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2002 Engine Coolant Temperature Sensor Problems

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Old 01-03-2010, 05:22 PM   #1
vaprtrl
Akron, OH
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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 chart 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
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Old 01-03-2010, 06:39 PM   #2
miker104
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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!
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Old 01-03-2010, 06:47 PM   #3
BrooklynBay
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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?
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Old 01-03-2010, 08:24 PM   #4
vaprtrl
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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.
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Old 01-03-2010, 08:27 PM   #5
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A dirty MAF sensor could give the PCM the wrong reading, and create problems.
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Old 01-04-2010, 02:50 PM   #6
jrford
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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?




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Old 01-04-2010, 08:31 PM   #7
vaprtrl
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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???
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Old 01-05-2010, 09:19 AM   #8
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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/reviewp...ct/441/cat/all
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Old 02-01-2010, 08:16 PM   #9
vaprtrl
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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 ECT resistance chart, 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?
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Old 02-01-2010, 10:10 PM   #10
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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.
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Old 02-11-2010, 06:16 PM   #11
vaprtrl
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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.
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Old 12-13-2013, 05:17 PM   #12
mjamesb80
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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
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