radiator is only there to use when there is extra heat, higher than 195degF

At 140degF on heater hoses the upper rad hose should be under 80degF, close to outside air temp, because there would be NO FLOW thru radiator hoses upper or lower because thermostat is still closed

Test your thermometer on another engine if possible, all gas engine run with same parameters for cooling systems