19mpg to 16.5mpg after trip, causes?

[cheezwizard]

Hi all,

Just to remind, bought my '99 3.5 GX (89k now, bought @86k) last april. Dig the car plenty.

I keep constant track of me mileage, filling the tank and resetting the Trip Comp. every gas stop. I'd been getting approx. 19-20MPG since I'd bought the car.

Last month, I took it on a trip up to Maine. 700 mioles there and back from Hartford. Got a bit over 26.25mpg, going around 75mph the whole way (travelled late at night, 0 traffic).

The trip was nice. I especially liked the Cruise Control. This is my first car with CC and barely having to use the accel was swell as hell.

Anyhoot, I get back from Maine, back to around-town driving, and I'm seeing the fuel needle fall faster. At the next fillup, I'm expecting to see the pump stop, knowing how many miles I've driven since the last top-off. But it keeps going. Naturally, that bugs me.

I've refilled the tank, probably 3 times since then. First calculation was 16.5mpg. The next two fillups, I've gotten 18.25 and 16.75mpg.

At this point, let me apologize for making a looong post about a simple question.

I had the oil changed (put in Mobil 1) less than 3k (4 months) ago, at GM dealer. Air and oil filters were replaced.

Before my trip, I stopped back at the GM dealer to be sure she was ready for the road. A very nice service manager at this patigula dealership checked my car out for no money. PS fluid was low, topped it off, all else checked-out fine.

I am auto-inept. In another thread, I think it was the 'how many miles on your intrigue' thread, a nice fellow mentioned he'd replaced several parts due to low mpg (filters pcv valve etc.) but then found it was the valve gasket seals which were to blame.

I may be wrong. I'd written it down but accidental-like threw it out.

So, at long last - is there anything simple I should do before bringing her in to the shop for a diagnosis? I've been blowing thru thousands lately and could do without another bill.

There's no 'check engine' light on, and she runs as well as ever.

Thanks a bunch for your time, much appreeshed,

CW

[jmlangeveld]

Try some fuel system cleaner. Chevron Techron concentrate or Gumout Regane are two of the better ones (some cheaper ones are not all that effective).

Also, one of the biggest drains on fuel economy is tire pressure. Check your car's tire pressure. 33 PSI for each tire is a good balance of fuel economy and ride.

Finally, with your car's mileage, it is probably coming up on it's first tuneup soon. New Sparkplugs can improve fuel economy.

[cheezwizard]

Thanks ALLOT for the fast response JML! I mean to remedize this problem ASAP.

Something I'd read, or heard on car-talk, maybe, in the past lead me to believe that fuel treatments were pretty much worthless. I don't recall exactly what it was, just a random googling probably.

But then, WTH do they know. I've seen posts a'plenty from seemingly knowledgeable car guys claiming that synthetic oil would be a major liability in a previously Dino-oil engine (even airline mechanics).

The Mobil-1 site said that's a myth, so I put the synth in there. Runs great, no leaks.

I've digressed, shame on me.

I Filled all tires up after I bought her. Both right tires had slow leaks (yokohama grand touring). They dropped from low 30s to low 20s PSI in about a week. I had that fixed before I hit the road.

They sealed the, umm, part of the tire that meets the rim for $8. They've stayed full since.

I'll try one of the FSCs you've mentioned. She's a great car. I'm a proud Papa. All systems are go other than the mileage drop.

And once again, thanks ALLOT for responding!

Cheez Wizard

[Ruley73]

Yes I would also suggest a fuel system cleaner. Some do work better than others. I usually suggest the Chevron Techron (black longneck bottle with red & blue label and white print) Lucas (longneck plastic see-through bottle with white label and blue & red print), or Seafoam (metal canister bottle with white child-proof lid, white label with red & black print on it). The latter two are our best sellers at work and I've used both with good results. The Chevron is supposed to be really good stuff too but I haven't used it before. You will need a funnel if you opt for the Seafoam.

If the fuel system cleaner doesn't solve your problem. Checking your spark plugs would be the next step. I recommend only double platinum plugs for this engine (and NOT Bosch!).

I believe I was the fellow that posted about the valve cover gaskets. It actually turned to be a bad balance shaft seal in conjuction with the leaky valve cover gaskets and it was fixed just yesterday (a dispute with the Warranty delayed the repair 3 months!!!). I doubt that either of these will affect fuel economy though except maybe if oil is getting in the spark plug chambers, even then, its hard to say.

Anyways, she's (it's my girlfriend's car '99 GL 3.5L) been averaging about 15 mpg (all city driving) and she doesn't drive it hard whatsoever. I've been suspecting a sluggish Oxygen Sensor for a while because no service light was coming on and I maintain it well. Sure enough, after about 6 mo. of the bad MPG her SES light now comes on intermittently. The code that comes up is P0141 - O2 sensor Heater Circuit Malfuntion Bank 1, Sensor 2. Which basically means that the heater circuit isn't working right on the Oxygen Sensor after the Catylac Converter so the sensor takes MUCH longer (several mnutes vs. a few seconds) to reach minimum operating temp (600*F I believe) and give the computer correct rich/lean readings. We're going to get this replaced and go from there. Her car had about 87K miles (95K now) on it when she first had this problem so I wouldn't be surprised if this is your problem.

Good Luck!

[Burnout524]

Hey alright, A fellow Connecticut Intriguer!

I think it's a combination of a lot of things. Things like weather which affects tire pressure (low psi, low mpg) and also cars run better with warmer weather.

I've noticed that my Intrigue (a '99 with the 3.8) is getting about 16.3 MPG (a helluva lot lower than the 19 MPG EPA city rating ). But I've only calculated the milage once and I let it warm up in the morning and have 'heavy' usage since the most I drive in a day is about 15-20 miles a day.

I highly reccommend checking out Tips to improve your Gas Mileage.

[paulkramer]

Fellow Connecticut Intriguer......yeah, I'll bet you all are a rare breed!! Domestic cars on either coast are pretty few and far between. Probably even more so than when I went to school in CT. I bought domestic out of fear for my life - here in the Rust Belt, people are very touchy about foreign cars, especially Japanese.

Wow! I got around 25 MPG the last time I checked in my 3.5. That was a weekend of around town and highway (80 - 90 MPH) driving. Slower than usual - I was consciously trying to improve my fuel economy. My next step toward that end will be to buy a smaller, 4 cylinder car. With Variable Valve Timing.

[cheezwizard]

Well dog gone it. Been a couple of months and at my last fillup I only got 89 miles out of 6.3 gallons. That's a whopping 14.1 MPG! Down 25 friggin percent.

Since me original post, I tried the Chevron Techron concentrate, then took to dealer. They checked the fuel filter and said it was clean, put in GM platinum plugs.

I'd thought the mileage had improved, as I'd taken another trip to Maine (after the new plugs). Got around 22MPG on the trip. Now I get my worst mileage ever.

Looks like I'm gonna be out some more cash, really don't need that right now, but can't settle for 14mpg either.

Any suggestions would be appreciated.

[Burnout524]

I managed 13.76 MPG on my last tank. Woohoo! But the reason it's so low is because I let it warm up a good 10 minutes every morning ever since I got the remote starter.

[cheezwizard]

Yeah, the service manager asked if I'd been letting it warm up. I never do though, despite being in a deep freeze. I just start it and go.

[jmlangeveld]

Originally Posted by cheezwizard

Yeah, the service manager asked if I'd been letting it warm up. I never do though, despite being in a deep freeze. I just start it and go.

http://www.valvoline.com/pages/produ...asp?product=21

I have had a noticeable improvment with gas mileage by using the Valvoline Synpower Oil additive (Synpower not Maxlife). You can get it at Autozone. It works well because it increases the Moly in your oil which reduces friction (Honda formulated oil uses a lot of Moly straight from the factory).

Also, again what about your Tire pressure and Alignment. Those can both greatly rob fuel economy. It would be worth getting an alignment to save the money you are spending on gas. Do you feel like you might need one?

Air Filter? Is it new / clean?

[cheezwizard]

Thanks jmlangeveld.

Hmm, I don't know if I need an alignment. The car drives straight w/ no hands on the wheel, not sure if that's a good test though. I do check my tires regularly, but maybe they need more frequent checks.

I was thinking of changing the air filter too. Put a new one in with the oil change about 4k ago (mobil 1, hoping to get 7,500k between changes). I thought they checked it when the new plugs went in earlier this month, but I guess I should check it myself.

Do you think the synpower would make a diff considering I'm already using synthetic oil? Was hoping to go 7,500 on the mobil 1, but it's looking a bit brown. Maybe I should change it sooner?

Could someone tell me how to change the air filter? I know it's simple, but I haven't done it yet myself on the olds.

Thanks again everyone, and sorry for all the questions.

CW

[jmlangeveld]

You should only need to change your Air Filter every 30,000 miles.

Also, something that can go bad and cause your engine to burn rich (i.e. burn too much fuel) is the Oxygen Sensors. Depending on the car, these usually last about 50,000 to 100,000 miles before they need to be changed.