OT - bad gas mileage

[whateg0]

My 89 F150 302/AOD recently started getting even worse gas mileage than before. Used to be, I'd get 13.3 no matter what I did. Then pretty much all of a sudden it dropped to about 10. I'm sure it must be a sensor for it to be that drastic, but it hasn't set any codes. It also seems to be very rich, which makes sense. If it idles for a bit, it seems like it loads up when I go to take off.

Ideas?

Dave

[xxjeepxx]

well it could be a # of things. 1st: fuel filter, or air filter, o2 sensors, old axel fluid, worn tires, exc..... here are some of the first things I would check! Good luck

[johnny_waz]

Fuel filter, air filter, and o2 sensors. One of them is likey the culprit.

Good luck

[imagineer]

Before I sold it, I got 14 mpg in my '89 F150 302, Auto, Long Bed, 4X4, (with cap and in 2WD).

The guy who bought it told me he changed the plugs and plug wires and is now getting 18 mpg.

[whateg0]

Before I sold it, I got 14 mpg in my '89 F150 302, Auto, Long Bed, 4X4, (with cap and in 2WD).

The guy who bought it told me he changed the plugs and plug wires and is now getting 18 mpg.

That basta'd! It seems like mine should be better, but who knows. Mine hasn't changed much over the 5 years or so that I have owned it. And it's been through tires, plugs, wires, overhaul, and so on and so on... I do know that I have had to drive a road that is 55mph a couple of times and have gotten somewhat better gas mileage than if I am on the interstate, but never close to 18. Did his story start with, "And this is no s***..."?

Funny, I hear people complain that their gas mileage sucks cuz they only get 20 or so, and I just roll my eyes and laugh. Oh, well. At least it's paid for.

Dave

[whateg0]

Fuel filter, air filter, and o2 sensors. One of them is likey the culprit.

Good luck

Fuel filter was changed fairly recently. Air filter is clean. O2 sensor was on my "I wonder if..." list. That'll be the next thing, I think. And it makes sense. Not that that matters.

Thanks.

Dave

[imagineer]

That basta'd! It seems like mine should be better, but who knows. Mine hasn't changed much over the 5 years or so that I have owned it. And it's been through tires, plugs, wires, overhaul, and so on and so on... I do know that I have had to drive a road that is 55mph a couple of times and have gotten somewhat better gas mileage than if I am on the interstate, but never close to 18. Did his story start with, "And this is no s***..."?

Funny, I hear people complain that their gas mileage sucks cuz they only get 20 or so, and I just roll my eyes and laugh. Oh, well. At least it's paid for.

Dave

I sold the truck on Ebay, but in the description on the auction, I didn't mention gas mileage (figuring that all F150's got poop for gas mileage). The guy showed up, took a quick test drive, paid me the balance of the $2861.00:), and as he was about to leave, speculated as to if he could make it to the PA line before needing gas. When I told him to factor on 14 mpg, he was less than happy. He had bought my truck to replace an '88 2wd 6cyl F150 that got over 22 mpg.

[Jaxom]

Is it fuel injected? Could be a bad injector if so. I had an 89 E150 with the same engine, couldn't figure out why I was going through so much gas. I had changed the plugs, wires, cap/rotor, air/fuel filter, check tire pressure. Then my buddy who rebuilds mustangs checked my injectors. And sure as Sxxt, that was it! Replaced that bugger, and my mileage jumped.

Jax

[Blacksmith]

That engine is port injected; the fuel is sprayed into the intake manifold. A simple explanation; the computer is calibrated to "expect" a constant pressure drop across the injector nozzles, but the intake manifold vacuum varies, depending on throttle position. There should be a sensing line from a manifold vacuum port to the fuel pressure regulator on the back of the fuel rail (passenger side if I remember right), a spring tries to close the regulator down and the vacuum and fuel try to open it - end result is a constant pressure across the fuel injectors. If that line is cracked, broken or leaking, or if there is raw fuel in it, the diaphragm in the regulator is busted; the injectors are acting like you have the pedal to the metal all the time (0" of vacuum) and it'll go rich. This is not a sensor, so no code is set, unless the O2 sensor finally fouls from all the fuel.

[whateg0]

That engine is port injected; the fuel is sprayed into the intake manifold. A simple explanation; the computer is calibrated to "expect" a constant pressure drop across the injector nozzles, but the intake manifold vacuum varies, depending on throttle position. There should be a sensing line from a manifold vacuum port to the fuel pressure regulator on the back of the fuel rail (passenger side if I remember right), a spring tries to close the regulator down and the vacuum and fuel try to open it - end result is a constant pressure across the fuel injectors. If that line is cracked, broken or leaking, or if there is raw fuel in it, the diaphragm in the regulator is busted; the injectors are acting like you have the pedal to the metal all the time (0" of vacuum) and it'll go rich. This is not a sensor, so no code is set, unless the O2 sensor finally fouls from all the fuel.

Sounds like I need to check the fuel pressure on the fuel rail then. I did change the O2 sensor this morning - it hasn't been changes since right after I bought the truck. I guess I always kinda figured if was rich, the computer would shorten the injections to put less fuel in and that was the reason for the O2 sensor. I guess I should also be able to vary the vacuum to the regulator and see a change in fuel pressure, right?

Thanks for all the help. Hope I get this fixed soon. The engine runs okay - not good, just okay - after it warms up, but cold, it's a chore keeping it running when pulling away from an intersection, or when getting ready to cross traffic on a busy road.:eek: Kinda reminds me of the guy who's car is stalled on the railroad tracks and he is franticly trying to get it started.

Dave

Dave

[Blacksmith]

Your fuel pressure gauge will reference the atmosphere, not manifold vaccum, so if all is well, I think fuel pressure should be high 30's at idle and if you "blip" the throttle if should jump and then drop slightly lower as the throttle slaps shut, then return to the previous idle readings. Don't quote me on the exact pressure, I quit teaching EFI in 1999, its the variation as you manipulate the throttle that shows this is working, or not.

[whateg0]

Well, the O2 sensor apparently is not the problem. I ran the truck for a day, and then filled up again. Got 9.7MPG. I know that there is some error from the fact that maybe the pump didn't shut off at exactly the same time, and the truck was on a different angle, and the... But it still runs like it did before, and it smells. I don't know if it is significant, but it seems to smell of gas worse after I shut it off for a few minute and then restart it. Anyway, as soon as I find the adapter for the shraeder valve, I'll check the fuel pressure regulator.

Dave

[Winger Ed.]

You got something going on with a sensor or something.

If the truck itself won't 'fess up a malfunction code, some auto parts stores will plug in thier diagnostic 'pooter for free. And it'll often find things that the on board computer doesn't.

Something else that I've seen is people will put on over size tires, and see thier mileage go down. They didn't factor in a correction for the new bigger tire going farther per revolution than what the speedo gear is set to record.

.

[sportsandspeed]

K.I.S.S. How about trying a new thermostat? If it's stuck, it wouldn't allow the engine to warm up to operating temperature and mileage would suffer.
Jim

[Bern_F150_4X4]

Hey Dave, go over here (http://www.ford-trucks.com/) and sign up. There are a ton guys at FTE with real expertice on our vehicles. They have helped me out of many a bind with my '85.

-Bernie

[whateg0]

Bernie,
I'm going right now to check out that site. I have posted a few questions on f150online, and don't seem to get many responses there. Seems everybody there is only interested in later model trucks.

Thanks.

Dave

[whateg0]

I haven't posted anything on the Ford Trucks forum yet, but I did get a new fuel pressure gauge. It starts out around 40 psi and stays there regardless of throttle position. Then after about 30 seconds, the pressure jumps up and pegs the gauge at 100psi! I don't think it is a problem with the gauge. If I release the pressure, and start the truck again, it does the same thing. And the needle bounces, so it isn't stuck there. It really appears that the pressure is excessive. I didn't even think that the fuel pumps were capable of that much pressure. Anyway, I think this means that I will be picking up a regulator tomorrow on the way home from work.

Dave

[Bern_F150_4X4]

Wow, sounds like you're on to something with that pressure reading. Sounds dangerious! :eek: I have never experienced a failed pressure regulator, but it sounds like a valid failure mode. Good luck and see ya on FTE...

[whateg0]

Wow, sounds like you're on to something with that pressure reading. Sounds dangerious! :eek: I have never experienced a failed pressure regulator, but it sounds like a valid failure mode. Good luck and see ya on FTE...

Conanski at FTE suggested it could be something blocking the return line. I'll check that as soon as I can. I need to find out when my wife is off work this week. Maybe I'll take a day off and mess with it. It would definitely be cheaper to unplug a line and check it than to replace the regulator. (Oreilly wants $50 for it, Autozone and Advance want $20.) On the other hand, if I find that the tank selector switch/valve (which I didn't think of until he mentioned a plugged line), that could be more expensive.

So, it looks like I have something to do this week.

Dave

[whateg0]

Well, I pulled the return line from the tank selector valve and stuck it in a gas can. When the fuel pressure went up, I pulled the line from the gas can and nothing was coming out. So I replaced the FPR. Doesn't load up now, and even though I have put 20 miles on this tank of gas so far, the needle hasn't moved. A week ago, after switching to this tank full, after putting 20 miles on it, the needle was already down to the first line. Now on to the transmission.

Dave

[Blacksmith]

Glad it worked - see, it isn't always the dang computer!

[whateg0]

Well, I think the fuel pressure has fixed one problem. Aside from the transmission, there still seems to be something wrong, though. The truck no longer loads up, but it almost acts like its starving for fuel at times, and it backfires. It sometimes also acts like the ignition timing is severely retarded. I checked it, and the base timing is about 8* BTDC, not 11, but with the SPOUT connected, it does advance normally. Unfortunately, as is often the case, it really only acts up when being driven, so it is under a load, even if it is a slight one.

I also noticed that there is a very prominent ticking - more so than normal. Could the extra-high fuel pressure have damaged an injector? I haven't had a chance to pull the plugs since replacing the fuel pressure regulator.

How do you troubleshoot a MAP sensor? Somebody else mentioned it before as being a possible cause of the erratic idle, which it still does. It's almost like it's hunting for the right rpm and then overshooting it and then just keeps overshooting it. The O2 sensor is new.

Sometimes it feels like no payments just aren't worth the trouble. I have to keep reminding myself that new vehicles can have problems too, in addition to the payments.

Dave