Bad Gas Mileage on Jeep GC 4.0

[AHarris]

Any ideas on why my wife's 2003 4. 0L Jeep GC goes from getting 17-18mpg to 12-13? Seems to run ok and the only CEL was an intermittent one for a Throttle Position Sensor which since went off by itself.



Some guy my wife works with said it was O2 sensors.



If the O2 sensors were bad enough to affect gas mileage, wouldn't the CEL be illuminated?



Any ideas appreicated.

 

[icman95]

I'm sure there are plenty of other reasons, but the O2 sensor could do it, I suppose. If it's giving a false reading of too much O2 then the computer will add fuel to richen up the air/fuel mixture and there you go-less mileage. Not sure about the CEL, though.

 

[Dieselnut59]

Has your station started selling ethanol blended gasoline? I have heard that can really hurt fuel economy.

 

[wcjp]

Time for Autozone. Check to see if the codes are still in the computer with one of their scanners. Is this GC equipped with full time 4wd? Could be dragging the front driveline. Fuel filter? Avoid Fram products.



On my 93' YJ 4. 0, I take the throttle body assembly off every once in a while for cleaning.

 

[BigPapa]

My wife's mileage on our '95 GC 4. 0 is down also. I have planned to replace the O2 sensors for a while, just never got that round tuit. We have noticed the ethanol blend gets significantly less mileage than straight gas as there are still a couple of places selling pure gasoline. I replaced the O2 sensor on a GMC 4. 3L V6 I had and it increased my mileage by 2-3mpg.

Scott

 

[AHarris]

Thanks

We'll try to change the O2 sensors and see what happens. Thanks for the info

 

[Matt400]

Any ideas appreicated.

A possibility and acording to Jeep service bulletin #09-003-03 If she does short trips (and or) the engine does not go above 3200 rpm there is a possibility carbon build up may be at fault.

Decarbonizing the combustion chamber & valves is the fix.



You might also want to pay attention to your temp gauge and make sure its at operating temperature. A stuck open thermostat can leave the engine management in open loop mode causing an over rich fuel condition.



A clogged converter will restrict exhaust flow and hurt fuel mileage.



I have replaced a few TPS on Jeeps so your previous fault code is suspect too.



Last is the 02 and before replacement I would check with a scanner to make sure its switching as it should.

 

[AHarris]

A possibility and acording to Jeep service bulletin #09-003-03 If she does short trips (and or) the engine does not go above 3200 rpm there is a possibility carbon build up may be at fault.

Decarbonizing the combustion chamber & valves is the fix.



You might also want to pay attention to your temp gauge and make sure its at operating temperature. A stuck open thermostat can leave the engine management in open loop mode causing an over rich fuel condition.



A clogged converter will restrict exhaust flow and hurt fuel mileage.



I have replaced a few TPS on Jeeps so your previous fault code is suspect too.



Last is the 02 and before replacement I would check with a scanner to make sure its switching as it should.

Good point--about 95% of her driving is short trips. I don't think she even warms up the engine driving from the house to work, which is1/2 mile away (she would walk but has to use the Jeep numerous times during the day for work). What is the best way to de-carbonize it?



I think the converter is ok--but I've already got a new TPS and am going to get it installed. Temp looks normal.



I don't think the RPMs on that thing have been over 3200 rpms since it was new.



Appreciate the tips

 

[PToombs]

I have a 96 and had the same mileage problem. Mechanic told me that O2 sensors can get slow, and won't trip a code. Scanner showed that they reacted fast, so I looked else where. The cat had also been rattling, and he said the muffler could be plugged. Well, that was it! I replaced the muffer, and gutted the kitty while I was there, and mileage went from 14 back up to 18-19.

 

[Matt400]

What is the best way to de-carbonize it?

From the Jeep dealer, pick up a can of #04318001AB "Mopar Combustion Chamber Cleaner" if there are no directions on the can I can fax you what to do but won't be back in my office till the 6th.

Posting it here would be a PITA as I would have to write allot with no way to just copy it in.

Its not hard though, basically an aerosol can you spray in the throttle body with the engine running but with a special 14 step procedure with 3 of the steps repeated 5 times.