Been investigating why the engine runs so rich, it’s been an interesting journey. Just to give you an idea of how rich it’s been running, since I bought the Jeep, I’ve put about 50 miles on it. Over that distance I’ve burned half a tank of gas, that’s 10 gallons. Time to fix that.

To start out, all the advice on Jeep forums was saying to check the engine codes. One does this by turning the key On-Off-On-Off-On and that’s the code for the computer to flash out the existing codes via the Check Engine Light (CEL). I did this and no light came on. Did some further reading and realized the CEL should come on at start and go away after 2-3 seconds. Mine never came on. So either I had a burned out CEL or none at all. I was already tearing into the dash to replace the dash lights with LEDs. I highly recommend this, it makes all the gauges more useable. While I was changing the ones out in the center console, I went ahead and checked on the CEL bulb. Turns out the holder for the bulb wasn’t even there. Not sure if the guy I bought it from did that or an owner prior to him. I was a little apprehensive about what I would find once I put a bulb in.

After putting a good bulb in, it’s a type 74, and replacing all the dash lights with LEDs, I put the dash back together. Side note: it might be necessary to clean the copper contacts on the bulb holders and the copper traces in the plastic sheet on the back of the gauge cluster. Turned the key and the CEL light lit up, and proceeded to check for engine codes. I was getting 12, 22 and 55. 12 is normal if the battery has been disconnected in the last 50 starts, 55 will always be there as that’s the end code to show they’re are no more codes to show. However, 22 is a code for the coolant temperature sensor.

Turns out that the 4.0 liter inline 6 that’s in my Jeep has two temperature sensing devices, a coolant temperature sensor (CTS) which tells the ECM (or whatever it’s called in this era of vehicle) the engine’s temperature and a temperature sending unit which provides information for the temperature gauge in the dash. Armed with the knowledge that I had a code for the CTS, I proceeded to find it and ultimately replace it. If you read my initial post about the Jeep you might remember that I mentioned a prior owner had installed an aftermarket temperature gauge under the dash. When I went to the location of where the CTS is located, I found the probe for that aftermarket gauge and not the CTS. I couldn’t even find the connector to tie the CTS to the wiring harness. I thought this was highly odd, so I started to pull back some of the cable shroud and sure enough, there was two cut wires that have the same color combo as what diagrams show to go into the connector for the sensor. I have no idea why someone would pull that sensor out, but that helps explain why the engine was so rich. The computer was effectively keeping the choke on the whole time the engine was running, because it thought it was below freezing.

Ordered a new sensor and connector, and proceeded to wire up the new connector. The sensor provides the computer a resistance reading, which means one wire is ground and the other goes to the ECM. I’m not sure if I got lucky or if the sensor is none directional with regards to the wiring, but it worked the first time I wired it up. This has made a world of difference. It runs better, it smells better and not the least of which has improved the appeal of the Jeep with the family. Unfortunately, who knows how long it’s been running so rich but that has issues of it’s own. I feared once I cleared this issue up, something down stream would pop up and now I’m getting an engine code (21 for those who want to know) for the Oxygen sensor. I’ve got one of those on order and we’ll see how that goes.