Drop in MPG....why?

[dalsox2235]

Hey guys,
I did a search on this but didnt come up with much.

Recently my Mountaineer has had a slight drop in mpg and I cant figure out why. Its a '99 5.0 AWD w/91xxx miles. I used to get around 18-20 mpg on the interstate cruising at 70mph. Now I am struggling to stay above 16 mpg, and its even worse in town.
Within the past two weeks I've done a few minor tune up procedures as follows:

spark plugs (Autolite platinum)
plug wires (Duralast, I know, I hate autozone too, but its all I could find on a sunday)
air filter (Fram extra guard)
PCV valve (Fram)
fuel pump & filter (Motorcraft)

Before I changed the PCV, my mileage was fine, afterward it took the dip, so I decided to put the old Motorcraft one back in, but no luck, so now the new one is back in.
Is it possible something like cheaper wires could have that much impact? I was expecting better mileage.
Also, before the drop I dumped a bottle of STP total fuel system cleaner in the tank. I just filled up with Mobile 93 octane tonight hoping it might help. Any suggestions would be great.

 

[BrooklynBay]

Welcome to this forum! Do you have any foul smells from your exhaust? A rich mixture could be the cause. Some other things to do are? replace the fuel filter, clean the throttle body, idler air control, mass air flow, air charge temperature sensor, and O2 sensors. A rich mixture could also be the result of a bad engine coolant temperature sensor. Do you have any problems starting? Check AlDive's thread called quest for 30 for more helpful fuel saving tips.

 

[dalsox2235]

I have no bad smells, the truck actually runs great, real good and strong. The fuel filter and MAF are both new, and I cleaned the IAC already. Did have problems starting, but a new fuel pump and FPR fixed that a while ago.
I'm just stumped, best I can tell, everything is working like it should.

 

[aldive]

Since your mileage issues began with the replacement of the PCV, it is possible that you dislodged a hose and subsequently created a vacuum leak.

Recheck all the hoses and vacuum connectors.

Good luck ....

 

[dalsox2235]

I checked all the hoses and none are out of the ordinary, but that did get me to wondering. When I took off the oricinal PCV valve it was wrapped to the hose with some sort of foam tape, mabye to seal the connection. Would not having this tape affect anything?

 

[dalsox2235]

Heres a quick update on my mpg. I installed an MAC air intake kit (got it on ebay for $59), looks real cool, but dont know if it helps at all, no increase in power or mpg really, btw, how do I go about cleaning the filter on this, would the K&N kit work? Anyways, I just seafoamed today and I'm waiting to get out on the interstate to see how it goes. Speaking of seafoam, I picked up this little injector that has a 1/4 inch detachable hose thats made for mixing oil with gas, I just put the hose in the seafoam can, sucked it out, put the hose in the pcv hose and injected. Really easy. I used about 3/4 of a can and put the rest in my tank. I have also reset my pcm by disconnecting my neg. battery cable for about an hour. Without seeing what the seafoam can do, I am still getting +/-16.5 mpg hwy. Cant figure why it has dropped from 18.5-19. Of course, I was only watching my info center screen for mileage, so mabye it was wrong the whole time. Is that possible? Anyways, heres a link to the injector if anyone wants to check it out .

http://www.hopkinsmfg.com/10111.html

 

[tlyman]

I don't know if this thread is still alive, but...

The initial poster said they put a can of STP fuel cleaner in right before the fuel drop. The exact same thing, with the exact same mileage drop just happened to me. Did not do anything other than add the fuel system cleaner. Only difference is that it was Chevron fuel cleaner.

Any ideas?