I have a 97 scab lariat 5.4 auto 4x4 with 167,000. Just a few concerns and not sure what to do next. First off, anybody else getting about 10 mpg city? Truck has 33's and 3.55 gears which i know are not ideal for 33's. Second, when the truck sits outside overnight (winter) depending on how cold it is it will either start normally but rev to about 1,500rpm for about a minute and then idle back down to normal? If its really cold the truck will not stay running without giving it gas for a minute or so? There is no engine lights or codes on my scanner. Plugs were changed along with fuel filter about 30,000 ago and all air intake componets have been cleaned as well from Gotts mod all the way up to throttle body. Not sure if the cop, fuel pump, or injectors have ever been replaced. Use fuel system cleaner every oil change also. Lastly, truck seems to miss at idle (year round) but drives fine? Thanks for any advice.

 

IAC valve needs cleaned

Your mileage is partially due to your tires, as you know. But even with those tires and gears.... and an average foot, you should be seeing 12+ on average.

Some may disagree but if it were me and with those miles I would check if the 02's are original. If so, change them.

My mileage dropped down to 8-10mpg when I had a misfire and washed the 02's out with raw fuel. No CEL for the 02's. Changed them and mileage is back to normal

What plugs did you put in it?

 

Just some comments on what you report.
The cold high idle is normal.
Sometimes if cold enough the high idle might remain longer and come down when the OX sensors begin to take over fuel control.
The OX sensors need to heat up to about 600 degrees to begin working.
They have electric heaters to assist heating the sensors faster.
This all takes place normally within the first minute or more of operation if there are no issues invovled.
Not staying at idle without help from you suggests the IAC is not staying open or controllable to maintain the correct idle speed.
.
I would clean the IAC to be sure the pintel is free to move, clean the inside of the throttle body then reboot the computer to begin to relearn from scratch and build the correct cold start functions because the function is distorted from the present way it has been working..
If you want to do this, come back and ask how to best handle this..
Good luck.

 

Plugs were replaced with motorcraft, could you give me a little more info on the IAC. Thanks.

 

All 4 02 sensors were replaced about a year and a half ago.

 

Idle air control valve. Sits bolted to the back side of the Throttle body elbow. Has 2 bolts and an electrical connector... It controls your idle. Common issue is they get gummed up. Remove and spray clean the inside with wd-40. Make sure the plunger/pintle is working freely before reinstalling.

Looks like this

 

ehhhh. i had the same tire set up as him, and i saw taht as well. the best it ever saw was 15. it's the NPI heads on the gutless 4.6. i saw your name and about had a heart attack! i thought i posted this and forgot about it!

 

Same tire, engine and gear setup I average around 10.5mpg city with my 2002.

 

First, if theres a misfire then we need a code. Have you tried just letting it idle for a while to see if it eventually throws a code? Second, and this may sound like a PITA, check your plugs, were they gapped correctly and do they look rich or lean? Can you see the timing mark on them?

If the IAC was bad or clogged... when was the last time the throttle body and upper intake was cleaned with Berryman's B12 or equivalent carb cleaner? These trucks are nasty about turning a nice clean intake manifold into a carbon-choked cavern.

 

That's not considered normal if you ask me.

 

Ageed, -definitely not normal, but w/33's everything's quite a bit more compounding. If somethings off just a little, that something can impact economy considerably.

Also, -the OP posts that all 4 O2's were replaced. It's very good replacing all 4 instead of just the two upstream. However, replaced with what can make a large difference as the AF's are tabled for Bosch and Motorcraft sensors only. Good sensors are louvered to spin hot gases around the ceramic vs others on the market that get blasted w/gases which set them apart. Bluegrass explained about the O2's a bit. Can be explained further off course, if warranted.