No worries - I always appreciate you chiming in!

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That does spark a lot of my interest. So ideally a 2001.5-2003 Duratec would heat to 208 as the "normal range" (i.e. Halfway mark) on the temp gauge?

I do know there was another member a while back I got into a discussion with about the temps and the sensors, etc. I believe the UltraGauge reads actual coolant head temperature rather than the water temperature, for example 265 degrees, 300 degrees. So if the OP hooks up an UG then that would pose a problem too.

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2007 Regular Cab - 2.3 DOHC - 5 Speed Auto - 4.10 Gears - Maxtrac 7" Lift - Fiberwerx 3" Fenders - Bilstein 5100 Series - 15x10 Ultra 164's - 35x12.50x15 Red Letters - Custom dents

I bought the Motorcraft RT1157, which is 190F rather than my original 208F, because I had it hammered into my head by various reading online posts that I need to replace the thermostat with Motorcraft - and ONLY Motorcraft - or else I'd eventually be sorry. And the 190F thermostat is the one recommended by Ford even for those earlier models that originally had 208F, because I believe they found that those ones were running too hot, and that 190F was optimal. I feel good about the choice to go with 190F Motorcraft RT1157.

For example, here's an interesting post on this subject:

"The 2001.5 through 2003 Ranger has a really crappy electric thermostat, and it runs the cooling system too hot. The thermostat runs from 208 to 235 deg F while cruising, and only operates as a 190 thermostat when you really get on the throttle. The problem is, Ford didnt get the programming right and the thermostat always opens too late. To make matters worse, it causes excess pressure in the cooling system which eventually causes things to break...........Fix: rent coolant system pressure checker from advance and put 16 psi on the system. Note what leaks and replace. Put new motorcraft cap on degas bottle. Go on rock auto and buy a motorcraft thermostat for a 2006 2.3l ranger and install (Fits exact no mods needed)."

https://www.reddit.com/r/fordranger/...ottom-comments

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Yeah, ok, I had actually ordered and used a RT1157 also. I found that even though it was listed as a 190* thermostat, the CTS temp would still hang out around 220*+ or so. I now have a non-electric 2006 Motorcraft thermostat in it and my temps range from 205*-215* on a fully warmed up engine and depending on whether I'm climbing a hill or not. The cooling system on these Duratec's really confuses the heck out of me. I would love to talk to a Ford engineer who designed this system so they could explain exactly how it works.

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2003 Ranger XL | 2.3L | 2wd | 5spd manual | standard cab | short bed | 112" wheel base | 3.73 rear (open diff) | no power windows or locks but at least it's got A/C!

I assume you've installed a RT1194.

I wonder if the 190F refers to the temperature that the thermostat is designed to begin opening at, rather than the temp that the engine is supposed to always remain stable at?

Also, the thermostat, the Coolant Temp Sending Unit, and the CHT are all in such different places under the hood that to me it makes sense that there are temperature differences between all those different areas.

I'm not into monitoring specific temps with OBD-II like you guys are - I feel satisfied if my gauge needle climbs to the right position and stays put there. So I'm very pleased with how my RT1157 is performing. Twice in the past, I've been driving a vehicle when that temp needle starts to climb and the engine starts going weird, and the vehicle craps out. Not fun.
At this point, the gauge needle staying put where it's supposed to is enough for me.

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My coolant level still slowly drops, my coolant level in the overflow tank drops down to the Min level about every 30 miles driven. Yet I find no evidence of a coolant leak, and I don't have any symptoms of a failed head gasket (no smoke from tailpipe, coolant is clear, motor oil is clear, no loss of power or weird symptoms...).

The mechanic never burped my coolant system 2+ months ago after draining & refilling my coolant. He WAS in the process of burping it but I literally said to him "Can you please stop burping it" because he was revving my engine, which is a pet peeve of mine. lol.

So because my system was never burped, I really doubt that the full 2.75 gallons (10.9 quarts) was put into my system, and my suspicion is that I have been continually adding coolant to it because every time the truck gets up to temp and the thermostat opens, more air is being worked out of the system, and so the level in the overflow tank is going down more and more. For someone who drives their vehicle a lot, this bleeding process would go on for a few days or a week maybe. But because I only drive this truck a couple times a week (and fairly short distances), the bleeding process is still occuring even now, and will continue to occur until the full 2.75 gallon capacity of coolant is present in my system.

This is my current theory, just thought I'd update.

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If you would like to perform the Ford burping procedure on your truck here's the procedure. The only thing I would add to it is to put the front end of the truck up on ramps so that it's at an incline but it doesn't necessarily have to be on the incline. Flat is fine. If after burping it correctly it's still losing coolant slowly then you've got seepage somewhere and I would highly recommend a proper cooling system pressure test.

Starting with the engine cold:
1. Fill the system through the degas bottle cap to the max fill level.
2. Start the engine and run for approximately ten seconds at 2,500 rpm to prime the heater circuit then turn the engine off.
3. Top off the coolant level to 0.6 inch above the max fill level.
4. Install the degas bottle cap.
5. Start the engine and hold at 2,500 rpm engine speed for approximately eight minutes until the thermostat opens.
6. Maintain 2,500 rpm engine speed for an additional three minutes.
7. Increase engine speed to 4,000 rpm and hold for five seconds.
8. Return engine speed to 2,500 rpm and hold for an additional three minutes.
9. Repeat the previous two steps.
10. Stop the engine and check for leaks.
11. Verify correct fluid level after engine cools for 20 minutes. Top off the degas bottle to "max" line.

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2003 Ranger XL | 2.3L | 2wd | 5spd manual | standard cab | short bed | 112" wheel base | 3.73 rear (open diff) | no power windows or locks but at least it's got A/C!

Thank you, Soledad! If the burping procedure requires revving the engine, then I don't want to do it because I've read that revving can put wear & tear on the engine, and my high-mileage engine doesn't need anything else stacked against it.

I think I will rent a pressure test kit from Advance Auto sometime very soon, and I might also need to get a dye kit and buy a small blacklight (because the reviews say the little blacklight pen that comes with the dye kit is basically worthless).

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Hey everyone,

So last night I drained some oil and looked closely at it. It's clear, there's no milkiness or signs or water/coolant. Thank goodness.

Today I rented a cooling system pressure test kit from Advance. I put my overflow tank under 16 PSI and it seemed to maintain pressure good, I saw no obvious leaks. But after about 10 minutes, I looked closely at the pressure gauge and saw that it was very slowly going down slightly in pressure toward 15 PSI, so it was not maintaining pressure. I climbed under the truck with my flashlight, and after a while, I found the leak. It's back near the firewall kind of: if you drew a straight line connecting the back of the front wheels, it's located along that line, but way down in there. The leak is right at the meeting of a black hose that's clamped in place. The black hose says "HEATER" on it. (Please note that I had a major coolant leak 2+ mos ago and the mechanic replaced my heater control valve. I'm not knowledgeable enough to know if this is the same area as that was...so not sure if I should ask him fix this coolant leak for free, or what).

I shined the flashlight way up there and snapped some pics with my phone. Can anyone tell me what this hose/connection is called and what I should do about the leak? It seems super inaccessable...Here are pics:



And on the left side of this image:

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Nice find! Unfortunately I can't get out to my truck now to look at that connection but I "think" the far right side of that hose connects to what I call the Octopus hose (I believe it's actually called the Coolant Tank Recovery hose) that runs behind the engine to the other side.

The leaky side there I "think" connects to the heater hoses.

It looks like it's a simple screw clamp. You should be able to just snug that up a little more IF you can even get a screwdriver in there.

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2003 Ranger XL | 2.3L | 2wd | 5spd manual | standard cab | short bed | 112" wheel base | 3.73 rear (open diff) | no power windows or locks but at least it's got A/C!

Thanks!
Yes, you're right - I need to get a long screwdriver in there and tighten the clamp. If I can manage to get some good access - either from below or from above (more likely) I think I'll loosen the clamp and try to slide it a little bit further up the nipple before re-tightening, just for a more secure connection.

Thanks again, Soledad, I'm glad I took your advice and got the coolant system pressure tester. So nice to know it's not a head gasket leak. ;-)

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I found that I can see the hose clamp from above with a flashlight so I bought an uber-long screwdriver today. It didn't work though. In fact, I couldn't get any of my screwdrivers to go into the flathead screw slot. The only thing that worked was sticking my hand way down in there and attaching my 8 mm socket, then sticking my hand back in and attaching my 1/4" drive ratchet. (I love my new socket set SO much! Stanley black chrome, baby!)

Anyway I gave the ratchet a few yanks to tighten the screw, then pressurized the system. It seemed to hold 16 psi of pressure. Drove 10 miles w/ AC on, then crawled under truck w/ flashlight. No dripping!! Woohoo!

Hopefully now I will not have to think about my cooling system any more at all for a long time!

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I want to put in here what I've found out about the (disastrous) cooling system in these duratec rangers.

The 2001.5-2003 Rangers have a "190" degree electrically-influenced thermostat. I quote 190 because really, its basically a 208 degree thermostat that can be 190 degrees if the PCM determines it necessary. In 2004, they switched to a conventional style thermostat that was actually 190 degrees (and cycled like any other thermostat would do).

In terms of temperature, pre 2007 Rangers report cylinder head temperature, while 2007 and later Rangers report coolant temperature (at the OBD 2 port). I cant speak for post 2007 Rangers, but pre 07 rangers have both a cylinder head temperature sensor and a coolant temperature sensor. The cylinder head temperature sensor is the primary temp sensor used by the PCM; modifications to mixture (add or subtract fuel) and spark are determined by considering cylinder head temperature, knock sensor values, MAF sensor values, and throttle position. The Coolant Temperature Sensor is used basically for the gauge on the dash... sort of. This is where it gets a little complicated, and understanding the original intended design (as implemented on 01.5-03 Rangers) is helpful.

On the 01.5-03 Rangers with an electrically controlled thermostat, the tstat mechanically is 208 degrees. However, it can be electrically influenced to open sooner if the PCM applies power at the thermostat. Engines are more fuel efficient at higher temperatures (to a point), so the general idea was this: in steady state closed-loop cruising with little load, power to the thermostat would be killed and the engine could run up to 208 degrees and cycle there. However, under high loads (as determined by the PCM utilizing other sensor values), it would apply power at the thermostat which would aid it in opening sooner... at 190 degrees, or fundamentally lower the temperature to 190 degrees to allow for more advance/power. This was great in theory, and a failure in reality. Problems with this design:

1. Coming out of steady-state closed loop operation into sudden high demand/load was disaster; because the engine was higher in temp, the engine would want to ping- the PCM would sense this via the knock sensor and drastically retard the timing killing acceleration (until the thermostat brought the temp down to 190 degrees). Basically, the thermostat couldnt bring the engine temperature down fast enough to allow for proper transition between cruising and having power available.
2. The constant changing of temperature- even beyond normal levels of expansion/contraction experienced in any cooling system- brutalized the nearly all plastic cooling system. With time and especially usage, the expansion and contraction would weaken nearly everything; the radiator (plastic tanks), thermostat housing, plastic cooling tee below the IMRC slave, and even the degas bottle all end up leaking/cracking/breaking at some point. It also is tougher on the radiator hoses, pounds the radiator cap (especially as it sometimes causes flash boiling), etc.
3. The thermostat was insanely expensive, and all for the privilege of it destroying the cooling system and periodically killing performance

As a result, some of us (like myself) decided to remove the stupid electrically-controlled thermostat and replace it with a 2004+ fixed mechanical thermostat (which bolts directly in place). All of the problems I mention above are gone, though without one further mod you will have a check engine light. As I live in an area where a vehicle must pass OBD2 inspection (no CEL allowed), I had to borrow a solution from another member- a resistor between the wires that formerly went to the thermostat. I used a 1500 ohm 1/2 watt resistor soldered between the 2 wires that went to the thermostat (I cut the connector off that formerly connected to the thermostat), and its worked for years now without a CEL. You can get a 5 pack of these resistors at radio shack for $1.50.

Basically, when you start the engine the computer checks to make sure there is "resistance" in the heater circuit for the thermostat. If there is an open, it will throw a CEL; if the two wires are twisted together, it will throw a CEL. However, the PCM isnt very smart- it doesnt check for an exact value; if you put a resistor between the two wires, that fools it into thinking the heater circuit is present and voila no CEL Since all fueling/spark considerations are based on the Cylinder Head Temperature sensor (and other sensor values) and NOT on what the thermostat is trying to do to engine temperature, it runs perfect and has no negative side effects. As an added bonus, the fixed thermostat is ~$100 cheaper while being BETTER by far.

Finally now that you understand the above, consider how a "normal" temperature gauge would look to your average driver- the temp with the electric thermostat would be all over the place. Ford doesnt want this because they dont want a bunch of service calls suspecting a bad cooling system, and thus enters the Coolant Temperature Sensor design. The coolant temperature sensor doesnt actually report directly to the gauge, but instead to the PCM. The PCM calculates based on many factors whether its reported value is "right" and sends a certain voltage to the gauge. This seems crappy and overly complex (and it basically is), but it sort of makes sense when you consider the electric thermostat.

When warming up and below 190 degrees (coolant temperature), the gauge proceeds up as the engine warms up (like most gauges would). However, lets say the engine is warmed up and you're working it- up hills, traffic, etc. As theoretically the PCM would be powering the thermostat thus lowering the temperature to 190 degrees- its not overheating and thus the PCM sets the gauge mid-gauge. Now lets say you get out of town and end up cruising 55 on the highway: PCM cuts power to electric thermostat and coolant temps start creeping up to 208 degrees. Rather than sending the gauge above mid (which people might fear is overheating), the PCM knows this rise is intended and thus the engine isnt overheating- the needle stays mid-gauge. Now lets say the electric thermocrap has extolled its punishment and the radiator lets go- you start losing coolant. Eventually, the engine is up to 220.. 225..235. The PCM realizes something has gone wrong and its unable to use the thermostat to control coolant temperature, and thus that the engine is now overheating: it sends the needle up from mid-gauge to communicate to the driver that the engine is overheating. This is similar to why they use a dummy gauge for oil pressure- avoiding panic probably to ease burdens on their service department.

The bottom line of all this is that the cooling systems on the duratecs suck. There is way too much plastic, the routing sucks, the 01.5-03 design had piss-poor implementation, and it requires much attention. Ford realized their two biggest fuck-ups with this engine design... and abandoned both of them in 2004- the IMRC and electric thermostat.

Sorry for the novel- I hope you find some of it useful in the future (as it seems you've identified your main issue already). FYI: get only motorcraft (especially the damn heater control valve- I went through 3 before I learned my lesson), and pray that the coolant recovery hose with the plastic tee doesnt go... you need to remove at least the catalytic converter and suffer HELL to change it if it does go; the service manual actually calls for dropping the transmission! Feel free to ask for clarification if I've been unclear about anything...

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2002 Ford Ranger XLT Supercab, 2.3L Duratec I4, 5-speed manual, 4.10 gears, ~100,000 miles
Power nothing with air conditioning; crank windows for life!
Throttle cable mod, retained accessory power mod, 2006 thermostat w/resistor mod
Headlight relay harness, Philips xtreme-vision bulbs
P235/75R15 Michelin LTX M/S2s

Thanks, GSF.
I have had to replace my heater control valve after it failed & sprayed coolant all over, and I replaced my CHT sensor & plug (the plastic had become brittle & broken), and replaced my thermostat to 190 degree electric thermostat. After that my truck began reaching full temperature. My cooling system is now stable & working great.

I probably should have changed it to the post-2004 thermostat with the resistor, but I'm hoping I'm fine just with the electric one I have, since things seem to be working great nowadays.

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