Taking opinions on poor mpg issue

[andrew_g]

my car is a 2000 pontiac grand prix n/a 3800 with 227k miles

have owned the car 4 years and has consistantly been capable of 280miles give or take per take

just had the common intake job done and car does not have in obvious running issues but my last tank of fuel only went 215 miles

possible o2 sensor issue due to coolant being blown thru exhaust due to intake? sensors are original to car as far as i know at very least i can confirm them to be 100k miles of used?

have used injector cleaner and almost always use 93 octane in it

open to other suggestions that your guys may have

 

[Clevy]

Is there a cel. If so get it read and start there. How is it running? Is the exhaust really strong with the odor of fuel?I'm just taking stabs in the dark here.

 

[JHZR2]

Do you really need 93 in that car? What does the OM request?

Good chance that the sensors are lazy or shot. Any CELs?

It also is wintertime, so your tires may be low on pressure, oil is thicker at startup, and IL may use a less energetic winter blend of gasoline.

 

[Falken]

What I would do, in this order, is:

1) Use Redline SL-1 or Gumout Complete Fuel System Treatment on 3 tanks of gas in a row (see dose recommendations on back of bottle) for a total of 3 treatments.

2) Change the fuel filter, it may be really clogged, or may become clogged after fuel treatments.

3) Change your plugs, cap, rotor, wires, coilpacks (depending on your igntion system) to make sure you have a nice hot, fresh spark for maximum combustion.

4) Maybe if it isn't too $$$ get new injectors to do along with #3.

5) Get a new O2 sensor.

6) Run 2 short OCIs of PYB of 3,000mi each.

7) Start using Pennzoil Platinum GRIII synthetic. Cleans and makes for easier starts. Also isn't a real syn so no worries about leaks.

If you do all of the above, you should have very good fuel economy. Hook up to a scanner if at all possible and research the codes, if any, before you start.

Fuel filters and researching your local gas is very important and very overlooked... Check owner's manual and see if you can use lower octane to save $$$ for the same mpg.

 

[08_SHELBY]

cold temperatures effect the mileage I get alot. I watch the tire air pressure closely and still lose 1-2 mpg. Some gas is just junk. I get the best results with shell regular. I get about .75 miles per gallon better year round and up to 1-2 mpg more on the highway. Cant explain it but it is just so.

 

[rshaw125]

I have the 3800 in a Buick. Mine actually gets decent mileage. But I check it by dividing out the miles. Much more accurate method than 'guess-estimate' miles per tank.
Easy on oil. Never used anything but 87 octane usually from Costco.

 

[sciphi]

Using 93 in a NA Grand Prix is always a roll of the dice. Some of those engines were very picky on what octane they liked, even the NA ones.

That NA 3800 ought to be fine on regular, but I've seen some oddballs that for whatever reason ran better on premium.

I'd check that the UIM is tight, and that there aren't any vacuum leaks.

OP, did you replace the upper intake manifold at the same time?

 

[bigjl]

Would second the tyre pressure thought.

I am a user of injector cleaners but not any one in particular.

I use Redex in the fleet due to the good price at Costco, it has been around for many years.

Also STP do some decent products for cleaning injectors and in light of the number of direct injected petrol engines around these days it also helps to clean up the valves.

I honestly wouldn't expect a high mileage petrol car to retain its economy but as it seems to have happened suddenly after work is down you would think they are related.

Does this vehicle have an ecu that would benefit from being out into learning mode with a prolonged battery disconnection?

I normally did that when I had work done, as otherwise it would take ages for the ecu to adapt to any changes.

On the diesel Pathfinder it is common practice on the forums to reset the fuel system after working on it.

The procedure was found in the workshop manual and is apparently performed by dealer techs.

 

[LeakySeals]

Yep, if you had a bad LIM for an extended period, swap the front/pre-cat air/fuel/o2(s). Use a Denso or an ac/delco, say NO to bosch universals!

 

[JHZR2]

Originally Posted By: Falken



7) Start using Pennzoil Platinum GRIII synthetic. Cleans and makes for easier starts. Also isn't a real syn so no worries about leaks.



Lol puhleeezzz.

 

[SwampSurvivor]

Alignment, too. That can damage gas mileage

 

[JohnnyZ52]

Mpg has so many factors to it. The 3800 engine is pretty bad on gas in the city yet decent on the highway. So maybe more city driving?

Winter fuel mixture? Colder weather? Backed up pressure or O2 issues could also do it.

Calculate mpg and use that as a reference. But a few extra short trips can make all the difference. I think it's rated at 19 city and 28 highway IIRC. I get 24ish combined in summer, haven't calculated in winter. The stock rims go bad and cause tire pressure to drop, I had to replace mine (on my '02).

 

[Spazdog]

Do you do almost exclusively city driving?

280 miles on a 17 gallon tank? Let's assume you fill up with 15 gallons. That's like 18.7 miles per gallon. 3800s usually get better gas mileage than that if you drive on the freeway.

O² sensors would be my first guess. Just a guess though. After 200,000 miles, any O² sensor can get lazy and lazy sensors = bad mpg.

Don't know that you really need 93 octane in an N/A 3800. My car actually gets better mpg with 87 (Mazda does have it in a relatively sedate state of tune presumably for that purpose because it is rated at 220hp...virtually the same Duratec in a premium fuel spec'd Jaguar is rated at 240hp)

 

[andrew_g]

plug wires and plugs were changed around 212k with ac delco oem equivalent parts.
no check engine lights or obvious running issues at all
it is running castrol syntec for the first time ever due to aap bogo sale and has always had fair quality and on time oil changes
in the past it seemed to get a slight increase in mpg with the 93 but i havent cared enough to check it in a long time

i believe ill start with pulling plugs to check them due to the intake having gone out and yes i replaced full intake and got the metal lower gaskets, and the fuel filter does need changed which i think ill follow with the regane for each tank afterwards for awhile and see what that gets me, then if still needs improvement ill tackle the o2 sensors

 

[andrew_g]

car has in my entire 4 years and 100k miles of ownership maintained an average of 23mpg and never seen above 28mpg on a long interstate trip

 

[mechtech2]

One tank after major surgery should be discounted.

 

[1999nick]

Before worrying too much, I would do a couple of actual mileage checks by filling up and checking exactly how much it takes to refill. Hard to be real accurate estimating by how much mileage yuo got on a tank refill, and not actually figuring using actual gallons required.

 

[jdeare]

This is not scientific by any means... but for some reason I've been getting terrible gas mileage with Shell gas. Every time I use it I get less than 30 mpg. Last tank of Shell I got 26.9 mpg, next tank on Mobil I got 33.7. My driving is pretty consistent but obviously there could be other factors at play here (temperature is the one I'm thinking of mostly; I check my tires every month so I'm pretty sure they're good right now). I'm in the Chicagoland area.

Also fwiw... I had a Buick 3800 previously. I noticed the gas mileage going down, and pretty soon after that I had a CEL for the thermostat/temp sensor (are these the same thing? not sure). So you may have something going on that hasn't revealed itself fully yet.

 

[MinamiKotaro]

I always get lousy mileage on Shell with my Saturn ... but I get a consistent 27mpg vs 24mpg in my '67 Beetle. The fuel pump is quiet, as well.

 

[Norm Olt]

Assume a 16 gallon fillup (out of 18 gallon capacity) since your o/p says nothing about the actual number of gallons in your "280 miles per tank"...

So, 280 / 16 = 17.5 mpg

& 215 / 16 = 13.4 mpg, a 24% reduction.

Your posting yesterday is talking about a "23 mpg average" over 100K miles and 4 years... That's a 42% reduction at 215 miles "per tank".

Something doesn't make any sense here: it just "don't add up"...

Uh, make that "divide right".

Maybe time for some professional help?

Cheers!