MAF clean or replace?
 

So, this is on my Echo. It's a 2000, auto trans 260k miles. A brief history, my in-laws bought the car new and put the first 200k on it in about 8 years. It became my wife's and then mine. Maintenance was horribly neglected, it ran very poorly. Among the things I did was to clean the MAF. It was dirty and popping a p0171, which that and a set of plugs cured. The car runs very, very well now, and were it not for my ultragauge, I wouldn't know anything was wrong at all.

I have noticed a problem with my fuel trims, which my UltraGauge supports on this vehicle. At idle I have good values, +3 or so on my long term. However, as the revs rise, so do my long term trims. At freeway speed, it's running anywhere from +13 to +17. I know just enough to be dangerous, and it looks like a classic MAF problem. There are no codes and as mentioned, the car runs very, very well. Should I attempt a more thorough MAF cleaning, or snag a new Denso MAF for $75? The car is all beat to hell so I'd like to avoid spending $75...

I wouldn't bother. Your O2 sensor should be determining air/fuel ratio unless you're in open loop when you see the long term trims go high. So, unless you have bad mileage to support that it warrants replacing, I'd leave it.

True. I wish I could watch the O2 voltages better, but the data resolution is so poor, it's hopeless via the UG. Kinda wish I had a good scope lol.

Still, it bothers me that the trims are so high.

Will it run without the MAF? I don't know about your echo, but the engines I am familiar with also have a MAP sensor, for comparison and backup. It'll throw a code (which you can clear after) but it might let you see if it'll stop enriching so much without it.

I don't know if it will run sans MAF. My guess is that it will use a substitute value in something of a limp mode. I am not positive, but I am pretty sure it does not also have a MAP sensor. I'm a truck driver out on the road and a couple of days from home so I can't test tonight.

How many miles are on the forward O2?
Messed up MAF and/or failing O2 a lot of times will not throw a code and reek all kinds of havoc.
Try cleaning the MAF since that is the cheapest quickest option, If that doesn't work replace the forward O2 sensor then if that still doesn't do it replace the MAF.

No idea how many miles are on the O2. I'd bet it's most likely the factory unit (the plugs I replaced at 192k were so worn that I think they might have been factory plugs, just to give you an idea of how neglected this car had been). Anyhow, that's why I wish I had a scope. It would be nice to actually watch the O2 switching.

Replace the O2.
People I know who are formally trained automotive mechanics, you know the kind who went to school for it for 2 years are fanatical about the O2 being changed out before 50,000 miles.
People in the know regard them as consumable items.

Save the old O2 until you can get a scope meter with high resolution and data log then you can always put it back in and study it later on, maybe even do an a-b-a test.

Well, MAF was indeed dirty again. I don't know what to make of this. Seems like it's getting way too dirty way too quickly, but after cleaning the fuel trims are all clustering back to around 0. Won't have a good solid FE result for a while though (I only commute 100 miles each way, once a month).

The air filter or piping is loose or has a crack.
If it has a crack then replace it.
If there is a loose connection seal it up with RTV. It works for air compressor air filters.