Hi. Probably posted already but I searched and didnt find anything. I have a 1986 F150 with 97K miles, 6cyl. I get maybe, on a good day, 10 mpg. It seems to run fine, smooth. I have replaced the o2 sensor and a couple others. Next I hooked up a vacuum gauge and it reads a steady 25. Is that normal? The "green" on the gauge shows 17-22.

Next I'll check the timing but thought I'd throw this out there for comments.

Thanks!

 

25? that is on the very high side. I suspect your gauge may be wrong. Have you checked for codes by running the KOEO tests and CM display?

 

20 inches of vac would be more normal your gauge may be off .You say you are only getting 10 mpg .Do you have a cap on your truck?I had two 300 6 cyl stick 4x4 F150 s.Both trucks were 1984s one got 18.5 on long trips the other 14.5 both had same axle ratio 3.08 both had same trans 435Np.I pulled all the emissions off and cat converters off and put thrush mufflers on them jacked timing 2 degrees made a big difference.If you left all your emissions on and opened the exhaust up upped the timing you should get 12 plus mpg.unless you have a high camper top .Back in the 80 s camper tops on Rangers and f150 six cyl auto trans would cost you a loss of 5mpg

 

If your guage is working correctly you may have the timing advanced too far, this can give a reading over 22 in/hg.

 

Thanks for the replies. Bad gauge. I'll pick up another this weekend and retest. In the meantime I did check the timing and it was spot on at 10 degrees btdc at idle. So maybe its a vac leak that may be causing the bad mileage? Or could it be a carb setting? I'll search that on the forum to see what I can find.