Tracking down issues

I have a 2003 V8 4WD with just over 200k miles on it. I'm having 2 issues that I'm hoping you guys can help with.

1st, no matter what I do, I'm only able to get 13mpg. This is manually counted, not via the vehicle computer. Its consistently 13mpg, regardless of city or highway driving mix.
Things I've checked/replaced:

• No stuck brakes/calipers (replaced rear calipers, pads, and rotors and checked fronts)
• replaced spark plugs with genuine Denso (iridium?) plugs

2nd issue - I have what sounds similar to diff whine, but only at ~60mph, and only occasionally. It doesn't seem to change volume or pitch with speed, and doesn't change when weight shifts from side to side, so I'm thinking not a wheel bearing. Could this be from the drive shaft needing to be greased? Something else?

Thanks guys!

 

Welcome

 

Welcome. With the understanding that AI sometimes pulls info right from its virtual a**, it can also be a good starting point. I pasted your mileage questions to Google Gemini and this is its response:

V8-Specific Primary Suspects (Highest Priority)

The V8 engine has four Air/Fuel Ratio (aka O2) sensors and two catalytic converters. When the fuel economy drops this severely, the problem is almost always in the fuel metering loop.

### 1. Sluggish Air/Fuel Ratio Sensors (Upstream)

* **The Problem:** The V8 uses two upstream Air/Fuel Ratio sensors. These sensors are critical for telling the Engine Control Unit (ECU) how much fuel to inject. After 15-20 years, they become **"sluggish"**—they respond too slowly. The ECU sees this slow response as the engine running lean and compensates by dumping extra fuel, trying to enrich the mixture.
* **Action:** Since there are no codes, simply cleaning the **Mass Airflow Sensor (MAF)**, located right after the air filter box, is the cheapest first step. However, the most effective solution is often to **replace both upstream A/F sensors** (Bank 1 Sensor 1 and Bank 2 Sensor 1). Use only **Denso** parts, as the ECU is very sensitive to aftermarket sensors.

### 2. Clogged Catalytic Converters

* **The Problem:** The 4.7L V8 is known for having the internal material of its catalytic converters degrade and sometimes break apart, leading to a severe restriction in exhaust flow. The consistency of the low MPG (13 MPG city and highway) is a classic sign of this. An engine with a restriction simply cannot breathe and has to work harder for every mile.
* **Action:** Have a mechanic measure the **exhaust back pressure**. This test is non-invasive and quickly confirms if the cats are clogged. If back pressure is too high, the converters must be replaced.

---

## ⚙️ Secondary Mechanical & Sensor Checks

If the MAF and upstream A/F sensors don't fix it, check these secondary areas:

* **Tire Size & Weight:**
* **Action:** Reconfirm the tire situation. The 4Runner community frequently adds heavy bumpers, roof racks, and large tires. These additions are the most common cause of sustained 14-15 MPG, but they could drop it to 13 MPG if the truck is heavily accessorized and running very aggressive tires.
* **Fuel Injectors:**
* **Action:** Have the **fuel pressure** checked and test the injectors. A partially stuck-open injector will cause the ECU to pull fuel, but if it is bad enough, it will run rich and waste a lot of gas.
* **Differential/Drivetrain Fluids:**
* **Action:** When was the last time the transmission and differential fluids were changed? Old, sludgy, or low fluid creates drag and resistance, forcing the engine to burn more fuel to maintain speed.
* **Fuel Cap/EVAP System:**
* **Action:** While usually accompanied by a Check Engine Light (CEL), a damaged or loose fuel cap can cause issues. Also, a rare fault in the **EVAP (Evaporative Emission Control) system** could cause a subtle fuel system error.

**Summary Action Plan for the 2003 V8:**

1. **Clean the MAF Sensor** (Easiest and cheapest first step).
2. **Replace both Upstream Denso A/F Sensors** (Highest probability fix for rich running).
3. **Check Exhaust Back Pressure** (The definitive test for clogged catalytic converters).
==

Hope that helps.

 

Interesting, thank you for that! I do not have any dash lights or codes (that I know of). I'm skeptical of the clogged cats because of no check engine lights as well as no sulfur smell. I cleaned the MAF when I replaced the air filter and spark plugs, but no improvement. I wonder if I'd get a code for bad air/fuel sensors...

 

I don't know about the V8, but the V6 won't indicate a code for the A/F sensors as long as the fuel trims are within +-10% on average. So no code if the sensor is functioning. But I guess it's like a bald tire, it's still rolling but the performance isn't close to when it was new. I just replaced my A/F sensors preventatively on my '07 Sport V6 at 278K miles, but it was just the other day so I don't have new MPG stats yet.

 

Greasing the drive shaft is super easy so you should do that anyways, and also see if it remedies the whine you are hearing. If not, no harm now you have a greased driveshaft.

 

For MPG how fast are you going highway?

Any mods, tires that are all-terrain or higher load rating than factory?

Fuelly has the 4th gen V8 (if I recall correctly) at around 14-15 MPG average so you’re not tooooo far off

 

Tell us a little more about your truck and the area where you live.

1) Do you have tires that are larger/heavier than stock?
2) Lift?
3) Any added weight like steel bumper(s), skid plates, RTT, etc...?
4) How fast do you usually drive on the highway?
5) Is your driving a mix of city/highway? Mostly highway? Mostly city?
6) Do you have a lot of elevation changes? Hills/Mountains - stuff like that?

 

I think my 04 v8 averages around 15-16mpgs unless it’s like a roadtrip with highway only miles.

 

Yeah, its for sure on my to-do list, but being a full-time 4wd, its difficult to rotate the drive shafts if the zerks aren't in the right position. I just haven't gotten around to lifting the whole vehicle yet.

 

My wife primarily drives the truck, but almost entirely city driving (she commutes ~5 miles/15 minutes, and then drives around town). The truck is almost fully stock. The previous owner did some cosmetic mods, but also has a roof basket and lightweight bull bar, as well as wheels off of a 3rd gen tacoma, maybe a TRD off-road?. The tires are the stock all-terrains from the Tacoma, and they are old but in great shape. I plan to replace them in 2026 just due to age.

The most interesting thing to me is that it seems to be consistent at 13 mpg, even if we drive entirely on the highway at 70-75mph.

 

1) no, stock size/weight tires roughly. The previous owner swapped on some 3rd-gen Tacoma wheels and tires, but I understand these to be the same size as stock wheels and tires (other than offset?)
2) no lift. as of now, I'm on the stock XREAS suspension from 2003. It's in surprisingly good shape for its age, but I intend to replace it with Bilsteins.
3) no extra weight - I've got a roof basket and a very lightweight bull bar, but all the skids are stock.
4) highway speeds around here are usually 70-75 mph.
5) Typically its city driving, which I know will cause MPGs to suffer, but even with a tank of entirely highway driving, it calculates out to ~13mpg
6) I live in Northeast GA, so there are some hills, but nothing insane on average. Probably a couple hundred feet at most of elevation change if that.

 

Another note, I typically drive a 3rd gen tacoma and have nearly identical driving habits and commute to my wife in the 4Runner. I average about 17-18 mpg in my truck (it's a V6/4x4)

I feel like there's room for improvement in the 4Runner, but don't want to just throw parts at it.

 

It's the city driving and doing 75MPH that is most likely the biggest factor in your MPG's. Try slowing it down to 70 or under while on the highway. I know, I know...you're going to say that you should go as fast as everyone else. But your MPG's will suffer if you drive faster.

Sometimes slight hills and a headwind can really effect the MPG's.

 

Sure, speed and hills can certainly cause poorer fuel economy, but I'm not sure that's the only issue. On the highway, the RPMs stay pretty low ~1500 so I don't feel like the engine is working too hard. Plus I've seen others with the V8 that do city driving and get 15-17 like gkomo above.

I definitely didn't buy this car for its fuel efficiency, but ~13mpg seems worse than I expected, even for a V8 brick on wheels. I'd argue my tacoma is far less aerodynamic and still gets around 20mpgs at the same speeds on the highway (though this is definitely not apples to apples).

I'm wondering if there's something causing a rich fuel mix...

 

If it was running so rich that it's emptying your tank abnormally, you would smell the burning fuel (like a top fuel dragster on sunday at the strip). As @Bluesky 07 mentioned above, no A/F code will show unless out of range at a certain point.
All the tune up things, like cleaning MAF, plugs, etc are always good things to keep up on higher mileage engines, but again would show signs of needing it beyond just mpgs.

Loco is correct, if nothing is wrong but gas mileage, then it's 95% the driver.

My Nissan Titan V8 was great at passing everything but a gas station, but at 10 mpgs, I just couldn't afford it anymore. And now I drive like a grandpa on a walker.
Nobody else out there pays for my gas, so they aren't going to dictate how i drive.

 

Well, I suppose my expectations might be too high for the vehicle, but I am surprised its as low as it is. How do people modify their 4Runners with lifts and tires and armor and not see single digits?

I've heard of sensors getting "lazy" with age, causing a rich condition. I certainly have smelled rich exhaust from the 4Runner, but usually only at startup which I'd think is normal. I have cleaned the MAF and throttle body as well as new engine air filter and spark plugs. Just general maintenance stuff that is way over due.

If its a case of managing expectations that fine, I just wanted to make sure there wasn't something obvious that could cause bad mileage.

 

13 does seem low. I recently replaced both my upstream and downstream O2 sensors, but i did throw a CEL unlike you.

The roof rack could be killing your highway mileage, but shouldn't affect your city driving too much. And as others have said, speed kills gas mileage. I notice a drop after going above and cruising past 70mph.