As Spook writes you providing wise guidance.

A cause of low fuel economy, as Nantanha indiates to you, is the failure of the fuel injector fuel pressure regulator (FPR).

At high air intake manifold vacuum (negative pressure we can call it? I dunno.), such as when you foot is off the gas pedal, the FPR opens allowing fuel to pass to the low pressure fuel return, lowering fuel rail pressure. The fuel ECU also changes the injector pulse duration. When you floor it, the intake vacuum is lowered closer (more pressure, yet a vacuum, usually, and the FPU responds by closing, reducing the amount of fuel to the low pressure return, increasing manifold pressure.

Blindboy, I'm unsure of your 1993 940 Wagon is Turbo or not. Bosch or Siemens / Bendix Rex (ignition) / Regina (fuel control). (Bendix Rex / Regina is like Bosch D-Jetronic a wee bit! Maybe with less fuss!)

Your 1993 940 includes the OBD-1 test ability. May want to check for fault codes as in in the FAQ for both socket 2 (fuel and emissions) and 6 (ignition and emissions). Does your instrument cluster display an illuminated CHECK ENGINE light?

One cause of fuel enrichment is a vacuum leak at any vacuum line connection at the manifold downstream of the throttle body (as well as upstream) as such a vacuum leak lowers the vacuum, so the FPR raises injector fuel rail pressure and injector pass more fuel into the engine.

The PCV on my factory stock (as I love it!) 1992 green 240 GL has been a vacuum leak headache since 2007. In the final round some months ago, I used new not the best quality PCV piping in the up to 1988 iPd relocation kit, a section of coolant line to secure atop the PCV breather box to the flame trap. I used hose clamps gently tightened.

Also, you may want to verify miles traveled using a cheapo GPS device or a mobile phone app.

I found odometers tend to lose accuracy with age and use, clocking more miles than actually traveled on RWD Volvo with electronic odo / speedo. I'm not sure of the 700-900 instrument cluster speedo / odo performance changes through age and use.

16 MPG in mixed use is a significant reduction in fuel economy in mixed motoring.

If you suspect leaks at the closed pressure side of the fuel delivery system, the leak would be significant and possible dangerous. You could wire the fuel pumps to run with engine off and body raised to see any fuel drips. You can wipe fuel line unions with a dry paper towel and check for gasoline wetness. Observe safety! I can suggest ways to jumper the Bendix pump or Bosch (both pumps) as you like.

Another cause for reduced fuel economy are faulty emissions control sensors:
- Oxygen sensor if exhaust header pipe outlet to catalytic converter union intake leaks. May not hear it. Yet between exhaust pulses, external air gets sucked in, the oxygen sensor encounters more 02 than should be normal with a properly sealed exhaust, and the fuel ECU enrichens the fuel to air ratio. Can also cause limp home mode. You can the the oxygen sensor using your digital electric multimeter (DMM). The 700-900 Series FAQ describes this procedure.

Yet do not test the ECU side of the oxygen sensor connector as cheap DMMs may not present the minimal resistance to do so and can cause fault in fuel ECU.

- Check Oxygen Sensor

Using a DMM with a disconnected 02 sensor connector and grounded to the engine, test the 02 sensor per the FAQ. Steady above .5 VDC indicates a rich exhaust. Steady at .5 VDC is a failed or failing 02 sensor. Below .5 is a lean exhaust. I'm better with Bosch than Bendix yet the values may be the same between the two. Bendix uses a very different 02 sensor.

- Check grounds and power.

Low voltage from the alternator, an old battery that does not hold a good charge or presentr improper load to the alternator, as in an old battery present very low resistance, reduces fuel economy. Poor found connections between the alternator, batter, and engine can be a fault.

On the engine control system side, poor signal, ground, and power connections can also limit fuel economy.

I guess that's all I got.

Hope that helps.

Questions?

Hope that helps.

Yes, practicing my tech writer skillz in these posts. Sorry for the long post.

Buttermilk MacDuffy's Tavern
(Glad and pleased and grateful Uncle OldDuke is BACK!!!!
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Jonathan Harshman Winters III: The Mightiest, Greatest, & Most Powerful North American Comedian & Comedic Actor in Perpetuity