Poor MPG

I was really expecting better gas mileage with this car. The highest I've ever seen is 25mpg going 55mph on flat road with the a/c off and no wind or passing. Currently it gets between 15 and 20 mpg with the cruise at 65mph, ac compressor not running, no wind, no passing. I've been battling this for a while. The car has new ac delco iridium plugs, ac delco wires, a new ac delco fuel filter, 2 new denso 02 sensors, a new 195 thermostat, a new pcv valve, cleaned the egr and reinstalled with a new gasket, and a new paper air filter. It also has a new alternator, 2 new belts, and all of the idler pulleys are new. I've also cleaned the MAF with MAF cleaner. I always use premium, I've sea foamed it once (half in the tank and half down the booster vacuum line) and ran a few cans through random tanks of gas, also ran a bottle of techron once. I keep the tires between 35 and 37psi. My boost gauge usually sits between 9 to 7 (not sure if in. hg or psi?) on the vacuum side with the cruise set at 65mph. The engine runs just fine and has plenty of power, doesn't burn more than 1/2 a quart between 5000 mile oil changes (syn. blend) even with a very miniscule pan gasket drip here and there when parked.

You aren't too far off the mark getting 25. Highest I remember getting with my '98 was 27 and I think there was a tailwind both ways. Higher can be had but it's tough. You should be able to get 24-25 at 65 mph if it's smooth/flat driving and no major headwind. I seem to recall averaging 22-24 hwy mpg, more like 16-18 city.

I get 28 with the cruise control set to 65. My best is 32 on a trip to Ohio and back. Left the cruise off and kept the engine out of boost.

Have you checked to make sure the boost bypass valve is working? If that is stuck closed(happened on my car) it impacts the fuel economy quite noticably in daily driving.

LARRY70GS wrote:

I get 28 with the cruise control set to 65. My best is 32 on a trip to Ohio and back. Left the cruise off and kept the engine out of boost.

28mpg is what I was expecting to get at about 65mph when I bought the car, especially since everybody I talked to that owned a series 2 praised the gas mileage.

th3fr4nchi5e wrote:

Have you checked to make sure the boost bypass valve is working? If that is stuck closed(happened on my car) it impacts the fuel economy quite noticably in daily driving.

It definitely is working, I get boost and have seen it move when revving the engine from under the hood.

stydel311 wrote:

I was really expecting better gas mileage with this car.  The highest I've ever seen is 25mpg going 55mph on flat road with the a/c off and no wind or passing.  Currently it gets between 15 and 20 mpg with the cruise at 65mph, ac compressor not running, no wind, no passing.  I've been battling this for a while.  The car has new ac delco iridium plugs, ac delco wires, a new ac delco fuel filter, 2 new denso 02 sensors, a new 195 thermostat, a new pcv valve, cleaned the egr and reinstalled with a new gasket, and a new paper air filter.  It also has a new alternator, 2 new belts, and all of the idler pulleys are new.  I've also cleaned the MAF with MAF cleaner.  I always use premium, I've sea foamed it once (half in the tank and half down the booster vacuum line) and ran a few cans through random tanks of gas, also ran a bottle of techron once. I keep the tires between 35 and 37psi. My boost gauge usually sits between 9 to 7 (not sure if in. hg or psi?) on the vacuum side with the cruise set at 65mph.  The engine runs just fine and has plenty of power, doesn't burn more than 1/2 a quart between 5000 mile oil changes (syn. blend) even with a very miniscule pan gasket drip here and there when parked.

given what you have checked so far:

- check your injectors.  One or more may be dumping fuel.
- check your alignment... any unusual shoulder wear on tires?
- how much do you weigh, and what do you normally carry around in the car?

albertj wrote:

given what you have checked so far:

- check your injectors.  One or more may be dumping fuel.
- check your alignment... any unusual shoulder wear on tires?
- how much do you weigh, and what do you normally carry around in the car?

How would I go about checking injectors? Wouldn't it run goofy if one was doing that?

The tires are pretty dry rotted so its getting new ones in a few weeks and also an alignment. I think I remember one of the passenger tires having ever so slightly more wear on either the inside or the outside, not sure if it was front or back but it wasn't anything horrible.

The car with me and a full tank of gas is right at 4,000lbs according to the scale.

Another thing that affects gas mileage is tire inflation. When the Riviera was new, the OEM tires had a maximum inflation pressure of 35 psi. The tire inflation decal from Buick says 30 psi. The newer tires today will commonly have a maximum inflation pressure of 51 psi. If you run 30 psi in them, they will be under inflated and that will affect gas mileage. The Yokohama Ecsta TRX tires on my Riv say 51 lbs. I inflate them proportionately at 43 psi. The ride is not hard.

LARRY70GS wrote:

Another thing that affects gas mileage is tire inflation.  When the Riviera was new, the OEM tires had a maximum inflation pressure of 35 psi.  The tire inflation decal from Buick says 30 psi.  The newer tires today will commonly have a maximum inflation pressure of 51 psi.  If you run 30 psi in them, they will be under inflated and that will affect gas mileage.  The Yokohama Ecsta TRX tires on my Riv say 51 lbs.  I inflate them proportionately at 43 psi.  The ride is not hard.

I do run them at about 35-37 psi, depends on the temperature since it swings so much here in the winter. I believe the max on the cheapo's that are on there is 44 psi.

stydel311 wrote:

I do run them at about 35-37 psi, depends on the temperature since it swings so much here in the winter.  I believe the max on the cheapo's that are on there is 44 psi.

Yup, that's about right. When you buy new tires, just check the sidewall. Lots of tires are 51 psi now.

If your tires are really old, new tires may help a lot. Make sure the new tires are "LRR" (Low rolling resistance) if you do a lot of highway miles and want the MPG to go up. I had sticky performance tires on my car and altered the gearing, so 24 never bothered me...

deekster_caddy wrote:

If your tires are really old, new tires may help a lot. Make sure the new tires are "LRR" (Low rolling resistance) if you do a lot of highway miles and want the MPG to go up. I had sticky performance tires on my car and altered the gearing, so 24 never bothered me...

The Pirelli's I am putting on in a few weeks are low resistance, these are kind of the last thing left to do before I can just drive it without worrying about anything.

Just updating this post, I ran Techron through a couple tanks of gas and that seemed to help a bit. Finally got new tires (Hankook Optimo H727's) and an alignment at the dealership, what a difference. The car is so quiet going down the road and rides so much better compared to the crappy tires it had on it. Just took a 400 mile trip at 60 mph with 3 people and quite a bit of idling and stop signs and got 28 mpg. Amazing what a set of quality tires can do, especially on an expensive car.

stydel311 wrote:

Just updating this post, I ran Techron through a couple tanks of gas and that seemed to help a bit.  Finally got new tires (Hankook Optimo H727's) and an alignment at the dealership, what a difference.  The car is so quiet going down the road and rides so much better compared to the crappy tires it had on it.  Just took a 400 mile trip at 60 mph with 3 people and quite a bit of idling and stop signs and got 28 mpg.  Amazing what a set of quality tires can do, especially on an expensive car.

You can also try using a good synthetic 5w30 or 10w30 oil.

albertj wrote:

stydel311 wrote:

Just updating this post, I ran Techron through a couple tanks of gas and that seemed to help a bit.  Finally got new tires (Hankook Optimo H727's) and an alignment at the dealership, what a difference.  The car is so quiet going down the road and rides so much better compared to the crappy tires it had on it.  Just took a 400 mile trip at 60 mph with 3 people and quite a bit of idling and stop signs and got 28 mpg.  Amazing what a set of quality tires can do, especially on an expensive car.

You can also try using a good synthetic 5w30 or 10w30 oil.

I have been running Valvoline MaxLife 10w30 Synthetic Blend since I bought the car and have been having good results. It helped with a small oil leak. I also use a Napa Gold (Wix) 1040 filter.