Getting only 15-16 MPG combined driving...

[Rocky]

What's your oil consumption like?
All that oil must be cleaned out of the intake system including the oil trap etc.
What other basic maintenance is overdue?

[mercuric]

"closed loop" simply describes the operating mode when the fuel injection ECU is actively using the O2 sensor input to determine fuel mixture. It's call "closed loop" because the ECU is essentially operating in a feedback loop -- A standard ("narrowband") O2 sensor operates over a very limited air/fuel mixture range, so what the ECU does is bounce between "lean" and "rich" (within the context of the very narrow range the O2 sensor can measure) to feel out where to set the fuel mixture. In fact, the sensor makes a very abrupt transition from "rich" to "lean" (and vice-versa) -- The line curve is not linear, rather the sensor is designed to flip between high (rich) and low (lean) voltages abruptly at the stoichiometric (14.7:1 air/fuel ratio) point. In closed loop, with a working O2 sensor that has fully heated, you should see the voltage bounce between ~0.2 and ~0.7v on a regular basis, a time per second or so. If the voltage is steady, it's either not heated or something is out of whack. Remember you need to use a high impedance digital meter with fast response, or an analog (dial) unit with a buffer circuit -- IE not a needle meter that directly attaches the needle to the circuit, thus loading it down. O2 sensors put off almost no current and are very high impedance devices. The best tool to use is a digital oscilloscope, so you can see the waveform, but unless you're a serious electronics geek or work/study somewhere you can borrow lab equipment from, that's cost-prohibitive. Personally I use a radio shack 22-174 digital multimeter, which has a (somewhat) fast-responding bargraph that reads quicker than the digits come up. I'm not sure on Motronic 4.x how fast the loop rate is -- I haven't had the fun of digging into my 850's fuel injection system much yet, but on a LH 2.4 car like a 740/940 I'd expect to see 1 or 2 cross counts (bounces between lean and rich) per second. If someone has been through this on Motronic 4.x, I'm honestly curious.

"open loop" describes the state when, for various reasons, the ECU is not in the feedback loop. This is usually because the O2 sensor hasn't heated up yet (~600 degrees) and therefore is not providing a valid signal... Or the O2 sensor or it's wiring have become problematic. Other faults, which should throw a diagnostic code, can also prevent the ECU from entering closed loop for one reason or another. Volvo documentation describes this as "fuel trim disabled". But don't worry about that, you'd be throwing a code if the ECU has disabled fuel trim on purpose.

If you google something like car o2 sensor loop or similar you'll come up with all sorts of various articles that go into more detail on sensor (and overall EFI system) operation.

And to add to cn90's conditions for almost 30mpg: flat grade One thing I really noted after getting an 850, my first car with a proper digital fuel consumption gauge driven from actual injector duration, is that hills _really_ kill gas mileage, especially when you love to power up them for fun

[Ben850]

I knew something was still not quite right with the way my engine was performing. Very inconsistent throttle response, and differences in power and mileage.

At this point, the ignition parts are all fresh. Plugs, rotor, cap, Bougicord, All the good stuff but still the inconsistencies.
This thread hit me like a brick. (ha) It makes perfect sense by the description of "open loop" my engine has been in this mode since I've owned it. (A year on the road now.) There would be an exciting period during the day when all things engine related would fall into place, and the drivability would be right on. Then moments later, the nice throttle response would be gone.

I've done the throttle plate modification, the VVIS modification, which both made a nice difference, but until installing a new Bosch front O2 sensor this morning, It could never be right.

Old Front O2 Sensor.

So I am pleased after doing an ECU relearn, and some city and highway driving today, it is acting like a normal, well behaved engine. I'm quite sure the mileage improvement will be noticeable, I don't have a trip, or mileage estimator on the '93, but the gas gage should tell the tale. It is a load off my mind, the "ah-ha!" moment I knew would come.

[Volvich]

When was the last time you changed your air filter? Let's keep it simple first!

I changed air the filter 3-4 months ago and it still looks clean.

What's your oil consumption like?
All that oil must be cleaned out of the intake system including the oil trap etc.
What other basic maintenance is overdue?

Oil consumption has been qt of oil in ~2-3 months. But recently I replaced bad oil cap seal and a broken pcv elbow and I expect it to be less.
I did an alignment 6 months ago. Tire pressure is good.

I'm going to get underneath the car to get to the O2 sensor connector.. I cant see it from above. Does any one one know an easier way to locate/reach the connector?

[JimBee]

I couldn't reach my O2 connector from underneath the car. I had to reach down from the top. Be careful when you separate the connector. You have to pull the red slip lock back. It has grooved tracks inside that will partially unplug the connector as you work the slip lock out. Mine was really stuck and took a lot of effort in a confined space, mainly because the turbo hose was in the way. It's easier to work on in non-turbo's.
I actually didn't end up changing my sensor. The connector contacts were quite dirty, cleaning them and adding a little dialectic grease made a huge difference in engine idle, which had been very erratic.

[Volvich]

I couldn't disconnect it from above. It's too narrow there. I got underneath, connected probe wires with "quck splices" on the black (signal) and grey (ground) wires. The voltage was 0.01v rarely getting to ~0.30v... when I touched the ground at the battery, voltage was bouncing between 0.9v to 1.6v but not 0.1 - 0.9 volts... Is this normal? If it is, then I assume the sensor has bad ground...

[abscate]

1999 V70 T5 at 65mph reporting 25 mpg

145k, original O2 sensors, 50f ambient

[cdm]

These cars do not like going fast. I get sub 20mpg at around 80mph but 35+ at 60-65 on flat ground.

with me doing 80% city combined ends up at 26-27.

[Volvich]

Thanks for your posts people. I did change spark plugs (NGK Irridium), put new bosch cap n rotor, fuel filter, took off the throttle body and cleaned it (wasn't that dirty), put a new gasket.

And still... getting only 15-16 mpg. Well, if I do lots of fwy it gets to ~18 never exceeds 20mpg.
Don't know what else could be wrong there. I had the lower control arms replaced w/ genuine volvo parts and alignment done. no check engine light, no codes.

Anyone having the same problem? Any ideas where should I look next?

[abscate]

Can you take a picture of the old plugs and post? Sounds like you are running rich.