My ‘98 C70 HPT, auto, 218k has for the past few months been returning poor mileage results. Usually around town 24.3, now 19.2-21.
No visible codes shown. I’ve been slowly replacing vacuum elbows & lines.
Any ideas? Thanks.

When is the last time you replaced the front O2 sensor? Checking for vacuum leaks is a good thing, but also check the intake hose from the air cleaner to the turbo for leaks, cracks. 20 years for hoses is a long time, they tend to crack and dry out.
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Keeping it running is better than buying new

I have never replaced the O2 sensors. I have ordered two Bosch sensors from Amazon just in case I have to replace them.
Thanks.

The front O2 sensors are good for 100-150K, depending on driving habits.

IPD has a 'kit' made up of 3 silicone vacuum lines, yellow blue/green, and red, to replace the lines to the turbo. Not a fun job, it took me a few hours with the air intake out, but then was a good time to clean the PTC valve, it needed it!
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Keeping it running is better than buying new

Klaus is on the money regarding the front 02 sensor. Lower gas mileage is a symptom of a tired 02 sensor. They can go on the Volvo 5 cylinder engines. The front sensor usually doesn’t last as long as your car’s has. The front one on my 95 850 certainly didn’t. However, the original rear 02 sensor was still going when I sold the car with 250k +. The rears normally last longer. Since there are no codes you might consider testing them both before replacing so you are not throwing parts at your car.

I also like the 20 year old hose idea. They definitely wear out...
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Will I buy another Volvo??? We'll see....

Thanks to all for the input on the O2 sensors. I replaced both O2 sensors with Bosch units this past weekend along with an oil change and tire rotation. Old ones looked tired. Gas mileage is almost back to normal.
Thanks again.

Besides vacuum lines, has the air filter, spark plugs, wires and fuel filter been replaced lately? Not sure if your car has a distributor cap and rotor, but if it does that too!
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Will I buy another Volvo??? We'll see....

I religiously maintain my C70, still looks new inside and out. I do the regularly scheduled maintenance myself. Mobil 1 oil change every 5k, plugs, cap, rotor = 30k.
No codes to speak of. Thanks.