[RowJ]

Been frustrated since Thanksgiving (colder weather) with very rough initial idle and hard starts, and occasional NO start.
Posting what I discovered because it has not shown up as possible problem area on any of my searches.
Problem was........
-Very rough Idle for 30-60 sec, if it caught
-When it wouldn't catch, it flooded easily
-No codes or CEL
Checked voltage to all sensors! MAP sensor was weak and replaced. Started easier for about a week (also warmer weather).

Finaly got a CEL on a particularly hard start.
Light went out on next startup but I went to friends auto repair shop and borrowed his fancy 'Snap On' code reader.
It showed defective Coolant Temperature Sensor!! Replaced sensor and TJ starts like new. Catches quickly and idles smoothly!!

RJ

PS: Did some research and I found an obscure internet reference on these sensors, from an pro auto mechanic!
He said they read coolant temps down to 30* below and read low when malfunctioning. The result is they tend to cause computer to pump more fuel than needed causing rich condition. Anyone know if this info is accurate?
All I know is....new sensor solved my problem!

 

[Jonny Jeep]

The reading low temp makes sense as when most resistors fail they increase in resistance or go open circuit if they fail completely. The lower the temperature a sensor is at, the higher the resistance. A failed sensor would show a high resistance making the computer think the engine is extremely cold.

 

[LJameson]

Yep, it sounds right to me. Did you by any chance notice your Jeep running cooler?

 

[RowJ]

No....not when it would start. Maxed out just below 210*.

RJ

 

[RowJ]

Also worth noting the Dealer Sensor did not fit properly. Guide bars on the outside of the sensor would not line up properly with the grooves on the plug.
Had to go to NAPA and get their more universal sensor...... no guide bars on it.

RJ

 

[TomZ]

Thanks for posting this solution.

 

[Jason D]

What code do you get on the reader?

 

[Jmerante]

had same problem i think..

 

[RowJ]

What code do you get on the reader?

Sorry....did not notice the number!

RJ

 

[Cjensenr]

Great information. I too, just got done trace a problem with mine. It turned out to be the temp sender also.

C.