Annoying thermostat behavior

[Shran]

'94 Ranger, 4.0, auto

It did this with the original engine and it continued with a different engine that I swapped in last year. The only part of the cooling system that has stayed the same is the radiator and hoses. Brand new water pumps on both engines and I have tried three different thermostats and several radiator caps. New fan clutch ("heavy" duty, the medium level one as opposed to standard, heavy, severe duty.) New fan blade.

The video was the best I could capture while driving but often the temp will quickly surge up to or just past the L on the gauge (my IR thermometer suggests that is roughly 230-240 degrees.) Then it will crash way down below the N - guessing that's somewhere in the 150-160 range. R is about where the thermostat SHOULD be opening, again my IR thermometer says around 190-200 degrees when the gauge is at that spot. I am measuring that temp at the thermostat housing. It will do this consistently every day, all day long, at a stop or driving around town or on the highway... the only difference is at a stop it takes a little longer to recover to a lower temperature. Constant big increase, then a sudden decrease, then it repeats.

My immediate thought was the thermostat was sticking/opening late or I had an air bubble. I didn't test the last thermostat but the latest one tested fine. I don't think I have any air in the system, I have bled a lot of these trucks and use the same process which works fine on them - I can't get this one to take any more coolant. I put several thousand miles on the last thermostat with it doing this the whole time and it didn't self bleed so I guess I am ruling out air in the system at this point.

Any ideas? Just bothers me that the temperature never stabilizes. I cannot imagine it's good for the engine to see sudden 80-100 degree temp changes repeatedly.

 

[maddyn]

I remember experiencing this in my '92, 4.0L. It didn't do it all the time, IIRC it was mostly in warmer weather. I think I remember hearing at some point, maybe here, that when the thermostat is installed, you need to make sure the little bleed hole is installed towards the top. But that could just be an old wives tale. Only other thing I can think is the location of the temp sensor for the gauge, and maybe getting a pocket of steam or something in there. Not sure if it is possible to watch both the ECU temp probe, and the gauge temp probe to see if they read the same.

Anyway, just some rambling thoughts to maybe help the mystery along

 

[Shran]

I was really careful to orient the bleeder hole at the top, that is an important part of the process.

This truck does it all the time, cold weather, hot weather, doesn't matter. The only difference there is in warm weather, the fan clutch kicks in fairly often when pulling away from a stop light. I tried a "standard duty" fan clutch and it seemed like it stayed hotter, longer, and didn't kick in very often - also tried a "severe duty" fan clutch and it was just running all the time with no difference in the temp fluctuation.

I'm halfways tempted to put a different radiator in it but I forgot to mention, that is also a relatively new part. Not by me but the previous owner put it in. I have had it out several times and it's not plugged up internally nor is there any mud/bugs/etc clogging it externally... no leaves between it and the AC condensor... all the factory baffles and foam are in place there so I don't think I'm chasing an air movement issue.

It is very irritating, I can go full throttle with the AC on and fully loaded with camping crap and people up mountain passes in Colorado in my Explorer and it just laughs at me. That rig probably weighs at least 1500lbs more than this Ranger does and is 1/4 as aerodynamic. I've had the cooling system in that truck apart a dozen times, same with my other Explorer, a bunch of 4.0 trucks I sold, my wife's truck, my old 4.0 swapped '89... it just doesn't make sense. I am hoping I'm just missing something.

 

[RobbieD]

I am convinced that there are simply some vehicles that are possessed by demons.

Now that that's out the way . . .

My first thought is air in the cooling system, but you know your way around these trucks. Colognes always fought me getting the air out, until I started topping the system off through the heater hoses disconnected at the core, and playing "squeeze me please me" with the hoses.

Another thought, since it sounds like you've replaced / inspected / cursed everything else, would be all-new hoses. I've seem where a hose would separate internally, between the plies, so maybe you have a hose with an intermittent or partial blockage?


And then there's always the "sacrifice a chicken to the demons" ritual.

 

[Brain75]

My first thought was a overly sensitive or under sensitive (dead spot) gauge.... or sending unit. Since you replaced everything but the gauge I lean towards the gauge but I also have to wonder what temps you are really getting.

Did you replace the sending unit?

Not as familiar with the 4.0 as the 2.9, where is the sending unit (for the gauge) in relationship to the thermostat? Is it buried in the side of the block or in the housing right next to the thermostat... If it is right next to the T-stat, I could see that behavior real easy - no flow till it opens, then suddenly hot/cold water quenching the sensor.

I plumbed up a redundant sensor in my '48 with just a common NPT T fitting and can see variance between 2 gauges... modern cheap gauges.

When you tested the thermostat did it snap open very quickly or open gradually?

 

[Brain75]

If you have forscan and a laptop etc, I would 2nd the vote for comparing the computer's version of temp and the gauge's version of temp... mount up your cell phone to record the gauge while laptop logs the ecu, go for a long enough drive to do the magic a few times then go home and time synch the logs.

 

[DeeRanger]

I wouldn't worry to much about the temp gauge, you're almost out of gas.....

 

[alwaysFlOoReD]

I had the same problem with, iirc, my 90 ranger 4.0l. I don't remember ever getting the problem sorted out, but RonD had some input in my thread about the issue. I'm not good at digging up threads on my phone but it should be there about 4-6 years ago.

 

[Shran]

My first thought was a overly sensitive or under sensitive (dead spot) gauge.... or sending unit. Since you replaced everything but the gauge I lean towards the gauge but I also have to wonder what temps you are really getting.

Did you replace the sending unit?

Not as familiar with the 4.0 as the 2.9, where is the sending unit (for the gauge) in relationship to the thermostat? Is it buried in the side of the block or in the housing right next to the thermostat... If it is right next to the T-stat, I could see that behavior real easy - no flow till it opens, then suddenly hot/cold water quenching the sensor.

I plumbed up a redundant sensor in my '48 with just a common NPT T fitting and can see variance between 2 gauges... modern cheap gauges.

When you tested the thermostat did it snap open very quickly or open gradually?

Two different engines, two different gauge sending units, same issue.

My usual thought process is "factory gauges suck" but I verified this by taking a reading with an IR thermometer at the thermostat housing while it was idling in my driveway. I could watch it spike up to 230-240 degrees and then drop down way low.

4.0 thermostat is in the same spot as the 2.9's, FWIW.

All of the thermostats I've tested in boiling water will open gradually including this one. I guess by gradually I mean a few seconds to go from closed to fully open.

If you have forscan and a laptop etc, I would 2nd the vote for comparing the computer's version of temp and the gauge's version of temp... mount up your cell phone to record the gauge while laptop logs the ecu, go for a long enough drive to do the magic a few times then go home and time synch the logs.

It's an EEC-IV/OBD-1 truck, I think my code reader will read live data but only in the moment, it won't graph it.

I wouldn't worry to much about the temp gauge, you're almost out of gas.....

Minor problems

 

[Brain75]

My brain damage for forgetting OBD1.... you got too many vehicles to keep straight (more realistically I need to re-read and proof better, it was right there in post #1)

Since you are actually seeing temperature swings that are violent (too fast) and extreme (too far) I would keep pursuing it.

Just curious, since this is a 4.0 and I am kinda inferring the original was a 4.0 as well - is this the double thick radiator found in explorers with 4.0 and AC? (the radiator suggested / I want to increase cooling on my 2.9.)

Possible the radiator is over-cooling the coolant in the radiator when thermo is closed and that's why you get the drop way way too low... old school fix partially block the airflow to keep the radiator from over cooling - we did this around here only when temps went into the negative numbers... but I wouldn't do that in this instance, not with it getting above 230 on the other end.

What is the thermostat rated at on these? 190 or 192 I assume...

If it is exceeding more than about 10 degrees more than that I would tend to assume the "usual" suspects, partially plugged cooling system somewhere (heater core, radiator, etc etc).

I like the one cause, one solution more than 2 coincidentally happening issues, but possible the radiator is just too big and there is a restriction/clog at the same time.

 

[Shran]

Yeah, it does have the big radiator. Every 4.0 that had an automatic also got the big radiator. 4.0/manual got one about half that size. I have swapped these big radiators into a lot of trucks and never had a problem with a 4.0 at least. My old Bronco II (2.9, auto) got one and it would really take forever for the engine to come up to operating temp.

I think your over cooling theory has some merit, I'm just not sure why that would be happening because I'm using all the same parts that the truck came with. I have had that problem with my '88 4 cylinder truck, in below zero weather the heater just simply does not work and the engine never gets up to temp. I always had a plan to mod the core support so I could slip a piece of cardboard or a plastic election sign in front of the rad to block it off but never got around to it.

Thermostat is rated at 198. I'm half tempted to put a 180 degree stat in it.

I do have the original 1994 hoses on it. I should just replace those but they looked fine.

 

[4x4junkie]

Mine started doing exactly this once before (moving up to almost past the "L", then suddenly dropping down to "N" within 10-15 seconds). Usually the biggest swing occurred as the engine was getting up to temp (after being parked), it would go up to "L", then swing way down, eventually settling to roughly around the "O" or "R", but would still continue to wiggle a little bit.
Mine was fixed by replacing the thermostat, along with the coolant.
With the new t-stat, it would go up then stop around center (as it should, between "R" and "M"), then settle back to around the "R" and more or less stay there, depending on outside ambient. The gauge however has never again been rock-steady like it was for the first year or two when the truck was new.

I know you said you've already replaced the t-stat though... Have you tried another brand t-stat? (mine was a Stant, but that was 10 years ago, and who knows if they are the same now or not). Could it be possible you got cheap chinesium junk 3× in a row?

 

[Brain75]

I started line by line sifting through RonD's posts.... I found 2 neat pieces of info - I am sure you "old timers" probably knew / forgetting at the moment...

2.3L ('83-'97) - Coolant gauge reading off

This is what my coolant gauge is doing. It never heats up to what it was before. Have lots of heat coming out of the vents when I turn it on. It does have a “new” radiator from a newer 2.3 Lima. an answer on this post or a link to another post works for me 93 2.3 regular cab

www.therangerstation.com

I'm gonna quick paraphrase the 2nd rather than link cause it was long... fill it full, put some fluid in the overflow, drive it around till hot, park it with the nose up / on jack stands, let it cool down overnight untouched, next day crack the cap if it is 100% full you have no leaks / air in system, if you have any void either you have a leak you don't realize or air in the system that just worked itself to the radiator cap.

 

[bobbywalter]

almost all of them seep at the seams when they get old.


i usually do the flush tee in the heater lines and then extend the radiator with a purge funnel.

gets it all out and aired off....


3 weeks later its aired off again.

i just make sure it is full. if it starts surging slower where the gauge starts hanging a bit......i burp it.

 

[Shran]

Still on the struggle bus with this.

I ordered a new Gates water pump and am going to try that next. I remember reading someone else's post about their water pump not working because the impeller was spinning on its shaft. I am probably grasping at straws there but I guess it is worth a try. It seems like the really hot -> really cold moment happens way more often right after I come to a stop. I'm wondering if the water pump is just barely working and that's why I see the spike at a stop light when it's idling. Again, grasping at straws.

I've tried burping it with the radiator cap lever and I don't get any air. Just coolant. Tried burping it after driving it, tried doing it cold, and right when the hot spike happens. Nothing but coolant.

I know you said you've already replaced the t-stat though... Have you tried another brand t-stat? (mine was a Stant, but that was 10 years ago, and who knows if they are the same now or not). Could it be possible you got cheap chinesium junk 3× in a row?

I don't know what the original one was but I replaced it with a Stant and then the latest is a Motorad. I have a Gates 180 degree stat on order right now... not really the ideal temp but I'm going to try it.

I'm gonna quick paraphrase the 2nd rather than link cause it was long... fill it full, put some fluid in the overflow, drive it around till hot, park it with the nose up / on jack stands, let it cool down overnight untouched, next day crack the cap if it is 100% full you have no leaks / air in system, if you have any void either you have a leak you don't realize or air in the system that just worked itself to the radiator cap.

I've done this and checked it a dozen times... the radiator is always full. I don't seem to be losing any coolant other than a tiny bit that seems to splash out of the overflow (assuming from rough roads.)