Average 8 mpg, need suggested fixes

[Clarythedrill]

To all, I have a 2001 TJ with the 3 speed auto and the 4.0 engine. I have put a 3 inch suspension lift on it, and it has 31x10.5 all terrain tires on it. I also have an AEV front bumper and rear bumper, and rocker guards. I know they add some weight, but over stock, not sure how much.

I currently get about 8-9 miles per gallon, regardless of whether I am on the road or in town. When I bought it, I replaced all fluids, put a new air filter in, IAC valve, new (and correct) oxygen sensors (up and downstream), and replaced all sparkplugs with new (and correct) ones. No, I do not drive like an idiot, I take it easy when stopping and taking off. Regardless, I am stuck at 8 or so MPGs.

I have noticed that the temp guage moves and does not stay steady at 195 like it should. If I turn the heater on, the guage will go down a good bit. So, I am not sure if it has a thermostat in it or not. Could the fact that it does not reach and maintain proper temp affect the mixture and cause it to run too rich and get the bad MPGs? I have a new 195 degree thermostat and new housing ready to be put on, but I dont want to waste my time if that is not a cause. I love the fact that it runs so cool, and living in Texas now, that will come in handy in the coming months.

So, any ideas what I might need to do to raise the MPGs? I feel I should be getting 12-13 max with my setup. Also, it got these bad MPGs before I did the lift and bumpers and sliders mod. Thank you.

 

[Chris]

When was the last time you changed the O2 sensors?

Are you aware they should be changed around every 80k miles (all of them)? They can affect MPGs dramatically.

See this thread to make sure you get the right O2 sensors, or you'll regret it: The Official Jeep Wrangler TJ Oxygen (O2) Sensor Thread

 

[Clarythedrill]

Chris, I changed them about three months ago. I followed your linked thread to ensure I got the right ones.

 

[xxdabroxx]

Unplug 2 fuel injectors. Instant 33% more MPG.


Your temp gauge should be at about the 210 mark. If it's under that it may have too low of a thermostat so that would be a good place to start.

Have you checked your speedometer with a gps? If you are going faster than the speedometer indicates you are getting more MPG than calculated.

 

[Clarythedrill]

Unplug 2 fuel injectors. Instant 33% more MPG.

Haha, I have had a TJ in the past with a 4-banger.........no thank you.

 

[DWR]

Myself along with other members have noticed that we get better gas mileage when using “name-brand” fuels. I try to always use shell myself, but I always try to use brands that are listed as a top-tier member.

Also, yes, the temperature has a lot to do with how well the fuel is burned/atomized and can also effect MPGs... definitely check your thermostat.

Here is the list of top-tier brands: https://toptiergas.com/licensed-brands/
Edited for grammar and the link.

 

[RMETeeJay]

Thermostat could definitely cause it. Before you replace, use a real-time OBD reader to determine what temperature the computer sees. Should be a little above 210 (mine stays around 213). I use a cheap bluetooth OBD dongle with Torque Pro.

 

[Feralkid]

Cats may be restricting exhaust flow.

I'd start with getting the temp stable at 210.

 

[Clarythedrill]

Thermostat could definitely cause it. Before you replace, use a real-time OBD reader to determine what temperature the computer sees. Should be a little above 210 (mine stays around 213). I use a cheap bluetooth OBD dongle with Torque Pro.

The temp pretty much starts climbing as soon as I start the engine, and never gets over 190 or so. I am fairly positive it has no thermostat.

I think I will go ahead and put the new thermostat and housing on this weekend....would not hurt one way or the other.

 

[RMETeeJay]

That would definitely cause your computer to assume a rich condition. This is detrimental to your engine long-term because it's dumping extra fuel all the time. Don't delay getting the new thermostat in and I'd change the oil soon too.

 

[Jerry Bransford]

Exactly how are you calculating your mpg? Are you going by the gas gauge for gallons consumed or by the number of gallons it takes to refill the tank divided into the miles driven since the last fillup?

 

[CharlesHS]

A few things no body has touched on...
You are running 31x10.5 tires when the originals were around 215/75.
Has the speedometer gear been replaced to calibrate the differential gearing to the tire size ?
Not sure if your 4.0 is running the original 1 hole fuel injectors, but I upgraded the fuel injectors in my 4.0L to the K Suspension 12 hole fuel injectors which not only cured the cylinder 3 heat soak problem, but also increased my gas mileage by 2 mpg.
Granted my Jeep is not built up, but I am getting 18-20 mpg on the highway and it is running 255/75x17 AT tires.
Yes..... I have calibrated my speedometer to the differential gearing and tire size.

 

[Rubicon John]

You absolutely have to get it running at the correct temperature. Running cool will richen your fuel mixture in theory. I would definitely pursue the thermostat. Mine sits right around 210 all the time.

The other thing you didn’t mention is how many miles your engine has and if you know what type of life it has had. If you have a good amount of miles on it, I would be willing to bet there is a good amount of carbon built up in your top end which would be causing pre detonation and causing excess fuel consumption.

It wouldn’t hurt to use some Mopar Combustion Chamber Cleaner through the intake and to also soak on top of the pistons to try and remove carbon deposits.

I know @Jerry Bransford also did a good write up on using water to clean carbon a while back.

 

[pagrey]

The temp pretty much starts climbing as soon as I start the engine, and never gets over 190 or so. I am fairly positive it has no thermostat.

I think I will go ahead and put the new thermostat and housing on this weekend....would not hurt one way or the other.

Your Jeep has a thermostat, it would run cold and you would get a check engine light without one.

Before you spend money read the above post written by @Jerry Bransford.

You might look into a cheap scanner like an ELM327, it can help to troubleshoot these issues. Especially looking at fuel trims and o2 sensor outputs.

 

[Painfultruth]

Yes, evaluate your fuel trims and open/closed loop operations.

 

[BlueFl.Wrangler]

Your thermostat could very well likely be broken-- happens more than you might think.
I also agree with a previous poster on brands of fuel. I buy shell most of the time and get about 2-3 mpg better than the cheeeep o's

 

[reddvltj]

So, I am not sure if it has a thermostat in it or not. Could the fact that it does not reach and maintain proper temp affect the mixture and cause it to run too rich...
I love the fact that it runs so cool, and living in Texas now, that will come in handy in the coming months.

1st thing is yes running under the specified operating temp your engine will stay in open loop and ignore the o2 sensors and run the engine rich (warm up mode)
2nd If in fact it does not have a thermostat inside the housing when summer time temps start creeping up there will not be enough restriction in the cooling system to allow the coolant running through the radiator to cool so you would end up over heating. A more possible issue would be that the thermostat is stuck open. still providing restriction but not allowing the engine to warm up completely.

 

[Clarythedrill]

Update: I put the new thermostat and housing on, but have not driven it yet to see how it reacts. I also changed the oil just for fun. Nothing looked wrong with the old thermostat or sensor, but there could have still been an issue with it.....so cheap insurance.

To answer a couple of questions, I figure mileage by dividing gallons put into the tank by miles driven. I dont really care about the tires being bigger, that should just be a bonus when compared to stock size tires. On paper, I should get 12-13 regardless when hand figured.

The jeep has 160k miles on it. I have owned it since about 150k. I have no idea about whether there are deposits on the pistons, I will look into the water trick if this latest change doesnt get the MPGs up. I have also ordered a scanner that lets you monitor perimeters, and it should be in Friday. I may also pick up a new temp sensor when I am in town today just to rule that out also.

As for gas, I only use name brands such as exxon, shell, BP, Valero and the like. As for one suggestion about changing the injectors, I am not sure that is something I want to take on, but is intriguing for sure.

Thanks for all the suggestions and advice, it is appreciated.

 

[BlueFl.Wrangler]

I am 99.9 % sure changing the injectors is fairly simple
The guy that runs the forum ( chris I think ) was suppose to post a how to on it

 

[TJ4Jim]

Update: I put the new thermostat and housing on, but have not driven it yet to see how it reacts. I also changed the oil just for fun. Nothing looked wrong with the old thermostat or sensor, but there could have still been an issue with it.....so cheap insurance.

To answer a couple of questions, I figure mileage by dividing gallons put into the tank by miles driven. I dont really care about the tires being bigger, that should just be a bonus when compared to stock size tires. On paper, I should get 12-13 regardless when hand figured.

The jeep has 160k miles on it. I have owned it since about 150k. I have no idea about whether there are deposits on the pistons, I will look into the water trick if this latest change doesnt get the MPGs up. I have also ordered a scanner that lets you monitor perimeters, and it should be in Friday. I may also pick up a new temp sensor when I am in town today just to rule that out also.

As for gas, I only use name brands such as exxon, shell, BP, Valero and the like. As for one suggestion about changing the injectors, I am not sure that is something I want to take on, but is intriguing for sure.

Thanks for all the suggestions and advice, it is appreciated.

The question was asked earlier but again did you correct your speedo for the tire size.