I just bought an '02 ranger FX4 5-speed manual with 60,000 miles in really good condition and drove it home from Dallas. It got terrible mileage (less than 18mpg) with a tailwind @70mph. Also I noticed that when shifting, the rpm's stay high for a couple seconds before coming down (and it's not because my foot is on the gas pedal). A little research tells me that the gas mileage could be helped by changing the mass air flow sensor and/or the oxygen sensors; and changing the throttle position sensor might help with the high revs. Are these common problems with the 4.0 engine? Any other suggestions? Thanks.

 

changing sensors is not going to guarantee an improvement in mileage, you could just end up throwing your money away on parts you don't need. although, air filters and spark plugs, basic tune up stuff can help assuredly with 60k miles. as far as the rpm and shifting, it seems pretty common in most engines regardless of make. i'm not sure the purpose but not a single injected vehicle i've ever owned dropped 'em fast, not likea carbeurated engine does, the computer lets it down slow to prevent damage and it also helps the tranny while shifting. the tailshaft is spinning at 90mph coming out 3rd why mate that to a clutch and flywheel spinning at 20? make sense? another thought for your gas mileage though... on the highway differences in tire pressure can make a lot of difference, more than you might think, usually 35psi is standard unless you have more aggressive tires, but to be sure just check the sidewall its printed there. and you don't necessarilly need to max the pressure either, keep in mind those are "fully loaded" maximums, if your not heavy you don't need that much, you'll just a wear a line bald right down the middle of your tires. i wouldn't go under 35 though.

 

With regard to my bad mileage, I did check the tire pressure and they were at 28 psi. The thing I don't understand about the tire pressure is that Ford recommends 30 psi for the original BF Goodrich tires that are standard on the FX4 and 28 psi isn't that much different. Does the mileage that Ford rates a new truck at come from actually testing it in real world driving or is it on a dyno in a shop? They say I should get 19-20 highway with 30 psi. I got 17.5 with a strong tailwind @ 28 psi; somethings not right. Oh, and I did replace the air filter and spark plugs when I got home. I aired up the tires also to over 40 psi (max 50 psi), but have not had a chance to test these changes on a long highway trip.

The problem I'm having with the engine staying revved up in between shifting gears is getting worse. It's not just a couple seconds, it's like 5-10 seconds before finally starting a downward trend. I changed the throttle position sensor, but I think it even got worse after that. I don't know if I need to calibrate that sensor or not. Any suggestions?

 

okay, found this for you. thought it would be worth checking out since your problem is getting worse.

"To a certain extent this activity is an emissions feature intended keep the fuel that is already in the engine from being denied air to burn completely when you close the throttle. It was used painfully with electronic carbs before fuel injection and somewhat effectively on early fuel injection. The computer knows when you are in gear and when you have the clutch depressed (manual trans inferred in update). For safety, they minimize the decel limiter when in gear to keep you from losing control while driving. Don't start unplugging things if you are going to have it looked at since you won't know what code was already set versus multiple fault codes set by your actions. A basic check of the throttle position sensor voltage may be all you need. Probes were known to have problems with the IAC (Idle Air control) solenoids sticking and causing erratic idle speeds. Your symptom though, usually comes from the manual adjustments that are attempted to remedy the IAC fault. Since the idle speed does come down but is simply delayed I'll assume that the IAC is not sticking at this point. Sticking IAC valves tend to be more erratic. The idle speed is not adjustable and is controlled by the PCM. You can however adjust the stop screw to the throttle plate thereby misadjusting the measured throttle position. This causes the PCM to think that you are almost closing the throttle which is when it is trying to burn that fuel before the throttle plate shuts completely. If the stop screw looks like it has been adjusted before (they usually put thread locker to keep it from moving) try adjusting it to a more closed position then start the engine and try it. Keep track of your changes by counting turns or marking the clock position of the set screw so that you don't create more problems in the event that you have to have it diagnosed by a repair shop. It will just cost you more money for them to wade through what you did to it before they get to the root of the problem. Also inspect for any vac leaks or holes in the intake tube between the MAF (mass air flow sensor) and the throttle body. Both will confuse the system and make it hard to reduce rpm on decel.A lot of newer Fordsuse a Mazda engine and is one of thoseengines that have way too many parts thrown at them so consider spending a few bucks for simple initial testing if the basic checks don't reveal your problem. One hour of diag from a qualified tech won't cost much more than the average sensor does but may help you replace one right part instead of several wrong parts." [/align][/align]I know its long but its useful info worth checking out. Hope it helps.[/align]

 

I have a similar problem but to a greater degree. '99 Ranger 4X4 regular cab, manual tranny and 3.0L V6. The engine stays at 3000rpm when I take it out of gear. Untill I step on the brakes to stop. If I coast to a stop, say against a curb or something and don't use the brake, the revs stay at 3000 untill I step the brake. If I am sitting at the top of a hill in neutral with my foot OFF the clutch, release the brakes and strart rolling, then the engine will rev up, by itself with both my feet on the floor - no pedals at all. It will hit 3000rpm at about 30 kmph. I can't use the tranny to slow down on hills or anything, I might as well have an automatic. It drives me nuts. Oh, and of course it's intermittent. Not there when the mechanic is there is what I mean.

Another thing I dislike is the AC. It's not needed here. It's used for cold on all heater settings. In defrost, foot vent whatever combo, when you dial cold, the compressor comes on and the revs kick up. I would love to unhook that stuff so when you dialed cold you just got cold outside air - not refrigerated air and crappy mileage. Can you just unplug the pump?