Sudden Loss of Fuel Economy??

[kestrel]

Hey folks - aside from the obvious possibility of fuel leaking from a bad hose/pipe, what would cause a sudden and drastic decline in fuel economy? My '89 van went from 20/22 to about 12 miles per gallon in a matter of a couple of weeks. I don't smell any gas with the engine compartment open, when it's hot or cold, or when I'm driving it.

Your thoughts are appreciated!
Kestrel

[trestlehed]

A bad O2 sensor will do that.
Your van being an '89... I think you have 2 (two) O2 sensors.
Also, a bad fan clutch will affect fuel economy, as it will reduce horsepower if it's engaged all the time.

[kestrel]

thanks, trestlehed! i'm not a mechanic, so forgive my ignorance if this is an obvious question...but wouldn't the O2 sensor have other symptoms that are pretty noticeable? i'm not having any symptoms besides bad fuel economy.

[trestlehed]

Well, I'm no mechanic either. :doh: I'm just passing on my experience of having lot's of different vehicles over the years.
A bad O2 sensor will cause your air/fuel mixture to run rich, as in too much fuel trying to combust. This causes the engine to pass unburnt fuel into the exhaust system which can cause another problem besides poor fuel economy: the unburnt fuel will cause your catalytic converter to get gunked-up faster than it should. And catalytic converters ain't cheap!

Maybe Tim can chime in here and correct me and/or add other ideas about your symptoms.
Good luck!

[timsrv]

These are good observations. I'd recommend checking your codes 1st as sometimes the answer is waiting there. Losing half of your fuel economy is pretty drastic. It could be a combination of several things. Things that reduce fuel economy are: sitting in rush hour traffic, jack rabbit starts, under inflated or oversize/aggressive tires, roof racks, head winds, driving up long steep grades, towing a trailer, driving in 4wd, poor fuel quality (winter blends come to mind), extra weight inside van, mechanical problems such as bad 02 sensor(s), bad TPS, (ECU will usually store a code for these), improper ignition timing, over-due tune-up (dirty air filter in particular), stuck clutch fan, restricted exhaust (cat converter), & fuel leaks. There are more but these are the ones that immediately come to mind. Some of these are worse than others. Combinations of two or more can really kill fuel economy. Tim

[kestrel]

sounds good, tim. none of the non-mechanical stuff is a concern (traffic, weight, headwinds, etc.). she's at the shop awaiting the rebuild, but i'm glad to know the range of possibilities.

[kestrel]

These are good observations. I'd recommend checking your codes 1st as sometimes the answer is waiting there. Losing half of your fuel economy is pretty drastic. It could be a combination of several things. Things that reduce fuel economy are: sitting in rush hour traffic, jack rabbit starts, under inflated or oversize/aggressive tires, roof racks, head winds, driving up long steep grades, towing a trailer, driving in 4wd, poor fuel quality (winter blends come to mind), extra weight inside van, mechanical problems such as bad 02 sensor(s), bad TPS, (ECU will usually store a code for these), improper ignition timing, over-due tune-up (dirty air filter in particular), stuck clutch fan, restricted exhaust (cat converter), & fuel leaks. There are more but these are the ones that immediately come to mind. Some of these are worse than others. Combinations of two or more can really kill fuel economy. Tim

okay, folks, so i'm some months down the road of trying to address this problem and i haven't had any luck yet. i've replaced the following:

1. rebuilt engine
2. fuel injectors
3. TPS
4. fan clutch
5. O2 sensors

i'm still only getting about 10mpg. there are no other symptoms at this point. i'm at a loss. the only thing that seems left is the cat converter. would there be any other symptoms if this were bad? i heard of a guy in vancouver, wa who works on these vans. any locals know who he might be? as always, thanks for your help.

[djshimon]

Is that just for city driving or is that highway MPG's?

[kestrel]

Is that just for city driving or is that highway MPG's?

both. there doesn't seem to be a substantial difference. got any ideas?

[timsrv]

Did you ever check codes?

[kestrel]

Hey folks! Happy holidays y'all! I posted here some months (years?) ago about a strange fuel economy issue my 1989 4WD van has. I still haven't gotten to the bottom of it and I am posting again to get some fresh thoughts on it. The story is: when I first bought the van, it was getting 19-20 mpg around town and 20-23 on the highway. About six months in, the van started getting about 16/18mpg suddenly. A year later, it lost another two mpg and now, it universally gets about 11-13mpg. I'm mystified.

I rebuilt the engine as soon as I bought it (so before the issue occurred) and since then have replaced both oxygen sensors, fuel injectors, filters, muffler, tires, and probably some other things I can't think of off the top of my head. I've also had the alignment dialed in to perfection. The van passes emissions with flying colors and runs perfectly outside of this issue.

I'm about to take it to yet another mechanic for diagnostics (it's not coding) and am at a loss. I'd love to have some suggestions to take with me. Dear van experts, please throw in your two cents no matter how unlikely. I feed the van the best gas and the only strange thing I experienced is that a tank of accidental regular gas upped the mpg by two miles, though may have been due to other factors. I've tried everything I can think of to try in terms of driving habits and conditions and absolutely NOTHING but that one tank of gas has affected any change.

Help!
Kestrel

[timsrv]

Welcome back Kestrel :wave1:. For organization reasons I merged this with your previous thread. Did you ever check codes?

[kestrel]

Hi Tim - No codes!

[timsrv]

Oh, yeah....just reread and see you mentioned it's not coding :doh:. Aside from the things already mentioned, weight and drag are usually the most likely issues. I only get about 13 - 15 mpg on my "mechanically perfect" 86 2wd LE, but I'm carrying around ~3k lbs of tools and cargo. I also have a roof rack with ladders & oversized tires. These things alone kill my MPG.

Of course there's the dumb stuff like having your hubs locked (manual) or perhaps having one (or both) malfunctioning (auto). Just to rule that stuff out I'd jack up the front and spin the wheels by hand. They should spin freely and independently. Also take a look underneath to be sure neither front axle spins with the wheels. Aside from the stuff we've already talked about, I got nothing.

If it doesn't have any stored codes, and it passes emissions, then that pretty much rules out engine mechanical issues. Do you ever smell gas while parked or while driving?

[kestrel]

Tim! Thanks for your response, as always. I'm not towing or carrying a load, so definitely not that. I actually hadn't thought of the 4WD issues. I have auto 4WD and I think it's entirely possible that something is up there. Some months ago, I thought I detected some drag on the van, but gave it up as my imagination. I will definitely start there. It's surprising no mechanic ever thought of it as a possible origin of the problem.

Lastly, I don't ever smell gas, except as my injectors were going bad last year. That was a brief time as I had them replaced right away. No gas smell since then. I'll let you know what I find out. Thanks for the tip.

[kestrel]

Oy, this is driving me nuts. The mechanic I took it to while traveling in Colorado was convinced I had a failing catalytic converter, so replaced it. And, it turned out one of my auto-hubs wasn't working properly, so he fixed that. But alas, absolutely no change in gas mileage. The van is running great, but jeez topping out at 13mpg? SMH.

[timsrv]

Have you tried gasoline as an additive? 2 gallons of gas added between fill-ups will get you about 26 extra miles per tank :yes:. Sorry to make light of your issue, but humor is all I have left (regarding this anyhow). If I think of anything else I'll let you know. Tim

[JDM VANMAN]

Uuummmm... Sorry... And I know I'm probably the last person to give advice on this after some of our reputable members have thrown out their thoughts and ideas, but... is your tripometer working properly or bad speedo???

Sorry just throwing out an idea:cnfsd:

[kestrel]

ha! you're funny tim. i'll try that as a last resort. what if THAT doesn't even work?? do you have the name of a good priest?


Have you tried gasoline as an additive? 2 gallons of gas added between fill-ups will get you about 26 extra miles per tank :yes:. Sorry to make light of your issue, but humor is all I have left (regarding this anyhow). If I think of anything else I'll let you know. Tim

[kestrel]

no, i appreciate the thought, JDM. we sometimes forget the simplest things. alas, i have verified the trip/speedo are working properly


Uuummmm... Sorry... And I know I'm probably the last person to give advice on this after some of our reputable members have thrown out their thoughts and ideas, but... is your tripometer working properly or bad speedo???

Sorry just throwing out an idea:cnfsd:

[kestrel]

back at this thread, just in case some fresh eyes can help. my van runs perfectly. like new, besides the fuel economy issue. we did get it bumped up a couple of MPGs when we replaced the MAF and a tiny expensive cracked hose in the airflow system (not sure that's really what it's called). just to refresh, we've replaced:

1. catalytic converter
2. fuel injectors (though i wonder if these may still be a root of the problem due to shoddy workmanship, though no fuel smell)
3. MAF
4. O2 sensors (also wondering about these)
5. tires
6. total engine rebuild
7. 4WD rebuilt
8. high efficiency radiator
9. muffler

the van passes emissions tests with flying colors. no strange smells or sounds. could cheap fuel injectors be the problem without any smell or would they affect emissions tests?

thanks in advance,
kestrel