Ok here is the situation, I have no clue to what is going on here...lol First about 6 months ago, changed the spark plug wires on my truck (96 f-150 4x4 5.8 EFI) everything was fine.. was getting around 565 miles out of both takes combined... ok well decided i need'd to get the plugs out, bought the truck from my father, and found out he never changed them.. So it’s a 96 with original plugs (93000 miles on the truck) so I figured I better switch them out.
Well changed them out with autolits and gas mileage dropped to around 480 out of both tanks... thought it was the plugs so bought autolight platinums and same results 480 both tanks.. so then bout the triple platinum +4's and still the same mileage, 480 ...lol so then changed the rotor, distributor, and pcv valve and STILL same mileage, also checked for vacuum leaks and found none, so I have no idea what to look for next, maybe I just need to put the old plugs back in there?...lol they were the original motorcraft that came with the truck off the line....lol
any advise would be very very welcome.. (And no my driving habits haven’t changed and tires are properly inflated and all the fluids are good...lol)

Thanks in advance guys.....

 

please delete this one, i have no idea why it tripple posted i clicked the submit once.....

 

ok dont delete this one,...lol this is the only one that posted what i had to say.. the other 2 just posted the title...lol have been getting a database error as well. so maybe that had somthing to do with the tripple post.

 

how do the wires look? you may wanna check them to make sure the resistance is still where it should be.

also, just use motorcraft plugs. OEM brand always works best, although Autolites are suitable since they're basically the same thing.

Mike

 

Ensure that you gapped the plugs correctly. Too little gap will not burn well and too much gap will not spark. Also, carefully check the firing order. It is easy to swap two of the plug wires and not notice a slightly rough idle. Finally, check the vacuum connections and hoses. These are easy to knock loose or break when changing the plug wires and cause a small vacuum leak that will hurt mileage. Also, besides the wires suggested above, check your timing.

 

went ahead a replaced the plug wired again, checked the firing order on the distributor to each plug and they are correct.. so now i need to re check for vacume leaks, any good ways to check with out taking it to the shop? last time i checked i was told to take some carbrator cleaner and spray the vacume lines and if the idle goes up then you have a leak, well did that and didnt find one, so is there any better way or more thorough way to check for these? also i need to change th u-joint in the rear, but i doubt that would effect the gas millage. Thanks for the advise guys......

 

As suggested above check timing - if it was reset after some miles and the plugs were worn, bigger gap, this will change your base timing. After setting gaps to spec, timing could easily change a couple degrees from the open gaps of the old plugs.

 

Try putting the old plugs back and see if your mileage goes back up. If so then you've got on hell of a puzzle to solve.