I'm fairly new to everything automotive. I just started working at a local quick lube and am gaining quite a bit of experience very quickly, but I'm getting burnt out on these problem with my truck. Before this all I knew how to do was change oil, but my knowledge of vehicles is still very basic, so the answers to this will probably have to be dumbed down a tad. I knew nothing about the amount of maintenance required for a vehicle before working where I am. I got it tuned up once, and the oil changed every 3k and that's about it.

The basics:
98 F150, 4.6, 4x4, automatic trans.

My problem:
I don't know exactly where to start with this, so I guess I'll list them, because they're all happening at once.

First is the weird rubbing sound:
Sounds like something in the front end is rubbing when I drive over about 20 mph, then it keeps doing it the rest of the ride. It does NOT do this sound when I turn more than 30 degrees or so though. I know it's not the tires or brakes. Drained the brake lines today, no change, the front differential oil is pristine, I have no idea what it could be. I was thinking that it could be the rear diff, but to stay on topic, I'll post about the rear diff below. It's been getting very slightly worse for about half a year. At first it was only a minor annoyance, but it's starting to get incredibly annoying, and who knows, perhaps even bad for the truck itself.

The rear differential:
I've still not had the oil changed in it and never have (Can't afford it currently. College kid. You understand I'm sure haha). I don't know if the diff had ever been serviced prior to me owning the truck, but I've had it for 45k miles and can't get the plug off to check it. I saw that it was leaking slightly around the seal on the back cover, so I went to check the oil in it and I rounded it out trying to check it. Still waiting on some money to get that fixed. I'm sure it's in terrible shape and I fear that I'll just have to install a whole new differential.

The HORRIBLE gas mileage (8.4 mpg):
This is my main concern, as I said I'm a college kid so I don't have much money to allocate to anything, especially gas. On 5.5 gallons, I got 46.2 miles, which comes to 8.4 mpg. My RPMs haven't changed a bit, and the truck rides fine with the exception of the rubbing problem. I have multiple theories on this situation, and weigh in as you will:

1) I've recently moved and currently live within 1.5 miles of literally EVERYWHERE I go. Which means over the course of a day, I start my truck about 5-6 times and drive 8 miles. I don't know how much gas starting it takes, and I had always assumed it wasn't much, but the mileage seems to coincide with the move.

2) 2 error codes I have with my Check Engine light. Every site I've checked on makes it sound like these wouldn't do it, but it's intake-related so who knows? The error codes are P0171 and P0174, and says "System too lean" as a description.

3) Rear diff problem

Like I said, no change in RPMs, I learned REAL quick about driving at high RPMs with a V8 and the gas mileage associated with doing so, so I've always watched it very closely since I've owned the truck. I usually drive at 1.5k, and when possible, I hit 40mph and it cruises at only 1K. It still does this, and has as long as I've owned it.

Sorry if this is too lengthy, but I can never be too sure with this thing.

 

Those codes point to a vacuum leak. Might be a reason for bad gas mileage

 

I've read about replacing the PCV valve, seems about the cheapest route to go with. It's got oil residue on it so I'm guessing it needs to go anyhow.

Would this help?

 

Check it out before you replace anything, you don't need to be spending anymore money than you have to. p0171 and p0174 has a lot of different solutions so don't go throwing money at it just to "try" to fix it. When was the last time the coil packs were changed?

 

What're the cops?

 

Probably never if I had to ask, unless it was done during the tune-up, which was upward of 3 years and 35k miles ago.

 

your truck does not have COP's (coil over plugs)

you need to get some air intake cleaner and chase down that vacuum leak. I learned from a very experienced mechanic how to get out the plugs on a differential that are seemingly frozen.

Step 1: get a half inch impact with a 6" extension. Put all your weight on it and hammer it out with that. If that fails because it is rounded out continue to step 2

Step 2: Take your 6 inch extension from step 1 place it in the plug the best you can. Weld the bastard on there without welding it to the case. Take your impact and hammer it out.

There is no step 3 because it has never failed.

 

I considered doing it (The welding) but I have no tools, all the tools I use are at the shop that I work at. I have very little income currently and it would be a little more cost-effective to just have the guys at our full-service shop drill it out I think. Thank you for the advice. I definitely about died laughing at the step 3.

 

Let me help you a bit on the codes and the reason for the poor fuel mileage.
The OX sensors in the exhaust detect oxygen levels.
They have detected to much oxygen and the signals they send back to the computer tells it to richen up the fuel trying to correct for the fault.
This is why you have poor fuel mileage.
The system trys to keep the motor drivable and not let you set for a relitively minor problem.
Somewhere on the intake system is an air leak.
It is air that has not been accounted for by the mass air meter so the OX sensors catches it and causes the fuel readjustment.
Look at all the rubber fitting for a split, rot or whatever that is letting air in.
It might even be your noise as well.
The computer works the same as any other except in this application it has sensors to tell it what is going on in the outside world and actuator controls to make corrections and do testing and monitoring.
There are many tables of fixed data that it compares the sensors' returning data to.
If the returning data falls outside the table limits a 'logic' code is set and a history is kept in a memory location.
The codes tell you what systems are affected but not the part to replace.
This being the case, you have to play tech. and know how that system works in order to track down the part that is faulty.
Except you come here and we tell you all about it. That's cheating.
Good luck.

 

So basically, fix the "Check Engine" light, fix the problem?? Hope so, I have a week before I'm paid and I have precisely 0 dollars. Good thing I only work a mile away hahaha