My last full tank produced nearly 200 milesafter mixed city and short interstate stretch driving, worse than 8 cylinder S class Benz! I was getting nearly 490 miles on this cars, same driving stile (1998 T5, manual transmission). Cleaned MAF sensor, but I think that did not help. I can’t see leaky vacuum lines. No CEL. Engine temperature is normal. No power loss, acceleration is excellent. However, the engine runs clearly rough, I can hear something that sounds like irregular spikes, maybe misfire? The spark plugs are factory ones and have less than 3000 miles. Wires looked fine (have 02/2004 stamped on them) when I replaced plugs and they are Original Volvo as well. I wonder if my valve seals are bad, but if they were bad the engine would be burning oil, which does not occur. Ran one large can of Exxon injector cleaner with Textron, but I think it is a joke. Can anyone suggest what else to look? I am dragging my feet but it seems a visit to dealership is immanent…

 

New to this. Had the same problem but with black smoke coming out of the tailpipe. Dealer in Daytona couldn't fix it and told me it was a bad computer ($1300). Problem want away for awhile till my MAF sensor went bad. Would run for 1/2 hour then die. I had bought a spare ECU on Ebay.

Ended up being an O2 sensor. I think the front one.

 

O2 sensor would be what I would suggest also. Also try pulling codes even though theCEL is not on. Have you cleaned the throttle body?