I have 140K on this truck and just went through a vacuum leak issue. Fixed that with a new PCV valve hose. Don't know if it is coincidence or not but after getting my vacuum back to specs my gas mileage went to hell. Was getting close to 17 and now I am down to 6 on a good day. Engine runs fine when up to temp. Until it gets there though it idles at about 1K. The idle speed will go from 1K down to 600 a few seconds after coming to a stop. When I come to the next light or stop sign it is at 1K again and will drop down to 600 after a few seconds. Have read numerous threads about coolant temp playing a part in how the "brain" controls the fuel and have changed the thermostat and will be changing the coolant temp sensor soon. I also changed the TPS at the same time I changed the PCV hose.
Any help is appreciated!!
6 MPG in a Ford is better than 18 in anything else!!

 

You may have two issues.
First the IAC is sticking too far open and causes the idle issue.
Remove it and do a spray cleaning so the pintle is free to move closed..
While your in there remove the throttle body inlet hose and spray clean the throttle plate and bore holding the throttle open so you get the whole area.
Next, I would reboot the computer by removing one battery cable for a few minutes then reconnect.
Do a cold start and turn on all loads, the headlites, fan and A/C and let it fully warm.
Go for a drive out and back covering about 5 miles.
Do several hot restarts.
The idle should be back to normal operation on both cold starts and hot drivabilty.
Then check out the fuel trim tables with a scanner if the fuel mileage has not improved.
If they are out of limits you still have another issue causeing it.
Good luck.

 

Okay, turns out I had also replaced the IAC back when I did the PCV hose. So I replaced the coolant temp sensor and while the air intake hose was off I cleaned the throttle plate and bore. It was gummed up pretty good. Reset the "brain", warmed it up and took it out for a five mile spin. Did three hot restarts and the idle drops down to about 700 after a second or two. Am going to do a mpg check this week. Will have to look into what functions my scanner has to see if it will display fuel trim tables.
Thanks again, Bluegrass 7!!

 

Did a mileage check the last two weeks and tried to duplicate the driving conditions both weeks. Work and back and the same destinations both weeks. The first week I got a little over 16 MPG. Second week was 11.7. Could I possibly have an injector issue??

 

Neighbor ... when was the last time that O2 sensors were changed? The advertised life is 100,000 miles today. If they have never been changed then the life for the old equipment is 50,000 miles. I have 2000 F150 Harley with 56,000 miles on it. Same problem as you. The O2 sensors were replaced with latest technology Bosch and fuel mileage jumped up to 16 around town from previous 12. I did not have any of the other issues that you mentioned because I use Lucas fuel treatment. your neighbor ... ron

PS ... I think that you should consider changing fuel injectors. The new ones have 4 orifices which makes for better fuel atomization.

 

Looking at repair manuals they say there are 4 oxygen sensors, two upstream and two downstream of the cat converter. Do I replace all 4?
Thanks Ron

 

If you haven;t fixed the clogged up exhaust look to that also.

 

What clogged up exhaust are you talking about here? I want to make sure I look into all possible causes.

 

Neighbor .... yes, it takes four. I mentioned Bosch, but upon checking, I bought 2 each of Denso 234-4071 and Denso 234-4127. I bought from Amazon because price was about half of what others were charging. Denso or Bosch are probably the best choice for brands because longevity is one of the most important issues. All brands will perform the same for the first year or so. If you decide to try to do the replacement yourself (it's a bitch) then you will need the special socket as well. your neighbor, ron :-)

 

If you have no codes stored, the rear OX sensors are still good and the cats are still within spec for oxygen storage.. If not you would see code 420 set.
The rear sensors have nothing to do with fuel mileage. They don't control fuel.
They are used to monitor cat system output performance, by the computer.
Fuel mileage sensitivity involves the front OX sensors, the gas quality, injector spray pattern, plug condition, cooling system operating temp too cold and other outside factors such as tire pressure, excessive drag on the motor from an bad battery causing the alternator to charge excessively, fan clutch froze etc.
For the cost, it might pay to use a bottle of injector cleaner in the gas but don't figure fuel mileage with it in the gas until you run the tank out and put fresh gas in.
I doubt at this time that you will find a huge improvement by replacing anything without a sign of drivabiliy issues.
We are entering a time that winter fuel mixes may be coming in in some areas.
This and cooler temps will cause a drop in mileage that is normal.
The fuel has some different additives in and the cooler air requires more fuel to meet the normal A/F ratios the OX sensors work at..
Be sure your figureing fuel mileage correctly each time.
Don't depend on the gas gauge but filling the same at the same place each time.
Good luck.

 

Excellent info neighbor!! Never considered Amazon for O2 sensors.

 

You ought to put all your knowledge in a book!! When I did my mileage check I made sure to duplicate the driving conditions both weeks. In other words I didn't pull a trailer one week and not the next. Will get back with results!

 

If you find nothing consider a clogged muffler, pipe broken off inside etc with exhaust back pressure killing your gas mileage. Your motor may seem to labor under torque conditions, like pulling a hill? Your cruise control may actually be very nasty acting.

 

I ordered the upstream sensors from Amazon.com. Got Bosch brand at half the price at Autozone. Took about 3 1/2 hrs to put in and noticed a drastic difference in the engine after the first startup. Will do another mileage check the next two weeks. I am almost certain that these were my problem. I will submit a separate post on lessons learned changing these out. By the way, the sensors that came out looked to be original. Made by Bosch but had a Ford stamp on them.
Much thanks friends!!

 

Neighbor .. Don't forget .... fuel injectors could still be spraying an improper pattern and fuel economy will be affected by that. Lucas fuel system cleaner is the one that I prefer, although there are probably others that work equally well. Please note that the less expensive cleaners use alcohol as the cleaning ingredient, which is an abrasive acting substance in a car fuel system.

One of our friends made a strong theoretical case against installing the rear sensors. I mean no disrespect to him, but it seems odd to me that your truck has been driven 150,000 miles with the OE emissions equipment. And, the system has never thrown a code??? I think that the cats are probably toast and the sensors as well.

Up to you, dude. I am just offering you experience derived information.

your neighbor, ron