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Unread 08-10-2014, 01:53 PM   #1
jordanj
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coolant sensor problem!

I have a 96 jeep cherokee 4.0L. It has been good starting and idling perfect only thing is that it shows its running hot. well i just had the radiator replaced and still is showing hot. so did a little research and replaced the coolant temp. sensor and ever since that, it has a hard time starting and it idles rough. and I can smell gas when i start it. any reason for it to start bad and idle rough after changing sensor???

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Unread 08-10-2014, 02:01 PM   #2
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no, can't think of any, BUT I would have simply checked/changed the Thermostat first.
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Unread 08-10-2014, 02:18 PM   #3
jordanj
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yeah thanks very helpful. still even If I change the thermostat still shouldn't make it start and idle rough.
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Unread 08-10-2014, 02:30 PM   #4
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check and double check the wiring and vacuum tubing
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Unread 08-11-2014, 05:48 AM   #5
SSellers
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Yes, the coolant temperature sensor is used for cranking enrichment and cold engine enrichment. If it is open circuited (or unplugged) it will go to basically -40 degrees and maximum fueling, or if otherwise too high in resistance cause injection pw to be too high for conditions. It should also throw a code if the PCM thinks the engine runs too cool for too long.

I had one once that wouldn't read below 60 degrees even in a cup of ice water, but that only caused long starts below 60 degrees and I just tuned around it before finally replacing it.
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Unread 08-12-2014, 06:06 PM   #6
jordanj
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thanks for for the reply. The reason I changed the sensor to begin with was the auxiliary fan wouldn't come on to cool the engine, so I changed the sensor and ever since it has hard time starting and idles rough. what should I do about that sensor put the old back in? or replace it with another new one?
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Unread 08-12-2014, 07:19 PM   #7
SSellers
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The auxiliary fan should not come on until 223 degrees unless the a/c is running. How hot was it getting and was it verified with an infrared gun?
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Unread 08-12-2014, 10:10 PM   #8
Paradise XJ
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Quote:
Originally Posted by jordanj View Post
thanks for for the reply. The reason I changed the sensor to begin with was the auxiliary fan wouldn't come on to cool the engine, so I changed the sensor and ever since it has hard time starting and idles rough. what should I do about that sensor put the old back in? or replace it with another new one?
Does the fan work now??
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Unread 08-12-2014, 10:19 PM   #9
1stockjeep
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Mine did same when my sensor was bad, either put in old one or get another new one. Check the pig tail for damage too
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Unread 08-13-2014, 03:53 AM   #10
JeepinInJackson
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If it is an obd2 equipped vehicle hook it to scanner and see what temp the scanner says compared to the dash. it may have two coolant sensors, one for engine management and one for dash gauge, now if you replaced the wrong one or damaged it on install it could cause issues. Your jeep is flooding because the coolant sensor is telling the computer that it's cold and needs more fuel I believe is how it works
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Unread 08-13-2014, 04:19 AM   #11
rajivkhandelwal
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Coolant sensor problems will manifest themselves during cold starts and during the warm up period, such as hard starting, erratic idle, smoke, stalling, etc, and will disappear once the engine has warmed up and the oxygen sensors have taken command, only to reappear again once the engine has cooled down.

Check the wiring before changing the sensor just in case.
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Unread 08-15-2014, 04:42 PM   #12
jordanj
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Yes the aux fan is working now. But runs at all times never shuts off. It threw a code for idle air valve so changed it but still starts rough but idles good now once it starts.
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Unread 08-15-2014, 05:10 PM   #13
jordanj
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Just put the old sensor back in and it starts great, but now the fan won't kick on at all unless the AC is on
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Unread 08-15-2014, 07:46 PM   #14
Paradise XJ
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Quote:
Originally Posted by SSellers View Post
The auxiliary fan should not come on until 223 degrees unless the a/c is running. How hot was it getting and was it verified with an infrared gun?
^^^This ... and one way to see if the fan itself is working and/or the sensor is at fault is to disconnect the sensor. If the fan turns on when the sensor is disconnected...the sensor is bad.

Quote:
Originally Posted by jordanj View Post
Yes the aux fan is working now. But runs at all times never shuts off. It threw a code for idle air valve so changed it but still starts rough but idles good now once it starts.
So it would seem that the "new" sensor is faulty. Until you get the engine up to temp...and OVER 220, that "old" sensor is not going to kick the fan on, unless the A/C compressor is running (which is how it's supposed to work).

Also, not sure if your 96 has the sender in the rear of the head (for the gauge). If so, different issues.
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Unread 08-15-2014, 10:41 PM   #15
calvin196
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I didn't know the temperature sensor could play that heavy of a role in how the engine ran. Anyway if you didn't buy a MOPAR temp sensor junk the current one and go head down to the nearest stealership and buy a mopar one. You shouldn't replace any sensors on a Jeep with anything but OEM (Mopar) parts. You will just have headaches and be wasting your money. It's worth spending the little extra for the parts required to do the job right..
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