VitaminH

Hi,

Over the past few months it seems like my 1996 Pontic Sunfire has been dropping in it's fuel efficiency. Considering gas prices are on the rise, it's not exactly what I'm looking for in my car. To be honest I'm only looking to drive this thing until this summer, when I will go out and purchase a new car. We're talking 3 more months, 4 max.

I first noticed a drop in Novemberish, I attributed to my increase in city driving, tire pressure (they were a tad low, I added) and that I'd heard gas stations put something in the gas during the winter to help prevent freezing that decreases efficiency.

This last tank it seems like it's dropping even more. I can remember getting roundabouts of 30+ MPG when it was mostly highway, and mid to low 20's city, now it seems like I barely made 20. I've already noticed on this tank that the needle is dropping faster than I remember it in the past.

I've checked my tire pressure, it seems fine.
I HAVE been letting the car warm up this past week, due to extreme cold, so this could be part of it.
It's still winter, so I assume I'm still getting the less efficient gas.

What I'm looking for is any other possible causes (that I could hopefully remedy myself) for such a decrease in efficiency. I know next to nothing about cars, but someone once mentioned to me perhaps it was my spark plugs getting dirty? I don't suspect a leak, I've never smelled gas while driving or around my car when it's been parked, but is there a possibility of a slight leak somewhere in one of my lines?

Any help is appreciated. Hope I've given enough detail.


Thanks!

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Borla

Cars almost always run richer in the cold, meaning lower fuel efficiency. That is probably part of the change you are seeing, maybe all of it.

That being said, do you have any idea when the last time the spark plugs were changed? Or what about the exhaust, is it in good shape? My first inclination is that the plugs and possibly the plug wires need changed.

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shakran

also oxygen sensor can have an impact here.

And lest you be like my buddy, who had a similar problem and spent 8 months replacing damn near everything attached to the engine to try and solve it before he found the hole in his gas tank. . .. .

cyrnel

disclaimer *I do not know that car at all.*

Has power declined as well as mileage?

Have you ever changed EGR or PCV valves?

How about the O2 sensor?

How does the air filter look? Don't worry if it's within 20K of new, but if it's full of mud it could cause problems.

Smell the oil. Any hint of gasoline? You could have a leaky injector which would lead to other bad things.

You could have a bad temp sensor or other leading to a rich fuel mixture. (it would always be in "warmup" mode)

Can you stop by an Autozone or other for a free diagnostic? They plug into the OBD system and dump codes. '96 means OBD I but it can still tell you about important things like emissions (mixture, exhaust content) depending on the car.

(duck - I'm throwing darts blindfolded)

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There are a vast number of people who are uninformed and heavily propagandized, but fundamentally decent. The propaganda that inundates them is effective when unchallenged, but much of it goes only skin deep. If they can be brought to raise questions and apply their decent instincts and basic intelligence, many people quickly escape the confines of the doctrinal system and are willing to do something to help others who are really suffering and oppressed." -Manufacturing Consent: Noam Chomsky and the Media, p. 195

Borla

If the oxygen sensor is bad it is probably making the check engine light come on. A 96 is new enough to be ODB-II.

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VitaminH

I'm honestly not sure how old the plugs are. I did get new head gaskets about 5 years, and 35k miles ago. Would they have replaced the plugs then? If not then, then I'd guess it's been about 8 years, and rouggly 45-50k miles ago.

Any ideas how much new plugs would cost? I guess I'd just hit up Pep Boys or something and go for a tune up and plug check. I know some people on here aren't real fond of Pep Boys (I think I've seen some negative comments about them in the past) but keep in mind I'm only looking to keep this car running for another 3-4 months.

I really hate this car.

I suppose there might be a hole in my tank, but I've never noticed a gassy smell nor any spots under my car of unexplained fluids.


Edit: Whoa people are posting faster than I can keep up:

The air filter is relativly new, but I'll look at it when I get a chance, and will also smell the oil. Should I smell the oil cold tomorrow before I drive or should I drive it first, then smell (after letting the engine cool a bit)?

There is no power declination, only mileage.

Never changed EGR or PCV valves

Nor the O2 sensor

I'll see if I can find somewhere that has a free diagnostic around here that I can get the thing too after work this week. I've been taking public transportation most of the time, but will be doing some minor road trips on the coming weekends and I'd like to at least get it checked out before I start driving all over.

__________________
"Glory is fleeting, but obscurity is forever."
- Napoleon Bonaparte

cyrnel

You're right. My bad. Has to be OBD II which means more good Autozone info.

Definitely get a free code dump. Your check engine light could be bad, who knows, but if you've never changed sensors there's a good chance you have one or a couple past their useful life.

Did we ask how many miles already?

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There are a vast number of people who are uninformed and heavily propagandized, but fundamentally decent. The propaganda that inundates them is effective when unchallenged, but much of it goes only skin deep. If they can be brought to raise questions and apply their decent instincts and basic intelligence, many people quickly escape the confines of the doctrinal system and are willing to do something to help others who are really suffering and oppressed." -Manufacturing Consent: Noam Chomsky and the Media, p. 195

VitaminH

No actually and I forgot to mention. I'm at 96500.

__________________
"Glory is fleeting, but obscurity is forever."
- Napoleon Bonaparte

cyrnel

I wouldn't suspect plugs with mileage dropping off somewhat suddenly. Still, at 96K in 11yrs I'd be changing them and the O2 sensor, and whatever PCV/EGR valves it has. That assumes you didn't buy it 2yrs ago from someone who gave you great service records... (hint, hint?) I'd be looking for vacuum leaks, too. There may be other wear items to consider. Don't buy everything but pay attention to the parts they offer to sell after dumping codes, and get the codes, not just the interpretations.

Declining mileage can be evil but it's usually a matter of replacing a couple wear items. I'm willing to bet you'll notice a power improvement, too.

__________________
There are a vast number of people who are uninformed and heavily propagandized, but fundamentally decent. The propaganda that inundates them is effective when unchallenged, but much of it goes only skin deep. If they can be brought to raise questions and apply their decent instincts and basic intelligence, many people quickly escape the confines of the doctrinal system and are willing to do something to help others who are really suffering and oppressed." -Manufacturing Consent: Noam Chomsky and the Media, p. 195

Borla

I agree.

I would change the plugs (plugs should be between $3-$6 each probably) and at least check the plug wires. The air filter should be cheap, but should probably be changed as well. And as mentioned, get it checked at AutoZone for any codes, and if any are thrown, let us know or ask the tech about them. It's very possible the sensors mentioned need changed. Depending on which ones they are, it might be pretty cheap and easy to do yourself.

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billege

He about summed it up.

I just wanted to add that's pretty much what I'd be looking to do also, given mileage and age.

FWIW, each time (three different cars) I've experianced a sudden drop in gas mileage that wasn't from anything obvious, it turned out to be a failing O2 sensor, and not once did the check engine light go on. I couldn't tell you if that was because the car was too old to have OBD-II, or because an O2 sensor can fail enough to tank (PUN!) your mileage without failing enough to trigger the dash light. Either way, a new sensor later and mileage was right back up where it was supposed to be.

02, plugs, air filter...yeah. When you go to sell you can advertise it just got a tune up!

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VitaminH

Well I called Pep boys out of curiosity, they told me it'd be $200 for new plugs and a tune up on my car. That seemed awful steep to me considering I see plugs online for <$10 apiece.

Anyways, I spoke with a friend of the family who is a mechanic and he made me break down how and where I've been driving. I definetly have put more city miles and during rush hour on the car in the past month than I have in the past, plus it's been colder here. He also recommended before I try the tune up to use some of that fuel system cleaner (Gumout is the brand I believe) as I had never used it before, and I might have some gunk built up in there, esp as I just put in regular gas. So I'm about 2/3 thru a tank with Gumout and I've noticed an improvment. But I've also had more highway than previous as well.

I really don't want to throw much money into this, but if I don't notice it's back to normal after the first tank post Gumout, I'll try to get it somewhere for a free diagnostic.

__________________
"Glory is fleeting, but obscurity is forever."
- Napoleon Bonaparte

cyrnel

If you noticed a change right out of the gate then it's either placebo or a change in burn pattern from the gas chemistry. If I recall, gumout is mostly kerosene with a light carrier so it won't clean as much as cause a brief octane-boost effect. Techron is a better cleaner. I think it's still ~$10/4 at Costco.

Without major mechanical problem your driving pattern & style will have the biggest effect on mileage. Tough to know if you haven't been documenting things religiously. Take any short-term changes in mileage with salt. You might be better off just leaving things as is before selling, or try a few jobs yourself? A cheap socket/tool set plus an afteroon might be very educational... just a thought.

The diagnostic we're talking about will be free and takes about 5 minutes. Don't ask for a garage check. You just want them to dump codes & make suggestions. Pep boys is more likely to keep you captive to stack labor costs. I'd try a parts store without a shop.

BTW, $10/plugs are akin to gold plated USB cables.

__________________
There are a vast number of people who are uninformed and heavily propagandized, but fundamentally decent. The propaganda that inundates them is effective when unchallenged, but much of it goes only skin deep. If they can be brought to raise questions and apply their decent instincts and basic intelligence, many people quickly escape the confines of the doctrinal system and are willing to do something to help others who are really suffering and oppressed." -Manufacturing Consent: Noam Chomsky and the Media, p. 195

VitaminH

The change I've seen was definetly not right from the pump. At first the needle was moving at the same rate it seemed on the last tank, but now I'm about 2/3 of the way thru the tank and It seems to be not falling so fast (I remember where my trip odometer was at certain levels last tank, and it's higher now at those same levels).

I'll try and avoid Pep Boys then for the diagnostic, I said them because there's no Autozones near me. I'll check the yellows for a parts store near me.

I should also reiterate that this efficiency drop was not sudden, sorry if I gave that impression, but it's been rather gradual since about November. November is also the same time I increased my city driving and I'd scratched it off to that, however it seems to have deresed more over the past months, but when I broke down my recent driving habits with that mechanic I talked too, it seemed like it could almost be chalked up to that.

Cyrnel (or anyone), can you recommend a good website with a tutorial on doing some work myself. I admittedly know next to nothing when it comes to this, and I'd hate to get in there and muck something else up worse. I'm generally fairly technically minded so I'm fairly sure I'd pick it up relatively fast.

Off to google...


EDIT: I found an autozone not to far from here and when I spoke with them I was told that the Check Engine light had to be on in order for their computer to get the code dump. Can anyone confirm/deny this? My light is not on, I told whomever answered the phone my gas mileage was declining and I just wanted a code dump to see if anything was wrong, and they told me the light had to be on else they would get no reading?

I also see that chanigng plugs is pretty dang easy.

__________________
"Glory is fleeting, but obscurity is forever."
- Napoleon Bonaparte

cyrnel

It probably could use a tune-up including sensors but the shotgun approach isn't cheap. That's why the code dump could be useful. Does your check engine light show when you first turn on the ignition? They can burn out.

Also, codes are thrown differently between vehicles. Many times they may register without causing the light. I haven't worked on a Pontiac since before OBD so couldn't tell you. The store requiring the light first tells me they're trying to discourage diagnostics that don't sell parts. Try another, another shift of workers, or stoop low and say it lights sometimes and your mileage is really bad. There's always the "pull a plug wire" method so it sounds like death on wheels.

Your best source of info will include people familiar with the vehicle. That usually means an enthusiast forum of some kind. As always there's good and terrible advice, but there's usually a core of people to guide you. FAQs for things like "declining mileage" and "basic DIY." I can't point you to any but google would be where I'd start. Google something like ("pontiac sunfire" forum enthusiast -dealer -sales) without the parentheses. Or just go pour coffee and sift through "sunfire mileage problem"...

Oh, and beware haynes & chilton. Better than nothing, barely. Look for factory manuals even if it means ebay. The best money you'll ever spend. (Assuming you're keeping the vehicle for awhile.) Even if they do cost a bit you can turn a used set back around on ebay for what you paid.

__________________
There are a vast number of people who are uninformed and heavily propagandized, but fundamentally decent. The propaganda that inundates them is effective when unchallenged, but much of it goes only skin deep. If they can be brought to raise questions and apply their decent instincts and basic intelligence, many people quickly escape the confines of the doctrinal system and are willing to do something to help others who are really suffering and oppressed." -Manufacturing Consent: Noam Chomsky and the Media, p. 195

moot1337

Bentley makes some good manuals, though I'm not sure if they do em for American cars

As far as your problem with mileage, if you've never had it done, replace your air filter! I'm not sure if your car has a distributor rotor/cap, but if so, do that too - all of these are fairly cheap and should be easy to do with a screwdriver and maybe a wrench or ratchet.

As far as changing plugs goes, they're really easy. Pretty much all you have to do is grab a ratchet and socket (there are only two sizes for plugs - 5/8 and 13/16 in), go to the local autozone and ask em for plugs for your car... I prefer bosch platinum plugs - you don't really need the 2 or 4 prong ones, and they come pre-gapped (though it never hurts to check the gap with a simple tool also available there and the guide for gap size in your manual or on the bottom of the hood). If your cars plugs are pretty tucked away, it can be helpful to buy a wobbly socket and perhaps an extension, while you're at the car parts store.

Once you're in there, all you've got to do is pull the plug wire off, take the old plug out, pop the included washer on the new plug if necessary, put the new one in with light-moderate torque (try to pay attention to how hard they are to get out, which shouldn't be too hard, as many heads are aluminum. Tighten until you encounter resistance, and go just over that. I'd call it just beyond hand tight, with your hand near the head of the ratchet.), and push the wire back on. If your wires are very dry, brittle, or cracked, look into a new set of those as well.

Also, put fuel injector cleaner through your tank every oil change or two. Techron is a good cleaner as mentioned, though you may opt for Redline if you think they're seriously dirty. Check out the section at your local auto store, and get something midrange, maybe by Gunk, STP, or Techron.

Hope things look up for ya, and you can pawn the car off on some other guy!

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banana-shaped.
This new learning amazes me, Sir Bedevere. Explain again
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Oh, certainly, sir.

VitaminH

The air filter is relatively new, maybe a year old I think. I will look at it and see if it looks dirty or clogged up or anything.

Yes, I've googled changing plugs and it does look emberassingly easy. I may give it a shot here if none of my other options work.

I've gone thru the whole tank on Gumout and I got roughly 27 mpg, which is what I used to get on about a 50/50 split of highway/city driving. Is it ok if I just try one of the Techron cleaners in my next tank right away or should I wait awhile? I don't know if it's ok to chase one with another (there will be one tank in between)?

Anyway I'll be out of town at my parents' this weekend (assuming the car makes it..heh) and there's a much easier Autozone to get at out there so I'll be headed in for a code dump to see what they say. I'll probably just tell them the light is coming on and off if they won't do it without the check engine light on. We'll see what that says. I'll post the codes as soon as I know.

Thanks again everyone for all your advice and help.

__________________
"Glory is fleeting, but obscurity is forever."
- Napoleon Bonaparte