2002 Engine Coolant Temperature Sensor Problems

Hello,

I did a little searching on the forums, but can't quite find the same situation. I have a 2002 4.0L V6. When the weather turned cold this year, I noticed the engine temperature guage would not come up to the normal range, and I got a check engine light. I don't have the codes handy, but they were 1) O2 sensor bad, and 2) Engine coolant temperature always low. (I've driven it for several months this way - not sure if that makes a difference.) I recently replaced the O2 sensor, and that code went away.

Even though I get heat I replaced the thermostat, expecting that it was stuck open. I also made sure the coolant level was correct. The code remained. I measured the resistance of the ECT when the engine was cold (don't know the outside temperature, but it was really cold), and after I had run some errands. The resistance values were around 1,000 kOhm cold and around 4 kOhm hot. (Not quite where I expect based on the Dead Link Removed I've seen floating around the forums, but still close.)

(In the mean time, the engine frequently dies when I put the transmission in gear after starting the engine from a cold state. I generally let the engine warm up for 10-30 seconds, or longer if I have to scrape the snow. Not sure if this is related.)

The resistances seem reasonable, but since the sensor is so inexpensive ($12.99 from AutoZone), I picked one up today and replaced it. The problem is, the truck won't start with the new sensor.

I noticed that with the new sensor (engine cold - it's around 18 deg F here, not running), the temp guage looks like the engine is warmed up normally (about halfway across the operating range). So I'm thinking that the fuel system thinks the engine is warm, and it won't start because there is no "cold start" signal.

So I unplugged the sensor, and the engine started for me. After a few seconds, I stupidly (I think), plugged it back in. The engine died almost immediately. So I unplugged it again, but I can't get the truck to start. Then I plugged it back in to the old sensor (hanging in the air), and it still won't start.

So I have a few thoughts.

1. The engine won't start again because it's so cold here, and I've flooded it with all this starting and stopping the engine. Can someone verify that this makes sense? It's so cold right now I don't want to pull the plugs to vent the cylinders if I'm mistaken.
2. The new ECT sensor is not working correctly. Why else would it be showing a normal operating temperature when it is so cold?
3. Maybe the original ECT was fine, and there is something else wrong. But this doesn't seem likely - new aftermarket parts generally work, right?

I'm starting to ramble, mostly because I'm really frustrated. The behavior of the ECT does not make sense to me. The long and the short of is that I have a three-week business trip coming up this weekend, and if I can't get this fixed, then my wife will be going bonkers stuck in the house with our toddler daughter in stupid Akron. So,

1. Am I on the right track?
2. What else should I be trying?

Thanks for any help you can provide.

Best wishes,

Alan

Try getting a new sensor from the dealer. Might cost more but may work. You want to keep the wife from getting bonkers. Three weeks in the house with the kid will definitely do that!

Welcome to this forum! Compare the resistance of the old sensor to the new one. Scan for stored codes, and print them here. Did you clean the MAF sensor?

I did not clean the MAF sensor - would that have made the temp guage display low? Or just cause my problem dying on cold starts (and not starting now)?

I wanted to check the resistance as you suggested, but I don't have a meter handy - will need to get it from work tomorrow, and will report the results.

Also don't have my own scanner, so will check codes if I can get it started and to the parts store.

A dirty MAF sensor could give the PCM the wrong reading, and create problems.

Yea, aftermarket, do take shortcuts and eliminate stuff.
About 10+ years ago, something called a TFI inginition, was a special sensor which had a high history of failure. The 'team' was asked to investigate, but the thing of note was what they found missing in the aftermarket parts, a resister (about $.02 worth) was eliminated, guess what that did? It prevented the sensor from shorting out the PCM, yes the PCM protection was eliminated in the after market sensor to save costs . . .

Since that decade, yea long time ago, i've only used OEM for PCM sensors. . .
I'm not saying 100% that whats wrong with yours and i hope i'm wrong. Just a guess.

I'd start by getting those codes read again if you can. . .isn't there two sensors, one for the temp gauge and one for the PCM?

No, this one has only one sensor that communicates to the temp guage and the PCM.

Forgot to bring home the meter today, so I'll either need to wait until tomorrow, or scrounge for my meter at home. Where the heck is that thing???

It's a good idea to carry a meter in your tool box. I keep one in my tool bag in my van. Here's a product review on it: http://www.explorerforum.com/reviewpost/showproduct.php/product/441/cat/all

Okay, brought the meter from home today (still not sure where I put MINE...).

122.2 kOhm on the new sensor, around 300 kOhm on the old sensor (I was holding it in my hand, and the resistance kept decreasing). It's 20 deg F (-6.5 deg C), according to the weather channel. The Dead Link Removed, does not show the resistance at this temperature, but extrapolating the resistance should give between 100-150 kOhms at 20 deg F. So it seems that the new sensor is in approximately the right range.

But wait, hold the phone! When I plug in the new sensor and turn the ignition to "on" (no starting the engine), the temperature gauge moves up to the middle of the normal operating range. So this cannot possibly be correct, as the engine has been sitting in the cold for three weeks (wife did not go bonkers; I worked out another solution for her...).

So what's the deal with this? There are no "engine running" sensors to cause a problem (i.e. MAF, as was suggested). So what am I to think? Is the replacement part good, but there is some other problem?

Is your coolant temp sensor located on the thermostat housing? On the '99 4.0 the temp sending unit is located behind the alternator bracket on some the others are located next to the CTS on the thermostat housing. I'm not positive on the '02 but are you sure the the gauge gets it's signal from the PCM? If their is no seperate sending unit then one thing to check is the pigtail for the CTS. Working in different shops over the years I have cleaned lots of MAF sensors and 90% of the time the only symtom is a lean mixture. I have never had one that wouldn't run because it was dirty, however it rarely gets below 20 deg F so it could be possible. You have more than one problem. I have seen more than 1 guy beat his head against the wall because he thought 2 problems had to be related to 1 sensor. I do completly understand your concern with the gauge and I don't want to sound like I think you're barking up the wrong tree, I believe you're on the right track with the PCM thinking the truck is warmed up so it's to lean for the cold weather you're having. I wish I could help you more but I'm not familiar with that new of explorer. I will do some research and if I get a chance before you get it figured out I will go to the dealer I used to work at and go onto ALL DATA and see if there are any TSBs. The one thing you need is a good scanner that not only will scan codes but check all the parameters and values so you can tell which sensor is haywire, but unless you know someone with one it'll be a shop deal. I know this has zero to do with the gauge but what do you think about a intake air temp sensor. In my years as a mechanic I have seen alot of sensors that were faulty from parts stores, once we got 3 in a row and that will really screw with you but you are doing everything right in my opinion, you are only replacing what you have confirmed bad and not just throwing parts at it so that says a ton about your ability as a mechanic. Sorry I couldn't be of more help to you.

Well, it makes me mad that this is going to wind up being a dealership fix. I just don't have the resources to keep checking things without knowing what I'm looking for. The last straw was that the vehicle died 5 feet from my driveway last night. (I'm sure it's not related to the temperature sensor, but it would sure be nice to know that that system was working before having to troubleshoot something else.) Please chime in if you have any thoughts, but I'll probably tow the truck to the dealer in the morning.

Thanks for the responses so far.

I'm having similar issues with the temperature gauge reading cold.
Code P0125
Anything to report after you took your vehicle to the dealer?
Thanks

Help

I NEED HELP FINDING THE COOOLANT temp sensor on a 03 V8 4.6 explorer. It is nowhere in sight and i have been searching the internet for hours... .someone have any info on that? please let me know....

http://www.explorerforum.com/forums/showthread.php?t=259795

The above linked thread has a few pictures that helped identify mine.

Looking at my Chilton -- try looking for Cylinder Head Temperature (CHT) sensor -- mounted on the left head in the valley between the heads. I've never dealt with it, just looking at my Chilton.

Apparently, for the V8, it's called the CHT sensor. For the V6, it's called the Engine Coolant Temperature sensor

fordysenior said:

... try looking for Cylinder Head Temperature (CHT) sensor...

Apparently, for the V8, it's called the CHT sensor. For the V6, it's called the Engine Coolant Temperature sensor

Click to expand...

In the Navy, the CHT is a much different animal, LOL!!! And a very smelly one.

I recently resolved an issue with poor starting and high idle speed on a 1996 Ford Explorer in the UK. The initial issue was that the temperature gauge read half way even when the engine was cold, so after changing the temperature sender and (whilst I was at it) sensor which sends info to the engine management, I had poor starting and high idle.

Looking on the forums here it appeared that one sensor should have a single wire and one two wires. However, both of mine had two cables. I tested both earth side cables for continuity back to the battery, no problems. I grounded out the temperature sensor cable and the temperature gauge went high, so the gauge was working. Although not likely to be failed parts, I bought two more branded Intermotor sensors 55530 and 55508. It seems that there are quite a few variants of these sensors (even though they show compatibility) and so the new temperature sender was an M12 thread, so did not fit into the standard hole on the thermostat housing.

To get to the thermostat housing the intake manifold needs to be removed. Once this 1/2 hour job is done, with a bit of wiggling around the thermostat housing can be removed. On my 1996 4.0 SOHC V6, the thermostat housing is a plastic body. The temperature sender has a back nut and rubber seal but this turned freely in the thermostat housing so the temperature sender could not have been removed without removing the thermostat housing completely.

In the end, I put a known good sensor in the original position with back nut to seal it and then drilled and tapped an M12 1.25. Before reassembly I wanted to get the temperature sensor back to the temperature gauge working properly. I boiled some water and put the sensor in the hot water and turned the ignition on to power up. No joy. I tried the other sensor, no joy. Thinking about other posts on this forum I had read that using PTFE tape to seal the sensors can stop them grounding and hence stop them working, although mine was a plastic bodies thermostat housing so this could not be relevant, I got a crocodile clip and grounded the sensor back to the battery negative - immediately the gauge comes to life and then the penny dropped. If the sensor needs grounding, perhaps the sensor back to the ECU does too. Although a little Heath Robinson, I used a multicore cable and some spade ends and cable tied them to the sensors, running this back to the battery negative.

Before fixing this, each morning I would have to turn over the car two or three times before it would start, then it would cough, splutter, backfire and then go to 1000rpm, rising to 1500rpm until warm. On a short journey without giving the car time to warm up, the car would again struggle to start.

This morning, I tried a cold start and it fires into life, settling to idle around 500rpm.

I hope this post helps someone else out there with similar issues!