hi gang, it's been a few weeks now (noticed it also when we got it back from dealership a few months back) that the gas milage has dropped. Yesterday morning, i had a full tank (neddle on F). i was in bumper to bumper traffic for about 500meter stretch. By the time i got on the highway, it had dropped between 3/4 and full tank! quite a big drop and i wasn't even moving!
the truck now has 112 000 miles on it and i'm sure it has original plugs. Might it be taime to change them? anything else i should consider? thanks
fab

 

OMG he said the taboo word "sparkplugs"! Yes, go ahead and change them. Select a good long lasting version (platinum, etc) Then pull your codes from the computer. That will give us something to work with. What year & engine do you have?

 

You didn't say what year but if its a Windsor engine (5.0 or 5.8) change the plug wires, cap and rotor while you are at it.

I really can't see it using 1/8 of a tank in less than half a mile unless it took you an hour or more to go that distance. If it was using fuel at that rate it would be running like crap.

I would be more inclined to think the gauge settled and it was just reading high when you started out.

 

Sorry, 2001, 4.6L engine.

 

I wouldn't go just by needle movement. You need to do the actual math to see if your MPG has changed. Have you calculated what it was getting before you noticed it seemed to be using more fuel? Fuel gauges aren't very accurate and they are dampened to keep them from bouncing. So it might stay at Full for a while even though you have used some fuel and then it will drop.

 

The thing is, I can also feel it Performance-wise. Often when driving I can feel a kind of stumble or miss when accelerating. At wot, it's not that noticable. Mind you, we're in dead of winter and don't wanna kill myself, so haven't floored it all that much. Was thinking maybe camshaft position sensor?? I show no check engine light on dash.

 

pull the codes

 

Yeah, check the codes.

Certainly could be plugs but a misfire, or an excessively rich condition, would set a code.

I have also seen a bad catalytic converter cause similar symptoms.

 

Hey gang, plugged in my odb2 reader tonight, no codes stored.
so what could cause these symptoms AND not throw a code? Thanks.

 

What brand gas ? You possibly could have dumped a tank of E85 into it, that could account for the rough running and the sudden decrease in MPG. And yea, change those plugs. But get ready to have some of them strip the threads out of the heads when they're removed.

 

Many times, a misfire won't set any codes. You might need to go to a shop and have them plug in their expensive scanner that can read misfires in real time. Here's a not so expensive scanner that can read misfires

 

Water in the fuel can cause a miss and not set codes; have seen em go putt, spit, spit, putt,, spit to PURR within 60-120-seconds by adding HEET to tank while running w/ known water issues in fuel