Ford 4.0 SOHC is significantly overcooling. Or, "Here's Why I Drink."

Hi,

First time poster, but long time reader. Forgive me for punishing you with such a long first post, but it's been a bit of an odyssey.

I have a 4WD 2003 Ford Ranger with the 4.0 SOHC V6. Not long after I bought the truck in the spring of 2014, I discovered that it would pop a P0128 "coolant temp below thermostat regulating temp" code whenever I drive it on the highway on a day that is t-shirt weather or warmer.

According to an OBDII live scanner app on my tablet, the engine warms from cold as expected, and the thermostat opens at 195. Then, however, the ECU never sees a temp higher than 160 again, even when I'm pulling a trailer on the highway up a hill. That's when the code appears. The temp gauge (which operates on a separate sensor in these trucks) shows exactly half, but I trust that thing about as much as a Nigerian Prince.

The truck feels fine around town, but sluggish when worked harder. It struggles to pull a 6 x 12 ~1900 pound enclosed trailer at highway speeds if even mild hills are involved, and seems to be quite thirsty even for an engine/vehicle combo that is known for being a high-teener at best in the real world. I suspect that both of these conditions are related to the ECU's reaction to a perceived low coolant temp, but I haven't been able to find irrefutable evidence to that effect.

Here are the things I've tried specific to this problem --

1) Tested the original thermostat in boiling water on the stove, and it behaved normally. Replaced it anyway with a Stant 195 degree unit. No change

2) Bled the cooling system and checked mixture strength. Mix was OK, and no unusual behavior during bleeding

3) Replaced the engine coolant temp (ECT) sensor and the thermostat again with an overpriced OEM FoMoCo 195 degree unit. No change

4) Checked engine vacuum, which was within limits and behaved correctly during throttle movement

5) Tested the clutch fan operation with a cold engine. There is some resistance, but it turns with one finger. The fan doesn't seem to run unusually often, and doesn't produce the "airplane propeller sound" of a stuck clutch as I drive around at operating temp

6) Used a laser temp probe to "feel" around the hot engine bay as the truck idled at operating temp, and couldn't find any spot that returned a temp higher than 160

7) Checked the easily-accessible sparkplugs, and saw no evidence of richness. If they'd shown overfueling, I'd have looked at the more difficult plugs. But, they didn't so I didn't

I've learned that the optimal temp range for this engine application is 195 - 205, and a 30 - 40 degree overcooling even during heavy service doesn't seem possible. When I described the situation, a Ford service supervisor stared at me like I'd arrived at his shop wearing only snowshoes and a lobster bib, and several hours of additional research have resulted in no solid leads. My next step will be to install an e-fan and a 210 degree temp switch, even though the clutch fan seems to be operating correctly.

So, I ask you... please, GRM forum patrons... does anyone have any suggestions before I build my own helicopter to transport this thing out to the nearest erupting volcano?

Thanks!

Bad news, I'm pretty sure your engine is insane (has a bad ECU).

Just the fact that the ECU thinks the temp is down around 160 when it's at 195, and you've physically checked temperatures and tried a different sensor, tells me that.

Edit: Also the fact that it initially climbs to full op. temp. normally tells me it's not a wiring problem. It seems that once the ECU enters open-loop mode, it goes nuts about the engine temperature reading.

GameboyRMH wrote: Bad news, I'm pretty sure your engine is insane (has a bad ECU). Just the fact that the ECU thinks the temp is down around 160 when it's at 195, and you've physically checked temperatures and tried a different sensor, tells me that.

I think the op said the laser returned an engine at or below 160 actual temp. Though it might be a good exercise to try that same laser on a normal good engine at operating temp just to see.

Is there any possibility the thermostat is incorrect? Seems like there is too much coolant flow to the radiator and that I would look at the thermostat to regulate. I know you said you changed it twice, but maybe it is physically incorrect? Mid model year change? etc.

rcutclif wrote:

GameboyRMH wrote: Bad news, I'm pretty sure your engine is insane (has a bad ECU). Just the fact that the ECU thinks the temp is down around 160 when it's at 195, and you've physically checked temperatures and tried a different sensor, tells me that.

I think the op said the laser returned an engine at or below 160 actual temp. Though it might be a good exercise to try that same laser on a normal good engine at operating temp just to see.

I'd say that sounds normal for an engine while the coolant temp is in the 190s. Even the water pump outlet and radiator tanks will be somewhere below the actual coolant temperature. Confirming on another engine is a good idea.

Edit: As an example, my Samurai's engine is 150-160F at the water pump outlet surface when the 180degF thermostat opens.

I know the tarus/stable efan is a good upgrade for lots of cars so it may work well for you, and an adj temp fan controller like from summit would be my next step but im not sure that would help. This maybe a dumb question for you but ive had a friend do something similar, your sure you replaced the coolant sensor for the ecu and not the gauge? Maybe turn the truck to on but not running when dead cold and check the temp through the obd2 to verify its correct.

What engine cooling fan does it have? If its a clutch type fan I'd say the clutch is on its way out, and is staying locked up once operating temperature is reached and not disengaging again till it's had a chance to cool down way past where it normally would. The other possibly could be an airlock in the cooling system, but that more often results in overheating than over cooling.

"6) Used a laser temp probe to "feel" around the hot engine bay as the truck idled at operating temp, and couldn't find any spot that returned a temp higher than 160"

^^^This seems weird. If the thermostat is functioning properly and it's not freezing cold out the thermostat should keep the engine coolant heading to the radiator at 195. Are you sure the laser thermometer you're using goes over 160?

Heater isn't playing combat with the AC is it?

Is it possible that a previous owner put in some kind of "bypass" hose that routes around the t-stat that shouldn't be there? Maybe some kind of "mod" to help with cooling? Is there anybody you know with the same engine/truck to compare all the hoses to?

I appreciate all the responses. Here's another marathon post.

GameboyRMH wrote: Bad news, I'm pretty sure your engine is insane (has a bad ECU).

I was a little worried about the ECU, but I haven't been able to determine with certainty that it's actually wrong. I'd hate to swap the ECU, especially one infected with the Ford PATS nonsense that would require a trip to the dealership, to find that the old one was right the whole time.

rcutclif wrote: Is there any possibility the thermostat is incorrect? Seems like there is too much coolant flow to the radiator and that I would look at the thermostat to regulate. I know you said you changed it twice, but maybe it is physically incorrect? Mid model year change? etc.

This thermostat part number is extremely common in the Rangers over a wide range of years up to the very end of production, so it seems quite unlikely that I've got some kooky mid-year switchover. I'll do a little reading to see if it's plausible, but I don't like those odds.

GameboyRMH wrote: I'd say that sounds normal for an engine while the coolant temp is in the 190s. Even the water pump outlet and radiator tanks will be somewhere below the actual coolant temperature. Confirming on another engine is a good idea.

It's probably true that I shouldn't expect to see 195 degree temps on the outside of the cooling system components, but I figured that it was worth a shot. I don't know anyone else who has a similar Ranger, but I suppose I could see if one of the Ranger forum guys would be willing to help me out. I'm not really into those forums, but one of them might take pity on an "outsider."

chiodos wrote: I know the tarus/stable efan is a good upgrade for lots of cars so it may work well for you, and an adj temp fan controller like from summit would be my next step but im not sure that would help. This maybe a dumb question for you but ive had a friend do something similar, your sure you replaced the coolant sensor for the ecu and not the gauge? Maybe turn the truck to on but not running when dead cold and check the temp through the obd2 to verify its correct.

Yeah, I've done two Taurus fan conversions before and I have all the parts to do this one too, but I'd like to figure out if that's actually the problem. The clutch fan sure seems to work as I'd expect, but it's really the last related item that I haven't replaced.

The two temp senders are directly beside each other and look exactly the same, and I admit that it was a bit of a task to make sure that I was swapping the right one. Finally I unplugged the other one and the gauge went dead, which is how I verified things before proceeding.

daeman wrote: If its a clutch type fan I'd say the clutch is on its way out, and is staying locked up once operating temperature is reached and not disengaging again till it's had a chance to cool down way past where it normally would.

When the thermostat initially opens at 195, the temp drops like a stone to 160 (+/- 3 degrees) and just sits there forever. Can fan clutches "partially" fail?

The fan spins by hand with a little resistance when the engine is stone cold, and runs for a few seconds during a cold start before spinning down until it's required for cooling. From the troubleshooting suggestions I've read, those are the characteristics of a healthy clutch.

Rad_Capz wrote: ^^^This seems weird. If the thermostat is functioning properly and it's not freezing cold out the thermostat should keep the engine coolant heading to the radiator at 195. Are you sure the laser thermometer you're using goes over 160? Heater isn't playing combat with the AC is it?

Oh, it's weird alright. I've used the same laser to test exhaust system temps over 500 degrees, so I do know that it's good well past 160.

Not sure what you mean about the A/C and heater battling each other. Clarify?

doc_speeder wrote: Is it possible that a previous owner put in some kind of "bypass" hose that routes around the t-stat that shouldn't be there? Maybe some kind of "mod" to help with cooling? Is there anybody you know with the same engine/truck to compare all the hoses to?

The hose routing is pretty easy to visually trace in this truck, and no such bypass contraption appears to exist. I'll run out to the garage tonight to double check, but it seems extremely unlikely.

Thanks again, everyone.

I didn't even think you could get the SOHC 4.0 in a Ranger.

I had one in a 98 Explorer and never had any problems with it.

Have you ohmed out the wires between the ECU and sensor?

The fact that nothing gets over 160 when checked with the non contact tells me something weird is happening with the coolant bypass. Greatly simplified: when cold the engine cooling system recirculates coolant through the block, heads, intake, heater core etc but not through the radiator. At 195, the thermostat opens and allows coolant to circulate through the radiator, at the same time the coolant bypass is blocked off. I'd go back and look real carefully at the thermostat; is it one which has the blockoff valve built into it? The valve is the 'disc' on the bottom of this one:

If it doesn't have a valve the bypass never gets blocked off. On Hondas which use this setup the wrong thermostat will make it overheat, the Ford may be different and thus not prone to that problem.

Also, a new thermostat isn't necessarily a good thermostat, BTDT.

Dumb question: what happens when you remove the clutch fan and drive around your neighborhood/street?

You say that when it hits 195 the temp drops like a rock to 160. By this, do you mean it drops like a switch was tossed or that it ramps down quickly?

I would pull the T-stat and drive around to see if it ever does go to 195. If it stays at 160, then you know that the T-stat is opening but not closing. For that matter, when you bench tested the T-stat, at what temp did it re-close?

In reply to iadr:

Mine was fixed early on, on a Ford factory recall.

"The temp gauge (which operates on a separate sensor in these trucks) shows exactly half, but I trust that thing about as much as a Nigerian Prince."

OK, if what you are saying is that the temp gauge only reads about half of what you expect, then perhaps you should investigate that as a related issue. Is the sensor(s) grounded properly? What is the supply voltage at the sensors?

Two things to try. Run it without the thermostat for a bit and see what temperatures you get. Run it without the fan (but the thermostat in) and see what temperatures you get.
And check your grounds. Always check your grounds.

Thanks again for all the suggestions. I'll spare you all the quotes, and just answer your questions.

• I have checked the wires between the sensor and the ECU for continuity (a short), and there was none, but I haven't checked the resistance. When I read your comment, I did some hunting and found a table of expected resistance and voltage values for this sensor at various temperatures. So, I'll haul out the multimeter and test those.

• I wracked my brain to recall if this thermostat has the bypass plate, and I am about 98% certain that it does not. I can't tear into it tonight for reasons that I'll explain in a second, but I'll find the "old" Stant thermostat and verify that it doesn't have the bypass plate

• Pulling the fan and driving around is a good idea. If I get to a point of starting on the e-fan, I'll do that before I actually install the electric one. This clutch fan requires a goofy tool to remove, which I don't yet own (or haven't made), so I can't do it on a lark

• When the stat opens, the temperature drops quickly... between five and ten seconds from 195 to ~160, according to the OBDII display. Pretty much what I've come to expect when a thermostat opens

• The temp gauge in the dash rises to half of its throw. Mid-way between the hot and cold display limits. There are no numeric values on the gauge

• I don't have any other electrical strangeness in the truck, but cleaning the grounds is always a good idea. I'll do that as well

To ensure that I wasn't misremembering anything that I posted in this thread, I just took the truck on a borderline abusive 20 minute drive, including roughly ten minutes at highway speed. I connected my tablet to the OBDII port via Bluetooth, and I saw a maximum of 165.2 degrees reported from the ECU while driving after the thermostat opened. That's actually about three degrees warmer than the last time I did the same thing. It's a cool, overcast day here.

I live only a few blocks off the highway, so I pulled it into the garage and immediately hit a few areas with the laser temp probe while the truck idled. At the thermostat housing (plastic) I saw a maximum of 168 degrees. At the rad (also plastic... sigh) just above the inlet from the engine I saw 170. At the waterpump, which was very difficult to hit with the laser with the truck running, I actually saw 182. That's by far the highest temperature I've measured on this thing after a few similar attempts over the last couple months.

So, I'm not really sure what to say here. These were higher readings than I've seen before, on what's actually a cooler day than the last time I did the same thing. The reported value from the ECU is clearly too low, but the measured values aren't as far off from being reasonable this time for some reason.

I can't see any easy areas to fudge a simple mechanical temp gauge in place temporarily, but I kind of think that might be the next move. I want to see a "dumb" representation of the damned water temperature before I can figure out which component is lying to me. When I popped the hood after the drive just now, the fan was running and the air being pulled through the rad was plenty hot. It made me wonder if it's actually running as cold as what I'm seeing on the tablet.

Oh, and one other thing. I found a web site that claimed the following --

When the OBDII port is connected, and the key is on but the engine is not running, a healthy coolant temp sensor (ECT sensor in the Ford lingo) and ECU should report between -4 and -8 F coolant temp. I ran this test before my driving the truck, and saw -4 F on the tablet. So, it appears that the ECT sensor and the ECU are communicating properly.

how 'bout the big tractor rigs trick of blocking off the radiator in the winter ? cardboard in front of the radiator and see if the temp comes up ?

have no idea what that would tell you … assuming any change in temps

plus the temp gauge on the dash sounds like the modern idiot light gauge … isn't giving any real info .. just dead cold, operating temp, and over heat … no fluctuations, just like a idiot light, three positions only

Pinch off one of the heater hoses with needle nose vise grips or something similar and see how it reacts. Im curious if the heater core is acting as a bypass for the tstat. Did those have a valve in the heater hose?

IMHO, the thermostat system is over opening. It's been pointed out before- but the system is supposed to not just open and close all the way- but to regulate the amount of water to keep the engine at a constant 195 deg. If you allow too much radiator water to get in- it will cool off fast. And if that by-pass just keeps things open- it will stay cooled off.

So either the stat is opening too much and staying that way, or the plumbing has been modified to not return water back to the engine, and push all of it to the radiator.

Thinking about it a little more- I wonder if something is in the line that returns water back to the engine from the T-Stat housing. Block that, and it will push too much water to the radiator.

NOHOME wrote: You say that when it hits 195 the temp drops like a rock to 160. By this, do you mean it drops like a switch was tossed or that it ramps down quickly?

I'd like to hear more about this. From what I understand it was dropping like a switch was flipped, which indicates a bad ECU. Also the fact that he's getting higher temperatures on the surface of cooling components than from the ECU's temperature sensor backs this up.

Way back in the Dark Ages, a goodly number of cars and trucks had a restrictor in the heater return hose. The reason was twofold: it slowed down the rush of coolant through the heater core which often caused a gurgling sound in the dash and also the heater would perform better due to the coolant being slowed down. I want to say Explorers had that and a Ranger is its kissin' cousin. Worth a look.

gearheadmb wrote: Pinch off one of the heater hoses with needle nose vise grips or something similar and see how it reacts. Im curious if the heater core is acting as a bypass for the tstat. Did those have a valve in the heater hose?

This was my thought in asking if the AC was battling the heater. The engine heat is going somewhere. If there isn't a problem with the thermostat opening and partially closing at 195 the heat is going somewhere else. Perhaps to the heater core > evaporator > condenser where the clutch fan pulls the heat out. Driving on highway would cool the condenser faster than the fan at idle further reducing temps. Easy test, unplug AC compressor.