Hello, my F-150 gets extremely bad gas mileage, like 5 mpg around town and 10 mpg on the highway. I did a full tuneup, new carb, 02 sensor, egr valve, set the timing, side gapped the plugs but nothing has helped. The truck runs great, smooth and alot of torque. I talked to a mechanic who says the fuel pump more than likely is the problem. But I thought if the fuel pump was bad the truck wouldn't run, or run as good as mine does. Is he right? By the way I have a 1985 with a mechanical fuel pump. Greg.


1985 F-150, Black with black fiberglass cap. 300 6. Aluminum mag wheels, BF goodrich tires.

 

what colr is the tip of your exhaust pipe? is real black? too rich maybe.
also, what is rear axle ratio. are brakes maybe dragging a little bit? is some body stealing your gas?( i actually had this happen to me!!!)

 

Exhaust pipe is black, no one is stealing my gas, rear is 3.08. Brakes are new and not dragging. Greg.




1985 F-150, Black with black fiberglass cap. 300 6. Aluminum mag wheels, BF goodrich tires.

 

If the mouth of your tailpipe is black, your engine is running rich. Is your truck FI or carbureted? If it is fuel injected, have the fuel pressure regulator checked ($10 to $20). When mine died, I could literally watch the fuel gauge drop as I drove around town...extremely low mpg. If your truck is carbureted, may be time for a carb rebuild. Quien sabe?

 

Hello. The tailpipe is black, but I have leaned the carb out till it starts to run rough then backed it out till it runs smooth again. The carb is new. Not a rebuild or used. What about the coil? Maybe not enough voltage, plugs running cold? Im running out of ideas to try. Greg.




1985 F-150, Black with black fiberglass cap. 300 6. Aluminum mag wheels, BF goodrich tires.

 

greg, what transmission is in your 85? also, did you put original replacement carb or aftermarket? how long have you had truck, do you know the history on it?

 

Hello. Im the second owner of the truck. My dad bought it new in 1985 and I bought it last febuary. The history is the truck had the check engine light on when I got it, but after replaceing the egr valve, oxygen sensor, full tuneup and a factory replacment carb the light went off but the bad gas mileage remains. The transmission is a 3 speed automatic that was serviced before I got the truck. It runs great but gets 5 mpg, 10 on the highway if I coast alot. It has 160K miles but dosent leak or burn any oil. My dad took real good care of it. He said it never did get better than 17 mpg even new. I would love to get that kind of mileage. Greg.




1985 F-150, Black with black fiberglass cap. 300 6. Aluminum mag wheels, BF goodrich tires.

 

If you do have a bad fuel pump, it's leaking gas into your crankcase, pull the dipstick and give the oil a sniff, if you can smell gas, you've got a bad pump. A bad pump won't necessarily make your truck run bad, mine ran fine, but if it sat for a few days, it would fire up, then almost die out, but once the pump picked up the fuel, it would run fine.
If that isn't it, if it runs good, and the tranny works fine, it's beyond me.
Evan MacDonald
82 F100 FlareSide ex. 2wd
MAF MPFI HD 300-6
Hedman Hedder
NP435(6.69 low)
NP205
3.55 Geared ARB'd 9"
Dana 44 TTB
31x10.50/15 Goodyear M/T's

 

greg, thanks for info on history of truck. it helps to know what to look for, and what not to look for. since you know it wasn't abused, axle ratio wasn't changed, tranny isn't slipping, aand it has fresh tune on it, i think you need to check the fuel pump for leakage as evan has said. if it is leaking through the diaphragm, you will have high oil level and it will be thin and smell like gas.
the other things i would check are the compression to see if the engine has to work harder duie to lower compression. also, did mileage change suddenly after carb change, or was carb change part of the attempts to increase mileage?

 

Hello. The carb change was because the engine idled like it was running on 4 cyclinders, before and after I tuned it up. I tried to rebuild the carb but it didn't help, so I bought a new one. The mileage didn't change from before to after and I was 100% sure it would. Thats why im running out of ideas to try and can't understand why after all the work and new parts the lousy mileage remains. One person suggested the catalytic converter might be clogged. But I was told the converter would put out a really bad smell, it dosent. One thing I was told to try was when the truck was running put my foot over the tailpipe and the exhaust pressure should push my foot off. If it does the converter isn't clogged. I tried it and the exhaust dosent push my foot off, but I tried this on my van and my freinds car and it didnt work on them either, so I dont know if that theory really works. I have a freind that has a 1971 F-100 with a 351 cleveland and he says it gets 16 mpg easy, must be nice! Greg.


1985 F-150, Black with black fiberglass cap. 300 6. Aluminum mag wheels, BF goodrich tires.

 

Do you have a lead foot ?

 

Even though you replaced your EGR valve, clogged gas tubes and passages will cause excess fuel consumption. Pull apart the egr spacer plate assembly and run brushes through the holes, and through the tube from the manifold.

 

Now that you mention the egr valve, I've noticed a fuel leak around the egr valve, the valve and gasket are new and bolted tight. It looks like alot of gas is comeing thru the carb and a very small amount seeping out around the gasket. Weird. Greg.


1985 F-150, Black with black fiberglass cap. 300 6. Aluminum mag wheels, BF goodrich tires.