Replacing coolant temperature sensor in 4.0L

I didn't see this anywhere else, and thought I'd pass it along.


2002 4.0L V6, 145,000 miles


My symptoms were -
Slightly low engine temp
Erratic high idle once in a while
Check Engine light with codes 118 and 125

Which lasted over a year, no big deal.

Then it degraded to hard starting, flooding, wouldn't stay running, then would after 5 minutes of coaxing. Time to fix it.

When you need to replace the coolant temp sensor, I learned the following:

Mine would not come out due to low height of the intake manifold. I had to loosen the front 10 cap screws (out of 12) to get enough room to get the sensor out. I did not need to remove the manifold, a half inch wobble is all you need. A T30 Torx bit on a long extension will be needed. 1/4" drive as the holes aren't large enough for 3/8 drive.

Mine was the clip style, not threaded. Removing the clip was problematic. I couldn't get needle nose piers to work, nothing else seemed remotely able to grab it in such close quarters. Then the light bulb went off. A paint can opener. Those little J hooked thingies. The hook was too thick to fit in, but a few seconds on a grinder to thin it down and voila, clip out. While we're on clips, DO NOT DO NOT DO NOT lose it. Both the Dealer and NAPA had no clips. The new sensor does not come with one. (Seriously?)


A little prying and cursing and out it comes. Mine needed to tilt forward as it came out.


Going back in, it's tight, and you have nowhere near enough strength in your fingertips. Vaseline made a huge difference, it didn't go in with glycerin. Don't even think about putting it in dry. Place the wire harness on first before installing.

While you're there, give serious thought to replacing the thermostat. The housing cover has to come off anyway, unless you removed the entire intake manifold.

There's some YouTube videos that cover the rest, but don't cover the above.

Immediately after sensor replacement all of the above symptoms were gone...

the thermostat has been in and out on my 4. 0 so many times, I replaced the bolts on the housing with studs and nuts. The short cut that works for me it's taking two bolts out of the alternator bracket taking the belt off swinging the whole bracket and alternator to the right (pass side) not tried changing the sensor "yet"

mister moose said:

I didn't see this anywhere else, and thought I'd pass it along.


2002 4.0L V6, 145,000 miles


My symptoms were -
Slightly low engine temp
Erratic high idle once in a while
Check Engine light with codes 118 and 125

Which lasted over a year, no big deal.

Then it degraded to hard starting, flooding, wouldn't stay running, then would after 5 minutes of coaxing. Time to fix it.

When you need to replace the coolant temp sensor, I learned the following:

Mine would not come out due to low height of the intake manifold. I had to loosen the front 10 cap screws (out of 12) to get enough room to get the sensor out. I did not need to remove the manifold, a half inch wobble is all you need. A T30 Torx bit on a long extension will be needed. 1/4" drive as the holes aren't large enough for 3/8 drive.

Mine was the clip style, not threaded. Removing the clip was problematic. I couldn't get needle nose piers to work, nothing else seemed remotely able to grab it in such close quarters. Then the light bulb went off. A paint can opener. Those little J hooked thingies. The hook was too thick to fit in, but a few seconds on a grinder to thin it down and voila, clip out. While we're on clips, DO NOT DO NOT DO NOT lose it. Both the Dealer and NAPA had no clips. The new sensor does not come with one. (Seriously?)


A little prying and cursing and out it comes. Mine needed to tilt forward as it came out.


Going back in, it's tight, and you have nowhere near enough strength in your fingertips. Vaseline made a huge difference, it didn't go in with glycerin. Don't even think about putting it in dry. Place the wire harness on first before installing.

While you're there, give serious thought to replacing the thermostat. The housing cover has to come off anyway, unless you removed the entire intake manifold.

There's some YouTube videos that cover the rest, but don't cover the above.

Immediately after sensor replacement all of the above symptoms were gone...

Click to expand...

Good write up, thank you

ECT sensor and sender for 4.0l SOHC is different than the 4.0l OHV

Pictorial of changing thermostat housing, and ECTs on 4.0l SOHC here: https://howtoreplacefordthermostathousing.com/

Just went through this on a 2004 B4000. My temp sensor was leaking fluid around the o-ring. Mine has two sensors, side by side. The leaky sensor was on the driver side. Presumably one is a temperature sensitive resistor to operate the gauge and the other a condenser fan switch. I did not know the sensors could be pulled w/o removing the thermostat housing due to the manifold interference, so I pulled the whole assembly. After I had it out, I checked the sensors with a meter, which looked and behaved identical and the connector key was the same for each. Both swept from ~33K ohms down to about 15K ohms with slight heating. The plastic housing had some chips on the hose fittings and I found a cast aluminum replacement for $89, which included both a temp sensor and a fan switch, a new bottom hose, all new H/W and clamps. When it arrived, I noticed the sensors were different from mine and also keyed different. One behaved more like a switch, the other like the temp sensor. The harness was keyed for the temp sensor in both locations, weird. Wanting to use the new sensors, I shaved the keying rib off the switch and installed the assembly with the sensors in the pockets as installed from the vendor. Soon after a test drive, she threw a check engine error for the cooling circuit. Here is where I discovered the sensor can be changed w/o removing anything else except the plastic 4.0 cosmetic cover. I concluded the sensors must be the same in both positions. I decided to replace the driver side with one of the originals, but had to first pull the passenger side to get access. I worked the clip out of the passenger side sensor with a pair of fine tip hemostats, keeping a white knuckle grip so not to drop it to who knows where. Once the clip was out, I could gently tease and wriggle the sensor up and out of the pocket then disconnect the harness. The driver side was similar, the clip was easier to reach but the sensor was further under the manifold and took more patience to tease it out. Replaced the switched sensor with an original temp sensor and the error code cleared. Residual issues: The A/C is now fast cycling. First thought, I had the fan switch in the wrong place or a disconnect somewhere and the fan is not working. However, the sensors seem to be as they were with the A/C working. I hooked up a low side gauge and sure enough there is a wide pressure swing with the compressor clutch in and out. Internet wisdom says it could be the condenser fan or low R134. I have not had the truck too long and have not been driving it much in the last month, the A/C issue could be a red herring. It may simply need topped off, or it may still be a fan issue. If I can't find any fan issues, I may get a set of proper gauges to dig a little deeper, now that summer is over the urgency on the A/C is lowered. Or I may just pop a can of 134 and see if that brings back the arctic breeze. It's always an adventure out there "under the shade tree".

bob-in-va said:

Just went through this on a 2004 B4000. My temp sensor was leaking fluid around the o-ring. Mine has two sensors, side by side. The leaky sensor was on the driver side. Presumably one is a temperature sensitive resistor to operate the gauge and the other a condenser fan switch.

Click to expand...

The drivers side was the sensor for the computer, senses low coolant temp for fuel mixture on starting and emissions control. That's why I was getting high idle frequently, it thought it was cold. I was told the passenger side drives the gauge, which for some reason needs it's own sensor.

I'm impressed you could wiggle out the sensor without removing/loosening the manifold, I tried and couldn't even come close. Makes me wonder if the manifold castings varied.

mister moose said:

Going back in, it's tight, and you have nowhere near enough strength in your fingertips. Vaseline made a huge difference, it didn't go in with glycerin. Don't even think about putting it in dry.

Click to expand...

Boy I wish I had found this post before skinning my knuckle to get that damned thing back into place!
The Vaseline certainly helped but I'm not confident I was able to get it all the way in.
Any tips on getting this in the last 1/8"?
The truck started up just fine, but I'm pretty certain when I turned it off I heard some air escaping from this general area.

There is a sensor and a sender. I replace me both just to not mess around. After replacing everything cooling system, my problem was not bleeding the air out. parking uphill and burping it out solved all of my problems.

Eddo Rogue said:

There is a sensor and a sender. I replace me both just to not mess around. After replacing everything cooling system, my problem was not bleeding the air out. parking uphill and burping it out solved all of my problems.

Click to expand...

Rookie here with common sense, but limited technical knowledge. How much of an upward slope and how do you burp the system?

Yankeroo said:

Rookie here with common sense, but limited technical knowledge. How much of an upward slope and how do you burp the system?

Click to expand...

There's some threads about it with varying techniques.
It just needs to be steep enough so the radiator cap is higher than anything else, slight incline is fine.

I start it up and while running, remove the cap before it gets warm...

Then spend the next 20 mins watching bubbles come out and adding coolant as it goes in.

Pop the cap back on and done.

To make it easier, I spliced in a bleeder valve up at the heater core line, which is the highest point when parked flat. Bleeds out a little faster.

Just remove either heater hose at the firewall
Fill radiator until coolant comes out the unhooked hose and heater core port
Reconnect
Air in the engine is gone

Put rad cap on
Put coolant in Overflow tank up to COLD line
Go for 20min drive, uphills are best to get engine hotter
Park it
Check rad 4 or more hour later after its COLD again
Rad should be full to the top no air
If there is air then you have an issue but not related to bleeding the air out of the system
Could be bad rad cap, clogged overflow hose or tank

Or failing head gasket/cracked head

Ranger's Overflow system purges any air on its own by driving, heat up and cool down, should purge any air in the system

I was hoping Ron would step in with a more detailed how to lol. Do what he said.