Complicated MPG/Emissions issues

[JojoRoll]

Hello fellow 4Runner enthusiasts,

I am currently having a complicated MPG issue on my 2000 4Runner Limited 3.4 V6 4WD. According to my calculations it is getting 11.19 MPG WHICH IS HORRIBLE (Same as an H2 Hummer) I got that number yesterday on my last top-up. Here are some pictures of the fuel level and the mileage trip:

Here it is midway...

And here it is when I stopped at the gas station yesterday on Top-Up...

133.7 miles nearly empty

This is based on 90% - 95% City driving on my last tank. I have read here in the forums that people have been getting up to 250-300 MPG to the tank (roughly 17-20 mpgs) Now I do have a current problem that may have to do with it, I will make it as short as possible.

Last year in October 2014 I have changed the catalytic converter to a new aftermarket one because it was throwing the infamous CEL Code P0420 & emissions testing was coming up. The one I got is from Eastern Catalytic. This May 2015 it came back on again 6 months later so I am going to assume that it can't be the cat since I just changed it. Maybe it could be the O2 sensors causing the truck to run too rich? Could someone that has had a similar issue chime in and provide some input on this?

List of things I plan on doing maintenance in regards to the MPG problem:

• Change air filter DONE!
• Clean the MAF sensor or change it CHECKED
• Seafoam the engine DONE!
• Add fuel injector treatment
• Check the PCV valve, change if necessary
• Check for exhaust leaks
• Check the O2 Sensors, or change them both CHECKED
• LAST RESORT: Change the Cat again.

Anything I should add or missing?

 

[Raintrees]

could it be you looked at the wrong trip metter? one pic says trip A and the other is trip B.

 

[Raintrees]

I would also start with cheap simple thing like clean maf,airfilter ect before looking into changing the cat or 02 sensors.

 

[JojoRoll]

LOL

They are both the same, I reset both when I fuel up.

One other thing I should mention, this 4Runner seems to be the California emissions spec although I'm not 100% sure.

 

[Rocky5000100]

And before those/in addition to, disconnect the battery for 15 minutes to reset the ecu.

 

[TheDurk]

You need to check the codes ASAP. It is a waste of time to guess at it. There are a couple of codes that will fail-safe the ECU to open loop (mainly o2 sensor codes) and open loop will destroy fuel economy. Open loop is the warm-up mode where the ECM ignores the O2 sensor input and runs deliberately at a pre-set rich fuel mixture. Leaving it like this will also destroy your nice new cat. Presumably all the extra fuel you are using is passing through uncombusted and this a cat killer. Cats don't die except from physical damage or bad maintenance.

If you meant you got a new P0420, instead of just the CEL,(I can't tell the way you wrote it) it could still be the O2 sensors, or an exhaust leak which will mess up the ECM mix and timing settings.

 

[JojoRoll]

If you meant you got a new P0420, instead of just the CEL,(I can't tell the way you wrote it) it could still be the O2 sensors, or an exhaust leak which will mess up the ECM mix and timing settings.

The CEL came up again this month, with code P0420 (same as last time). I cleared the CEL with my OBDII Scanner and see when will it come up again. Now that you mention, I should add *Check for exhaust leaks to the list.

I just changed the air filter to a new aftermarket K&N filter, the truck still has a whole lot of miles to go so I figured why not? When I opened the package the filter was oily to the touch now I understand these things are supposed to be oiled but I didn't think it'd be this much. Anyways, it is now installed and will see how it affects the mileage.

Old filter

 

[satek]

Be careful with the oiled air filters! Sometimes they can pollute the MAF sensor and throw some codes.

 

[WallyWest]

Yeah, that's pretty bad. The mileage you have at below a quarter tank is what I'm at right now with the needle just below 3/4. That's entirely highway driving, but still.

I know some engine codes will also cause the engine to run rich, which is probably what you're seeing. I wouldn't drive it much until you get this fixed, it will destroy your cat.

My wild guess is a sensor or wiring fault causing the ECU to either get bad data or no data at all.

Also, I wouldn't use that oily K&N thing, they're notorious for fouling MAFs.

 

[sportscarfan]

So how many gallons did it take to fill with yesterday's top up?
My 4runner will throw the low fuel light when there is 4-5 gallons left (holds 18.x gal?). It has routinely made 18.5 MPG over the past 4.5 years I've had it, but mine is has a manual trans. Most of my driving is in/around the city as well.
Unless you go crazy oiling the K&N, I wouldn't worry too much about the MAF. I've had a K&N filter in mine since I bought it. I swap in a clean stock filter when I wash/dry/oil the K&N.

Your mileage is believable considering it's an automatic and nearly all city driving. Maybe you've got a heavy foot? There are a lot more losses with the automatic drivetrain and stop and go is a mileage killer. There are lots of other things you can check before changing the cat.
Have you checked the tire pressures?
Jack up the rear end and spin the rear tires - are the brakes dragging? Does the driveshaft spin freely?
Pull out the 02 sensor and look at it - is it fouled carbon black?
Longshot - maybe it's stuck in 4wd?
As theDurk alluded to, might be worth checking your coolant temp sensor - one thing that will keep the ECU in open loop...

 

[4-Ripcord]

that fault code is the O2 sensors, bad ones cause really bad gas mileage.

From the buyer's Guide:

O2 Sensor:
The most common reason people see really bad MPGs (<15mpg) is due to a faulty upstream O2 sensor. Most of the time it will throw a code if its bad, but sometimes it won't..
You can find your O2 sensor's part number here:
Oxygen / AFR Sensors - Underdog Racing Development

And get your part from amazon for the best prices.

 

[JojoRoll]

So how many gallons did it take to fill with yesterday's top up?
My 4runner will throw the low fuel light when there is 4-5 gallons left (holds 18.x gal?). It has routinely made 18.5 MPG over the past 4.5 years I've had it, but mine is has a manual trans. Most of my driving is in/around the city as well.
Unless you go crazy oiling the K&N, I wouldn't worry too much about the MAF. I've had a K&N filter in mine since I bought it. I swap in a clean stock filter when I wash/dry/oil the K&N.

Your mileage is believable considering it's an automatic and nearly all city driving. Maybe you've got a heavy foot? There are a lot more losses with the automatic drivetrain and stop and go is a mileage killer. There are lots of other things you can check before changing the cat.
Have you checked the tire pressures?
Jack up the rear end and spin the rear tires - are the brakes dragging? Does the driveshaft spin freely?
Pull out the 02 sensor and look at it - is it fouled carbon black?
Longshot - maybe it's stuck in 4wd?
As theDurk alluded to, might be worth checking your coolant temp sensor - one thing that will keep the ECU in open loop...

To answer your question sportscarfan, I retrieved the calculation I made on my calculator. See below:

I divided the total miles driven on the trip odometer per tank by the number of gallons I used to top up (all the way until the pump automatically stops) and the result is the MPG.

And I definitely won't be changing the cat right away, I still have to check all those things listed + your suggestions. I am going to do an engine oil change later and will check on what you mentioned.

that fault code is the O2 sensors, bad ones cause really bad gas mileage.

Then I am going to have to definitely check on those O2 sensors ASAP.

Today's progress so far:

Check engine light came on again with same code, no surprise there.

Added the SeaFoam treatment today, noticed a very slight improvement in acceleration or maybe that's just the Placebo effect, no way to know for sure without graphical data involved.

I'm gonna keep on updating as I go through the list, but I am going to jump straight to the O2 sensors next.

 

[JayRolla]

To me it sounds like o2 sensors. Car is stuck in open loop and running super rich. Do you get any smells from the exhaust, like sulfur build up?

 

[JojoRoll]

There are sometimes when it smells like rotten eggs. ainkiller:

 

[gamefreakgc]

Some aftermarket cats aren't up to snuff and they tend to not do a very good job. But definitely check on the state of the O2 sensors first since they are only $45 each opposed to several hundred dollars.

 

[4-Ripcord]

I don't understand what is so "complicated" about this.

You fault code says O2 sensors
All the symptoms say O2 sensors
Everyone posting says O2 sensors

You have fixed everything but the O2 sensors.. Just replace them and be done with it.

 

[JojoRoll]

Exactly, that's why I jumped straight to the O2 Sensors today 4-Ripcord, and yes we were all correct all along. I just found the culprit when I did the oil change tonight.

As it turns out the truck is Cali-Spec emissions so I looked up the diagram and it has only 1 O2 Sensor and a A/F Sensor. I checked both.

The resistance seems to be up to spec here for the O2 Sensor

I checked the A/F sensor and didn't get any resistance at all, not even continuity. Soaked it up in PB Blaster and will order the new part.

Also checked the MAF Sensor, nothing bad here:

I will keep you guys updated once I get that A/F Sensor and report back with the new MPGs.