Can one bad injector be causing this 3 to 3.5 MPG?

It could be, I couldn't tell you. Some people just don't have any luck with Volvo. Try to replace it and see, I believe you can disable EGR, you may look into that too. Some people report good fuel mileage, but you have issues. Check your fuel and air filters, replace your muffler, do an over head. Then, go from there.

 

if they say the injector is bad. i certainly wouldn't keep driving it with a bad part. you're only going to end up with more problems. and kicking yourself in teh butt for not fixing what was wrong in the first place.

if your getting low mileage. i'm guessing your buring up a lot of fuel. which means diluting the oil inside the engine. and that's not good for the engine.

from what i've seen of my muffler. i put a high flow on. and the inside is really no different then the muffler i pulled off. it's basically a hollow casing. only difference is the high flow is shorter. therefore a little noise compared to quiet. but certainly nothing in there to get plugged up. causing an exhaust restriction.

i figured my muffler would have been clogged from all the smoke i was blowing from the dead turbo. but the muffler was totally empty of soot.

air filters can decrease mileage but they'd have to be totally plugged off to see that low of mileage. my w9 had a dead turbo and i was down to 4.5. i'm up to 5.5 with the turbo and muffler and air filters. finally got the oil changed from a dead turbo and now i'm up to 5.9.

but that's over on the east. i'm back in the mountains now. i'm expecting to see 5.5 now.

 

dont know about 3-4 but i can tell you it will make you loose over 2 mpg

 

I'd go in and put in a complete set of 6.

If the one is dead, how soon before the other 5 start giving you fits as well.

 

Probably an open in the injector coil. Is it skipping? If it is a dead injector, I would think you would hear the miss. And yes, it probably would get terrible fuel mileage.

 

Take one of those temperature guns and check the exhaust temperature on the exhaust manifold at that cylinder. If it is much cooler than the other 5, bad injector.

 

Don't make a harsh judgement on 140 miles results. Give it 1,000 or so to make sure.