[Bradleyp932]

I have a '99 JGCL. 188K mi. NOT modified. It has always sucked on mileage. ALWAYS a tank average of 16.3 mi/gal. 14 city/21 hwy. A couple weeks back, I took it to my guy for CE light. He determined it was O2. He replaced UP O2. The only other things he did was front brakes, rotors, inspection. From the moment I got it back I could tell something wasn't right. In the past, from a dead stop, My "Instant" mpg on accelerating was always 7-9 mpg at the absolute worst. Now? It is 2!!!! I've been through two tanks of gas now and my avg mpg is 12.8. My highway MPG is still 21 avg. But city driving is literally down to 5-7 mpg. Sitting at a light or even in park is sucking gas at a rapid rate. My Instant MPG doesn't get back above 12 until I am cruising and not applying any gas pressure at all.

Any ideas?

Please don't say the Overhead Console is unreliable and don't pay attention to it. I've owned this thing for 23 years and know every single nuance of it. And please don't begin giving me a laundry list of fuel filter, plugs, fuel injectors, air filter, PCV valve, etc. You failed to read the part about "from the instant I picked it up from having an UP O2 replaced" that this problem began happening. There is NO smell of gas or rotten eggs to indicate it is running rich. I've researched other people having the same issue and it keeps coming back to contaminated sensor, calibrating the ECM, and even the wrong manufacturer. Seriously? And yes, this was the original O2.

TIA

 

[DaingMaing]

Your '14 city' was trash to begin with. If this a bone stock GC, you have something else afoot there. I know. I own one.

I would highly suggest you consider a replacement OEM (read MOPAR) Crankshaft position sensor, and a MOPAR cam shaft position sensor. Especially if your approaching or over 200k miles.

Next, what brand was it that he put in there? Bosch sucks. So does just about everything else. NGK is the closest to MOPAR your gonna get aftermarket. Also, did he replace downstream as well? Usually a good idea to replace both at once. Also be sure to clear out your PCM after changing them. You do so by disconnecting ther PCM and touching the cable ends together(not hooked up the battery) for about a minute to de-energize the capacitors in the PCM. This will put it back to the base OEM fuel mapping. After that, it will take about 40 start/cool down cycles before the PCM learns the most efficient way to run that engine and injectors. So yes, seriously, your research was right.

Do or do not: that is up to you. My shitty 'round the town MPGs stopped and improved after the MOPAR Crankshaft position sensor. It ran soooo much smoother. I have been through about 5 differring brands of CKPS before then, and have had them come bad out of the box. Always go MOPAR on those two indicated.

In short, I'm betting the up O2 was something other than NGK or MOPAR(not even sure they sell em anymore), and wasn't replaced as a pair. Then the PCM was not cleared/reset. You may get better MPGs after about a month after the repair. The engine adapts to it's environment, and it takes about that long.

Just my 2 pennies from another grand survivor.

Good Luck and let us know how it turns out.

 

[warriorsuspensions]

I would think that if you were actually getting that kind of milage that your tail pipe would have nothing but black smoke coming out. How did you check your milage?

 

[Bradleyp932]

Thanks all for the responses. NO black smoke or residue out of the tailpipe. Messaging my guy now on what brand O2 and why he didn't replace both. I do remember replacing the CKPS back in '16 but not sure now why they determined it needed replacement. All I remember is them cutting a 4 in X 4 in hole the floorboard about 8 inches above the gas pedal. Then stuffing a mechanic rag in it to dampen the sound. LMAO

 

[DaingMaing]

I do remember replacing the CKPS back in '16 but not sure now why they determined it needed replacement. All I remember is them cutting a 4 in X 4 in hole the floorboard about 8 inches above the gas pedal. Then stuffing a mechanic rag in it to dampen the sound. LMAO

LOL.... 😆😂🤣

It is a PITA to get at, but with a ratchet with about 3 feet of extensions, a swivel extension, and a 6 point 11mm (yes 11mm) socket: the CKPS comes out pretty easily from under the vehicle. Them cutting a hole to replace it, should tell you all you need to know about your mechanic: run while you still can.

The CKPS is a hall effect sensor and the PCM allows a little bit of 'play' in the frequency of the pulses coming from the CKPS, and when the sensor is poorly made or is starting to go bad: the 'gaps' in the signal get longer or erratic, so to speak. At that point the PCM is all but guessing at the timing or it just plain won't run. The symptoms start out being shotty MPGs, but then moves on to timing symptoms like: hard starts, chugging/missing when the accelerator is pressed, and a few other things. All of these start occuring without throwing any trouble codes or check engine lights. Some times the issues with accerlation and power are mistakenly attributed by mechanics to the TPS and other timing assorted sensors. It can be quite the expensive dragon to chase.

I hope you figure it out, and she's back to getting good mpgs soon enough.

 

[Bradleyp932]

Thanks Daing. Actually, the CKPS was replaced by another shop in another city many states away. Based on your response you did remind me of something else. I have noticed a couple times since picking it up after the O2 replaced that she seems to be revving up a bit while driving without stepping on the gas. Or it seems that way. Hard to explain. Could this have something to do with the PCM searching as you described?

Something else I'm noticing is that cruising at 35 mph is giving 25-28 mpg now on the overhead and I have NEVER seen that before except downhill. But ANY acceleration whatsoever is under 10 mpg.

Bottom line, I'm seeing MPG increasing on the steady rides from 35-70 after the UP O2 replaced. But accelerating has destroyed City mileage to 5 mpg. Wouldn't it be wonderful if replacing the DOWN O2 would fix the hirrible City mpg? LOL

 

[warriorsuspensions]

One thing I have observed about the Jeep ECM is that they are kind of stupid and learn slowly

 

[DaingMaing]

^ X2