Expected MPG on a 1998 V8 AWD

[CDW6212R]

Read the list of items in this thread which suggest poor mileage, and then compare those to a typical later sedan. Those cars don't have 4500+ lbs, 4WD transmissions, four cats, huge frontal areas, and hauling a lot more stuff regularly.

There's things you can do, but removing 1000-1500 lbs of constant weight isn't one of them. Most of these we discuss here have 4WD or AWD, they have probably double the HP loss due to the drivetrain as 2WD cars with 2WD. A lot of people drive manual trans vehicles, so these 15mpg SUV's have glaring fuel use issues.

They aren't as efficient as vehicles 10 years newer, which gained a lot of advantages with emissions changes, and variable valve timing becoming more common. Get an engine/trans from a brand new vehicle and swap that in, there would be a big improvement there. But nobody is going to do that with such an old vehicle, given it's current value.

 

[Xylene]

I hope the rebuilt engine does great for you. I also hope you have replaced all of the fluids with the best synthetics you can. That and good tires can help a lot.

If you haven't done it yet, consider a computer tune to dial in the combination, there's a good bit to be had for better power and economy.

Well after all of that work, my MGP is still around 12. So perhaps my car just isn't meant to get any better than that. I have an OBDII reader tethered to my phone so I can see live data from the ECU. On the freeway I seem to average about 18 MPG but in the city I am getting about 8 MPG.

Also any suggestions for a P0420 code (CAT system efficiency below threshold bank 1) I would see this code about once ever 3 month before the engine rebuild, and it just came on again this morning. Could my cat actually be bad, my feeling is it's trying to tell me something else is wrong, but most everything else upstream from the cat has been replaced. Any suggestions?

 

[CDW6212R]

I think it's time for some deeper diagnostic tests. Search well for that CEL code, or find a shop who can track that down. I'd be guessing and it could be the same as my Mountaineer issue is, I'm thinking my mileage is high enough that the cats are plugged. I now have another 98 302 truck, so near future I'll build a cat pipe set and swap the old one into my Mercury.

Most shops can pressure test the exhaust by drilling a tiny hole in a pipe etc, but they will expect to install a new OEM cat if the pressure is too high. Be careful of that depending on what shop you might ask to do that.

 

[Xylene]

The O2 sensors seem to be reading fine, but I think I will pull them and clean them off just in case they got dirty from before the engine rebuild. After that I will have to take the car into a muffler shop and have them look at the cats.

 

[CDW6212R]

If the O2 sensors are old, or could be fouled from poor running, replace them when you know the air/fuel mixture is good now. That can help a lot, but I doubt those would have anything to do with the P0420 code.

I have an exhaust shop here that would install a universal cat if I wanted them, and if they drilled a test hole, they would weld it shut if needed. Some shops once they get the vehicle inside, they will only install new OEM level parts related to the cats. Ask ahead before you go.