1997 850 Wagon NA Still getting crappy MPG

dairyqueen814 · Post by **dairyqueen814** » 18 Mar 2014, 07:06

Currently still getting around 20-21mpg in my NA 850

Things I've done:
Cleaned out PCV/replaced vacuum lines
Started running mid grade gas instead of regular
Full synthetic oil change about 500 miles ago
Removed my pre-heat hose/sealed off the pre-heat port/removed thermostat flap
K&N air filter
I keep my tires inflated
I have stage zeroed it except for the plug wires as they were relatively new when I bought the car (yes they are bougicord)
I drive 60mph like an old lady on the highway and drive for mpg in the city (yes I know all about hypermiling)
Car idles between 950 and 1000rpm
I even put a new fuel filter in
And the only extra weight is the spare tire and the widow maker in the back.

Should I switch to premium gas? Will it pay for itself?
The only other thing I have heard people suggest is O2 sensors but I'm not getting any codes

Am I missing something?

Thanks
Nick

**EDIT** when I did the oil change I used 10W 30 full synthetic and replaced one of the quarts with MMO

Post by **rspi** » 18 Mar 2014, 08:23

Where are you located?
How far is your average drive?
Have you checked compression?
How old/many miles on you front o2 sensor?
How old/miles on your MAF?
Have you cleaned your MAF?
AUTO or manual trans?
What brand spark plugs?

Post by **rmmagow** » 18 Mar 2014, 08:30

Boston traffic? Not bad MPG. O2 sensors can get lazy but still not pop the light. Premium gas, say 89 octane would be better than the 87 or so regular, but not much. On a good day, you'd be hard-pressed to get much beyond 24 with a lot of light pedal work. In town, in traffic is death to gas mileage. Raise your tire pressure to high option inside the gas flap, that will help some.

dairyqueen814 · Post by **dairyqueen814** » 18 Mar 2014, 08:35

rspi wrote:Where are you located?
How far is your average drive?
Have you checked compression?
How old/many miles on you front o2 sensor?
How old/miles on your MAF?
Have you cleaned your MAF?
AUTO or manual trans?
What brand spark plugs?

I'm in central Mass

Drive to school is about 25 miles. About 80% is highway at a leisurely 60 mph (actual math not just estimation)

I have not checked compression

I have not touched the O2 sensors at all. They could be stock.... which is likely... the guy I got it from knew nothing about cars and the car only has about 100,000 miles on it.

Not sure about miles on MAF but I just cleaned it hoping it would help (forgot to add that to my list)

Automatic transmission

I THINK they are NGK plugs. I asked the guy at consumer auto parts who is a seasoned 850 veteran and he recommended them. Always open to new advise though.

dairyqueen814 · Post by **dairyqueen814** » 18 Mar 2014, 08:41

rmmagow wrote:Boston traffic? Not bad MPG. O2 sensors can get lazy but still not pop the light. Premium gas, say 89 octane would be better than the 87 or so regular, but not much. On a good day, you'd be hard-pressed to get much beyond 24 with a lot of light pedal work. In town, in traffic is death to gas mileage. Raise your tire pressure to high option inside the gas flap, that will help some.

Not in traffic
Running 89 octane
Should I just spend the $70 on the front O2 sensor on amazon? (yes it's Bosch)

Post by **erikv11** » 18 Mar 2014, 09:12

The thing about the O2 sensor is, getting a new one is the only way to be sure. But 105k is really not bad for one of those, your call.

Cleaning the MAF is rarely helpful but the fact is they almost *always* throw a code when bad: I wouldn't worry about that.

There is no reason to worry one iota about compression.

I would swap the plugs as I have zero experience with NGKs (maybe someone else does). Bosch coppers are cheap and they always work.

Post by **rspi** » 18 Mar 2014, 19:48

Any 15+ year old sensor will be sluggish in my opinion.

The calls for 91+ that may make a difference.

You should be getting 27+ mpg with that trip.

Do you have a trip computer in the car to calculate mileage?

Since you are having mileage issues, I would get Volvo plugs. I have seen as much as a 20% difference with OEM Volvo plugs.

How old is your thermostat? Is it OEM? A car running a little cold in your climate can make a big difference. The gauge read 3 O'clock at 173 degrees. The car should be running around 188 degrees on the freeway. Do you notice that temp gauge move at all on the highway?

You should consider checking compression. A worn motor will still fire (not throw a misfire code) but will NOT be as efficient as a strong motor. My buddy has a '96 R and his compression numbers are around 125 - 138 (155,000 miles). My numbers are 172 - 179 (245,000 miles) and he tells me that my car runs stronger than his and he gets around 19 mph while I get around 22 mpg around town.

When I do 20/80 driving on a 18 mile trip going around 65 mph on the freeway, I get between 28 - 32 mpg.

Being that it is cold has heck in your area, I would really lean towards the thermostat. Check your temp with a scanner while driving on the freeway.

Post by **rspi** » 18 Mar 2014, 19:53

Here is a thought, if you can get your mileage up to 28 mpg, you will increase your mileage by 35%. Imagine if you were at your gas station and one of the pumps was priced at $2.28 instead of $3.50, which pump would you use? Also, how long would it take you to save $100 or $200 if you were using that pump weekly instead of the $3.50 pump?

Buying a set of $45 OEM plugs and a $18 thermostat may pay for itself real quick.

dairyqueen814 · Post by **dairyqueen814** » 19 Mar 2014, 05:41

rspi wrote:Here is a thought, if you can get your mileage up to 28 mpg, you will increase your mileage by 35%. Imagine if you were at your gas station and one of the pumps was priced at $2.28 instead of $3.50, which pump would you use? Also, how long would it take you to save $100 or $200 if you were using that pump weekly instead of the $3.50 pump?

Buying a set of $45 OEM plugs and a $18 thermostat may pay for itself real quick.

Right and I'm almost positive I had my NA automatic 850 up to at least 26mpg regularly.....
I'll look into getting plugs next pay check (genuine Volvo from FCP?) Free shipping and I like to give them business.
Also how many tanks before you see the boost from switching gas types?... Just filled up after I posted this and saw a pretty strong 1.5mpg increase... This will be my third tank on mid grade (89 octane)
I might also just get the front O2 sensor so I can have it on record that I've done it.

Post by **rmmagow** » 19 Mar 2014, 07:30

My car's a 98 turbo.
I found the Volvo plugs made a world of difference in my overall performance. My mileage went up a tiny bit going to 93 octane, not enough to justify and I am running fine w/89. My drive is 100 miles a day, rt 95 from PVD to rt 128 and about 2 miles of 128 in traffic. Currently reading 23.5 mpg with the thing having been reset back to zeros about 1500 miles ago. My O2 sensor could be original and at 200K + way overdue for replacement but no codes. Although I'm not a true lead-foot I do like to drive at near 80 MPH when I can and generally am at 70+ on the highway. I run the tires at 36 front, 41 rear.
I have 98 s70NA but I don't drive it enough to figure out gas mileage. My wife says she fills it only once in a while and doesn't complain that she thinks it runs bad or anything. The thermostat and ECT are a big deal with these cars as rspi stated. A real time recording OBDII reader could be used to gather running engine info. You would be interested in seeing ignition timing, fuel trim info, air temp sensor, air volume going through MAF, O2 sensor action, front and rear. Ignition timing could conceivably be retarded by faulty knock sensor operation, unlikely, yeah, impossible, no. I can't actually think of any one single big change to make for improved mileage. Your tranny isn't slipping and it is locking up on the highway right? Lock-up function is fairly important to highway mileage.