Bad MPG
- Thread starter NKlekas
- Start date
One of the best things you can do to understand your vehicle's performance is getting a OBD2 scanner so you can see what is happening while it's running.
Running a vehicle only requires fuel, air, spark, and compression to happen; but a scanner will give insight as to why it's not running optimally.
Until then, some things you can do:
Look at the air filter, is it clean or clogged? Replace if clogged. Clean the idle air controller (IAC). Disconnect the air intake off the top of the throttle, start the vehicle, and slowly spray throttle body cleaner into the throttle. Don't do a heavy spray hoping for faster results, just slow and even so that when the engine starts lugging you can stop spraying and allow it to recover. The spray gets sucked through the IAC and cleans as it goes. (Thanks @Jerry Bransford for this technique.)You can also clean the mass air flow (MAF) sensor with an appropriate spray. (This sensor tells the computer the amount of air passing by on its way to the intake, allowing it to begin fuel trim management. More on this later.)
Move on to the fuel system. Check injector function by disconnecting one while the Jeep idles. If the RPMs drop once disconnected then that injector is working. Reconnect it and go on to the next. If they all cause an RMP drop they are all, at least, working. Fuel pressure is also a consideration, but requires more specialized tools/hardware. (More on this later.)
Moving on to the electrical system. When were the spark plugs installed or last replaced? If you don't know the answer to either, disconnect and remove the coil pack, and then check each plug's condition and gap. Regap or replace, as needed. If all good, reassemble.
I'm not going to cover engine compression because it requires a more thorough engine breakdown and some specialized hardware.
Other simple things that may affect overall mileage:
Tire pressure. They may look good to the eye, but they may be low in pressure. Check with a gauge, refill as necessary.
Oil. Did you change to a heaver weight oil prior to the drop in MPGs? If so, try lower weight. It may make a difference.
Intake/Exhaust leaks. Check for holes in exhaust metal, gaps in exhaust connections, and disintegrating rubber connectors or splits in vacuum lines.Remember, the air going into the engine was measured by the MAF. If the rubber boot where the crank case ventilation (CCV) hose connects on top of the manifold is split, then it is allowing extra, unmeasured (or un-metered) air into the intake. (More on this later.)
Once you get (or borrow) a scanner you can find out a bit more without much hassle.
Oxygen Sensors: Obviously they read the exhaust gases and tell the computer if it is running the engine too lean or too rich. They have a 'normal' range of voltages. By looking at these voltages, you're better able to tell what's happening before and after the catalysts. If you've replaced the O2 sensors and their readings are still out of 'normal' range while operating then you know you have problems elsewhere.
These numbers may show you how the pre-catalyst O2 sensors sense a lean condition and are telling the computer to dump more fuel. This is where the cracked boot on the CCV connection can mess things up. The extra air it allowed to enter the manifold wasn't measuredat the MAF location, so the exhaust gasses will appear to as 'lean' to the pre-catalyst O2 sensors. As a result, the computer dumps more fuel to correct the mixture. In reality, this particular air/fuel mix is most likely now too rich. This will end up damaging the mini-catalysts. (A mixture that is too lean is also bad over the long term.) If the post-catalyst O2 sensors' voltages are always out of wack, meaning they never change, you may be needing new mini-catalysts. Been there.
When the media in the catalysts are damaged it may break up, and make its was down the exhaust. Ultimately this debris may clog the larger catalyst and reduce exhaust flow. If the mini-catalysts rattle while the vehicle is idling, the media inside is loose. A pretty good sign of some internal damage.
I also monitor my fuel mileages and began seeing a decreasing trend. I also had engine hesitation at acceleration and going up hills. So I thought it was simply fuel system related. I did all the things above and after a new fuel pump, new exhaust, and new CCV tube, have my O2 numbers back in the acceptable range. This may all sound and look like a lot but it's all manageable.
Once you've better identified your situation, you'll be able to start targeting the fix(es). You're on this forum and you can do this.
(Edited to strike references to MAF. I was thinking about the Intake Air Temperature, or IAT, sensor. On my 2005 4.0 its in the intake tube, above the throttle body. I understand it to have a similar function as a MAF, but it's not a termed 'MAF'.)
Running a vehicle only requires fuel, air, spark, and compression to happen; but a scanner will give insight as to why it's not running optimally.
Until then, some things you can do:
Look at the air filter, is it clean or clogged? Replace if clogged. Clean the idle air controller (IAC). Disconnect the air intake off the top of the throttle, start the vehicle, and slowly spray throttle body cleaner into the throttle. Don't do a heavy spray hoping for faster results, just slow and even so that when the engine starts lugging you can stop spraying and allow it to recover. The spray gets sucked through the IAC and cleans as it goes. (Thanks @Jerry Bransford for this technique.)
Move on to the fuel system. Check injector function by disconnecting one while the Jeep idles. If the RPMs drop once disconnected then that injector is working. Reconnect it and go on to the next. If they all cause an RMP drop they are all, at least, working. Fuel pressure is also a consideration, but requires more specialized tools/hardware. (More on this later.)
Moving on to the electrical system. When were the spark plugs installed or last replaced? If you don't know the answer to either, disconnect and remove the coil pack, and then check each plug's condition and gap. Regap or replace, as needed. If all good, reassemble.
I'm not going to cover engine compression because it requires a more thorough engine breakdown and some specialized hardware.
Other simple things that may affect overall mileage:
Tire pressure. They may look good to the eye, but they may be low in pressure. Check with a gauge, refill as necessary.
Oil. Did you change to a heaver weight oil prior to the drop in MPGs? If so, try lower weight. It may make a difference.
Intake/Exhaust leaks. Check for holes in exhaust metal, gaps in exhaust connections, and disintegrating rubber connectors or splits in vacuum lines.
Once you get (or borrow) a scanner you can find out a bit more without much hassle.
Oxygen Sensors: Obviously they read the exhaust gases and tell the computer if it is running the engine too lean or too rich. They have a 'normal' range of voltages. By looking at these voltages, you're better able to tell what's happening before and after the catalysts. If you've replaced the O2 sensors and their readings are still out of 'normal' range while operating then you know you have problems elsewhere.
These numbers may show you how the pre-catalyst O2 sensors sense a lean condition and are telling the computer to dump more fuel. This is where the cracked boot on the CCV connection can mess things up. The extra air it allowed to enter the manifold wasn't measured
When the media in the catalysts are damaged it may break up, and make its was down the exhaust. Ultimately this debris may clog the larger catalyst and reduce exhaust flow. If the mini-catalysts rattle while the vehicle is idling, the media inside is loose. A pretty good sign of some internal damage.
I also monitor my fuel mileages and began seeing a decreasing trend. I also had engine hesitation at acceleration and going up hills. So I thought it was simply fuel system related. I did all the things above and after a new fuel pump, new exhaust, and new CCV tube, have my O2 numbers back in the acceptable range. This may all sound and look like a lot but it's all manageable.
Once you've better identified your situation, you'll be able to start targeting the fix(es). You're on this forum and you can do this.
(Edited to strike references to MAF. I was thinking about the Intake Air Temperature, or IAT, sensor. On my 2005 4.0 its in the intake tube, above the throttle body. I understand it to have a similar function as a MAF, but it's not a termed 'MAF'.)
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Certainly not advocating to use synthetic for break in, but I can only say my experience with synthetic has been nothing but positive for decades. I have synthetic in 5 of my current vehicles spanning back to 1964. Now my driving does not include things like rock gardens though.
Funny I can say the same thing about my use of conventional lubricants since the late 60's. Never an engine problem. I only run synthetic transmission lubes and power steering fluids. 
Well, if it makes you feel any better, at this point you’re just out the cost of the new O2 sensors.
I spent 35 grand on a new engine to get better mileage, and I still get 10…
Good luck getting it sorted. The good news is that there is something causing the bad mileage - you just need to find it. Lot’s of geat advice you’ve gotten here so far.
Did you notice the decline in fuel mileage before or after you installed the Garvin rack?
O2 sensor code usually means it's detecting something wrong, not that the sensors are bad.
Well, if it makes you feel any better, at this point you’re just out the cost of the new O2 sensors.
I spent 35 grand on a new engine to get better mileage, and I still get 10…
Good luck getting it sorted. The good news is that there is something causing the bad mileage - you just need to find it. Lot’s of geat advice you’ve gotten here so far.
Lol that's tough man! For sure, lot of great replies and help on here. I really Appreciate it.
I actually just got rid of my rack...That was my last-ditch effort to help get the MPG back...Stayed the same. Now, it's only been one tank and it did go up slightly...but not much. Maybe it will keep getting better with more tanks.
The good ones! The ones from the 02 Sensor threads, the NKT's. The code it was throwing was only the 2 upstream ones but I went ahead and replaced all 4. That certainly helped. When they were bad and throwing the code, I was getting like 7-7.5 MPG. I'm getting closer to 9-9.5 now but still bad.
Definitely bought the good 02 sensors, the NTK's. I think the catalytic converter is my next guess. It's not throwing any other sensor codes and all my filters are just fine. Im at such a loss because before my 02 sensors went bad, I was getting pretty solid numbers, 13-15 MPG. I replaced them, did regular maintenance, and regeared it... and it's worse. My Jeep runs great, not throwing any codes and my MPG is pretty much at 9-9.5. I even brought it into a shop to look at things more in-depth and professionally. Things like; filters, seals, sensors fluid levels etc. The shop couldn't find anything wrong. Said the jeep looked and ran great. Like I said, I think ill try and look at my Cat Converter next.
I glad you got a CEL to lead you in the proper direction. Does Nevada use a summer blend of fuel? Some of these blends can reduce mileage.
LJMolina
TJ Enthusiast
My 06 Jeep is in the same boat...getting terrible MPG, replaced all o2 sensors and replaced the cats with the flowmaster kit and still get about 9-10mpg. On the highway its has a slow response and sluggish at best....and when going over slight inclines on the highway shes slows down like something is holding her back...pressing down on the gas pedal to accelerate 9/10 has no response until I floor it.
Im at a loss.