2.4L reg cab Tacoma getting awful MPG

I just took my 98 Tacoma on a 300 mile drive to and from East Texas this weekend after getting new tires, an alignment, changing the oil and ATF. On this trip, which was on pretty level land while the truck was practically empty, I got 18 MPG . This is well below what it's rated for on the highway.

Where do I start here? I have an occasional P0420 code for the O2 sensor that gets thrown, but it didn't ever come on during this trip. Other than that, there are no issues that I know of that would be contributing to this awful mileage.

 

Did your gas mileage get worse after you replaced the tires?

 

Plugs look good. I've already cleaned the MAF when I first got it. I'll replace the 02 sensor and see what happens.

Tires are slightly taller (215 75R14 instead of 215 70R14), but that should increase the mileage slightly if anything. More circumference= less motor revolutions, right? Of course on a lifted truck with much larger tires, weight and wind resistance becomes an issue

 

This depends on how you are measuring mileage. If you are using your odometer, the apparent mileage goes down when you get larger tires.

When I switched from 225/75/15s to 31/10.5s, I checked by GPS and the odometer was off by 14.5% afterward.

So if I drive 250 odometer miles on 15 gallons of gas, it looks like I'm getting 250/15 = 16.67 mpg.

But in reality, 250 miles on my odometer is 250 + 14.5% = 250 + 36.25 = 286.25 miles. That means I'm actually getting 286.25/15 = 19 mpg.

 

As for your mileage problem, replace your upstream o2 sensor with a new Denso sensor, and I'll bet it will help. Also make sure your throttle body is clean.

 

Is the upstream sensor Bank 1? That's the one that's throwing a code, though it hasn't come on in about 2 weeks. Throttle body has been cleaned.

I'll check my mileage via GPS next time I go a long distance and see what happens. These tires are barely larger, so I don't expect much of a difference, but now I'm curious.

 

Your motor will only have one bank (4 cylinder with only one exhaust), and it's the sensor upstream of the catalytic converter. It can be nasty dirty or old and not throw a code, but still not read well enough for the ECU to give you the best air/fuel mix to maximize your mileage.

The P0420 can also get thrown when you have an exhaust leak upstream of the o2 sensor - a cracked exhaust manifold or a leak at the junction between the manifold and the Y-pipe can cause this; both conditions are very common on these trucks. If the upstream o2 sensor reads too much o2 in the exhaust because outside air is getting in, the ECU injects more fuel to try to get the best burn. That causes a rich condition, and lower mileage.

 

What was your average speed on the trip?

 

This is totally possible. My exhaust seems a little louder than it ought to be, though I do like its tone as it is. I'll take a close look at this area and see if I can find a leak.

 

My average speed was probably 65

 

So I've replaced the pre-cat O2 sensor with a Bosch (b/c Denso doesn't make one that fits my truck; stock exhaust as far as I know; this was covered in another thread of mine). The truck ran noticeably better, pulling stronger and smoother than before. However, about 2 weeks later, P0420 came on again and I'm still getting about 18mpg.

I don't believe that the tires are the issue because they're barely larger than the previous ones (and they were mismatched sh!t anyway).

Also, on further thought, I don't quite get the logic to what MrRiverMan said about an exhaust leak causing this. The exhaust is a pressurized line so how would outside air get in? I could see this being an issue at the intake manifold side, but I've been unable to detect a vacuum leak (despite the presence of a high-pitched whine clearly coming from the throttle body area, which I've also cleaned thoroughly). Correct me if I'm wrong here.

 

The exhaust gasses leak out, not clean air leaking in.

P0420 is thrown when the catalytic doesn't appear to be functioning in the normal bounds that it is supposed to function in based on the conditions of the vehicle. (That statement sounds redundant but work with me here.)

So the truck knows how much air is going into the cylinders... what goes in must go out, amirite? Based on what goes in, how much fuel is sprayed, it has a round-about idea of what the CO2 concentration should for the upstream O2 sensor.

When the upstream sensor is in spec, the truck knows you're running right. So it knows how much of a CO2 concentration should hit your downstream O2 sensor.

When something goes wrong, the downstream concentration falls out of spec with regards to what the upstream sensor is detecting. If its too dirty, the catalytic is typically shot (thus not properly removing some of the smog). If its too clean then it knows something else is going on (a leak...). But there are several reasons a catalytic could fall out of spec, so the code doesn't say exactly what is going on.

It sounds to me like the truck has been acting up for some time and ended up shooting some extra unmonitored crap down your pipes. Since the front O2 sensor was lazy, you haven't noticed it an no codes have been thrown. Before you waste your time (and money) replacing the catalytic, go through and make sure the spark plugs are in good shape. If you have the patience, clean and check the fuel injectors (how many miles are on your taco?...) Run a can of seafoam through it, pour a can in the gas, and put some 91 octane in the tank... recipe for decarbonization. Make sure the taco is running in tip-top condition; then tackle the P0420. It could be that the engine is dirty and just needs to be burned clean.

 

Thanks for your input, rogue909, this makes sense.

I'm pretty sure now that I don't have an exhaust leak. I'm still getting a P0420 code and still having some power issues. Most pronounced when starting from a stop, but now I feel like it's lost a bit at speed, too. I think I'll try new plugs and wires next, maybe a fuel filter if I feel like getting into that, and see what happens.

 

actually, no, its opposite

bigger tires mean less bearing rotation but it takes more power and burns more gas to turn those bigger tires, smaller tires burn less gas because there is less power used to turn then but that raises rpms to go the same speed.

with my 5 lug 2.4 automatic with 14" tires I used to get 19/21 mpg and when I changed to the 4 hole upgraded injectors and a red permanent dry air filter (not k&n) and did the cut the 90 off inside the fender mod, it went up to 21/25 mpg.
The elbow mod is cutting off the back side of the 90 degree elbow inside the fender. it on the left side of the air box (if your standing at the front of the truck looking in)


with a prerunner or 4wd 2.4 if you see 18/20 mpg I think your doing good and if its lifted from there it often just gets worse.

replace your plugs, maybe replace the wires (use NGK), put in good permanent air filter, do the elbow mod, replace upstream o2 sensor, clean throttle body, clean MAF sensor, then switch to 4 hole upgraded injectors and reset EFI, check exhaust manifold for cracks in center and if its cracked replace with the stock oem 02-04 exhaust header instead of the cast iron one. that's about the best you can do to improve and get the best mpg unless you go back to stock tires.