MFA MPG too low?

[jamesoff 10]

My MFA seems to be reading too low recently - an average drive where I'd have expected to get around 25-30 MPG, say, is coming out more like 15-20 according to the MFA. Because I use the car infrequently, I haven't been able to verify that this is actually wrong, but given how much out of my last tankful I've used, and the distance since (I zero the trip counter when I fill up), I'm on track for a bit over 300 miles from a tank, which is what I'd normally expect.

 

I've had this before, and the problem was one of the vacuum lines had disconnected from a T-piece. Reattaching it fixed everything, but in this case that doesn't seem to be the problem. I've checked everywhere except the back of the clocks (I hate getting in there, but I wouldn't expect that connection to have failed) and it's all securely attached and in good condition. I replaced quite a bit of it after the previous problem. The car feels like it's running well too.

 

I popped the MFA into the diagnostic mode where you get the vacuum pressure (?) and instant MPG values, and went for a quick drive. I couldn't get the instant MPG over ~20, and even coasting down a gentle incline with my foot off the throttle at 30mph didn't do much over that - certainly not "99.9" which I'd expect at that point. The pressure reading hung around 220, never lower than 190 or higher than 240, but I couldn't see that it was definitely falling or rising under hard acceleration/coasting. I've no idea what range that value is supposed to have, or how it should behave.

 

Anyone have any suggestions for things to try?