Averaging 5 mpg

Tahdig_siah

Tahdig_siah

TJ Enthusiast
Joined
Apr 22, 2023
Messages
130
Location
Ohio
This is ridiculous. I am averaging 5mpg as a daily driver. Noticed it for a few weeks.

2000 TJ 2.5L Manual
No lift and (same size tires as they have been on for many years).
New spark plugs and distributor
New exhaust manifold
New Downstream O2 sensor
New air filter and tires are all balanced and up to the recommended PSI.
I have run seafoam twice in the gas tank since last month as maintenance and to clean any possible injector clogs.
The most recent thing I have done to the jeep was changing out the U-joints on the front driveshaft.
No transmission slippage either, I replaced the fluid myself.

Help me with this issue please! Is it worn piston rings? did running seafoam (recommended amount based on the instructions on the bottle) mess with something?
 
You replaced the wrong O2 sensor, only the Upstream O2 sensor affects the air-fuel mixture and thus affects the engine power and mpg. The Downstream O2 sensor only monitors the catalytic converter and has no effect on mpg or engine power. And make sure you're only installing NTK, NGK, or Mopar O2 sensors. Avoid Bosch and avoid store brand O2 sensors.

A bad Upstream O2 sensor can erroneously report the air fuel mixture is too lean which will cause the PCM to richen the mixture. When the O2 sensor doesn't respond correctly and keeps saying it's too lean eventually the PCM will set the air fuel mixture to full rich which will destroy the mpg.
 
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Jerry Bransford said:
You replaced the wrong O2 sensor, only the Upstream O2 sensor affects the air-fuel mixture and thus affects the engine power and mpg. The Downstream O2 sensor only monitors the catalytic converter and has no effect on mpg or engine power. And make sure you're only installing NTK, NGK, or Mopar O2 sensors. Avoid Bosch and avoid store brand O2 sensors.
Click to expand...
I replaced the rear one because the code was for the downstream O2 sensor. I did that a few months back. Check engine hasn’t been on and no codes are pending. I used the recommended sensor (NGK) on this forum.
 
Hooked up OBDII just now. No stored or pending codes.
Here's live data I captured on idle:
IMG_5432.jpeg


IMG_5434.jpeg


IMG_5433.jpeg
 
Your coolant temp is low. Is it fully warmed up during this capture? If not, what does it show after fully warmed up?
 
a little digging around, found out that 0.9v or higher indicated a bad sensor. S2 is a downstream sensor. its bad again!? i replaced it with NGK a couple of months ago.
 
Tahdig_siah said:
A tad over the middle mark
Click to expand...

Sounds like it's running hot enough, but what is the actual ECT value after a full warmup?

I agree with @Jerry Bransford to replace the upstream O2 regardless. Even without a code thrown, they're wear items that should be replaced periodically (every ~75-100K). I would also recommend resetting the PCM after replacing any FI sensor so it can relearn fuel maps for your system.

Are you certain that you are calculating MPG correctly? Have you installed a new speedo gear to match a non-stock tire diameter? If not, use a GPS to calculate distance instead of the odometer against gallons pumped into the tank.
 
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Tahdig_siah said:
I replaced the rear one because the code was for the downstream O2 sensor. I did that a few months back. Check engine hasn’t been on and no codes are pending. I used the recommended sensor (NGK) on this forum.
Click to expand...
The Upstream O2 sensor can be bad without it generating a code. I'd change it as that sensor is commonly the cause of bad mpg due to its ability to cause the PCM to set the fuel mixture full rich.

The PCM is not very smart when it comes to generating O2 sensor codes. It can say they're bad when they're not and it can say nothing when there's a bad O2 sensor.
 
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Tahdig_siah said:
I replaced the rear one because the code was for the downstream O2 sensor. I did that a few months back. Check engine hasn’t been on and no codes are pending. I used the recommended sensor (NGK) on this forum.
Click to expand...

The smart thing to do would be to replace them all at once. Even if they aren't all throwing codes you should generally replace them all around 80k miles or so as part of a preventative maintenance plan.
 
Jerry Bransford said:
The Upstream O2 sensor can be bad without it generating a code. I'd change it as that sensor is commonly the cause of bad mpg due to its ability to cause the PCM to set the fuel mixture full rich.

The PCM is not very smart when it comes to generating O2 sensor codes. It can say they're bad when they're not and it can say nothing when there's a bad O2 sensor.
Click to expand...

Thank you Jerry. I will order a new upstream O2 sensor. I was out of town for 2 weeks, and completely disconnected both battery cables. I keep taking the negative black cable off on weeks that I am out of town so as to not drain the battery. Maybe removing the cable(s) causes PCM to reset, therefore, it takes a while for it to "learn"?
 
Tahdig_siah said:
Hooked up OBDII just now. No stored or pending codes.
Here's live data I captured on idle:
View attachment 453866

View attachment 453867

View attachment 453868
Click to expand...

The o2 reading will not matter you are in open loop the engine is running off set fuel tables not the o2 sensors. Check coolant temp sensor or cooling system to get it up to operating temperature. Once you verify it is going into closed loop then check the map sensor and associated hoses and look for any engine vacuum leaks. Then check o2 sensors. A good check for vacuum leaks is spray some brake or carb cleaner while watching your LFT when it change you are getting close.
 
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