1997 850 Wagon NA Still getting crappy MPG

[rspi]

I'd check operating engine temp before anything else. Even when I drive "hard", with no highway travel, I get 20 mpg. Oh, I always drive in S mode.

[dairyqueen814]

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.

I'll get a hold of my dad's OBDII reader and try to get something off of it.
in regards to my transmission what do you mean slipping? And I'm assuming lock-up is staying in high gear when cruising?
Im pretty sure everyhing in the tranny department is fine. The car drives great... even better since I switched to 89 octane. I'll definitely switch to Volvo plugs before anything else.

[dairyqueen814]

I'd check operating engine temp before anything else. Even when I drive "hard", with no highway travel, I get 20 mpg. Oh, I always drive in S mode.

I'll check the engine temp. I know its on the colder side since I removed the pre heat hose and it's winter but when I'm driving around town it's right at 3 o'clock. Also I keep it in sport mode because people have said it improves MPG if driven correctly. All it does is change the RPMs required for shifting right? Should I switch to ECON for highway?

[dairyqueen814]

Also another thought. I replaced the flame trap hose and the brake booster hose. all the other PCV hoses were up to snuff and I'm pretty sure there were no leaks. Are there any other systems that have vacuum lines I should look at? Or does that pretty much cover everything?

[rspi]

Unless the ambient temperature is UNDER 0 degrees Fahrenheit, the engine temp should stay between 183 - 220 (locked in at 3 o'clock). The air box thing you blocked only effects the intake air temp, not the coolant temperature. A cold motor runs rich, more fuel and could cause burnt valve issues.

You should get a engine code if you have vacuum problems.

[rmmagow]

On the highway, press on the accelerator about 1/4 or so and rapidly go from say 45 to 60 mph or so while watching the tach. The car should hit 60 at say for example 3000 RPM then there will be a little drop in RPM to say 2800 indicating the lock-up engaged. Another way to "test" is to be cruising along at 60 and very gently press on the gas. The tach shouldn't move much and speed should gradually increase. If the tach jumps about 100 RPM right away that would seem to indicate lock-up isn't engaged. Now, I don't know if Volvo reacts the same way as my other car but if I was cruising along at 60 and gently pressed the accelerator then very gently pressed the brake pedal you could actually see the rpm jump as the lock-up disengaged as the brake action would tell the tranny to unlock. Lock-up function can be pretty subtle if you're not looking for it and without lockup the car "feels" pretty good as it accelerates gently, but lock-up is critical to improved mileage. Lock-up function is what has made the MPG penalty of automatics virtually disappear. PS none of my RPM numbers mentioned here are real, just used to help give an example.

[dairyqueen814]

Unless the ambient temperature is UNDER 0 degrees Fahrenheit, the engine temp should stay between 183 - 220 (locked in at 3 o'clock). The air box thing you blocked only effects the intake air temp, not the coolant temperature. A cold motor runs rich, more fuel and could cause burnt valve issues.

You should get a engine code if you have vacuum problems.

OH I misunderstood your question. Where is the thermostat you are referring to? What does it monitor? Is it an easy fix? because when I'm driving on the highway the temp is pretty low on the gauge..

[thedancingfiddle]

Then there is your problem! The engine temperature thermostat went bad on my 850 a couple years ago and I was getting really really bad milage. I also ran it this way for too long, and it ruined my spark plugs as well. So I would get it fixed asap.

The thermostat is on the right (passenger side, or 'front') of the engine. I'll let someone with more knowledge explain exactly how to get at it. I had my mechanic do mine - they ended up having to replace the housing and temp. sensor as well, so it was quite a bit of money (the thermostat housing is over $200 I think), but worth doing asap!

[rmmagow]

Rspi's got a great video showing how to do this job. It can be easy but plan for the worst. Buy a thermostat housing just-in-case, but new thermostat bolts no matter what, buy the ECT sensor and genuine Volvo thermostat. FCP's got everything and don't hesitate to call them, very helpful folks. I think you can find rspi's videos on you tube by just searching rspi. Buy new radiator hoses too while your at it. Wouldn't be a bad idea to flush the cooling system and putting new volvo coolant in there too. Yeah, you really do want to fix this first. The temp gauge should always be right at 3 o'clock and not budge. A higher or lower is bad. My 98 never moves no matter how hot outside or cold for that matter (once it's up to temp). Don't skimp here, bad cooling system has killed a lot of Volvo's.

[dairyqueen814]

AWESOME
genuinely pleased
This is the most deductive and helpful place on the internet.... at least for volvos!
Ill look into it and report back when I can say for sure. Could be a few weeks but all the evidence points to this being the problem. Thanks for the input and help guys.

Nick