Gas mileage on my 90 F150 SC with 300 I6 recently dropped from 19 highway and 15-16 in town down to 11-12 regardless. My gas gauge is also bouncing around a lot, especially when braking or accelerating, but does read across full range of the tank. O2, ACT, ECT sensors are relatively new; cleaned plugs, wires and dist. cap last weekend; clean throttle bores and new air bypass valve...still no effect. Anyone seen good MPG results from changing fuel filter? How about the fuel pressure regulator? How do you check it? Any replies appreciated

 

Tm ,
I can see why you want the mileage back ...Same truck here and the best Ive ever got is 17 highway . Any change in preformance , if you can call it that with a 300 ? Whens the last time the Air filter got changed ? Could it be the gauge is just throwing you off ? I can go about 200 miles on 12 gallons or so , combined city and highway . I always reset the trip Odometer on fill up . I know I had problems w my fan clutch , but I doubt that would cripple your mileage that much ? Plus it being winter , its rarely engaged .
Paul

 

Is your catalytic convertor plugged up? This will back things up.

 

My plugs and mouth of tail pipe are sooty black. Does that indicate a toasted CAT? Something else?
Ran engine self-test last night and it posted a 22 code...MAP or Air bypass sensor out of range. When it did this before, I changed the MAP and got no effect...then discovered a toasted air bypass valve solenoid and changed it. Maybe my MAP is bad, too. Anyone know a good, CHEAP way to diagnose MAP operation besides vacuum pump method mentioned in manuals. Don't own a vac pump. thanks again.

 

TM,
After you replaced the Map Sensor , did you clear the code for it ? Im not 100% sure on this , but I think unless you clear the code , the computer will still operate thinking its recieving faulty or no data from the sensor . My exhaust is sooty black too , thats pretty normal for any engine as long as its not excessive . As far as I know , the only way to test the Map sensor is the vacuum gauge way because thats what it works off ... engine vacuum . You might try taking it to a parts store and see if they cant check it out for you . Good Luck ,
Paul

 

Thanks for the replies guys. Yes I cleared the codes so don't think I'm picking up an old one. I'll just have to invest in a vacuum gauge set this weekend and test MAP,TPS and CAT (see info below that I found on the web today)

When the engine is unable to exhale as it should, a positive pressure will develop insde the intake manifold each time the intake valve opens. Rather than combustion gases leaving the cylinder, they back up and wait for another way out. The path of least resistance then becomes the open intake valve, which allows the gases to enter the intake manifold under pressure. This condition lowers manifold vacuum to a point that causes the MAP sensor output signal to be out of sync with the throttle position and the rpm inputs to the computer. Obviously, the end result is a fault code. With a vacuum gauge connected to a source of manifold vacuum and the engine at normal operating temperature, run the engine at 1000 rpm and record the vacuum reading. Next increase engine speed slowly to 2500 rpm and note the reading. If the vacuum reading at 2500 rpm gradually decreases more than 3 in. Hg from the initial reading, the cat convertor should be inspected for gagging the engine.
source: Alta Vista search engine query on MAP Sensor testing.

Sorry for the long post, but found this interesting and wanted to share it with others. I'll vacuum test mine this weekend and report back what I find.
Thanks again for the feedback.

 

The color of the inside of tailpipe should be approximately "tan". If it's black, your running rich &\or your burning oil. GOOD LUCK