I went to get in my 94 ranger 4.0 4x2 today and the truck ran really rough and wouldn't run well enough to go anywhere. If felt like it was starting out in a high gear (I have a manual). I opened the hood and checked plugs, wires, ect and everything seemed OK. I switched out the fuel filter because I had been meaning to do it anyways. While idling, it would stall out after a couple of minutes.

I ran my code reader and got code 159 which relates to the mass airflow sensor being out of range. This summer, I replaced the mass airflow sensor to get rid of a check engine light. I read on the internet that if you unplug the MAS, the engine should stall. Mine doesn't. I'm wondering what peoples thoughts are on this. Should I try cleaning it (best method?), or would you take it apart and return the part. I believe the part should still be under warranty at AutoZone. If anybody has ideas or comments, it would be welcome.

 

if you unplug it, one of two things will happen. The engine stalls, or it runs better if not the same. If it stalls, the sensor is good. If it runs the same or better, double check your wiring and then return it. If you got a reman- it has a 1yr warranty, if new, Lifetime Warranty. Double check if its in stock first though. Most stores will order it in ahead of time if you let them know you are going to return it that way you have a lot less down time since it typically takes at least 2 days and ordering it tomorrow would mean you get it Tuesday at the soonest.

 

Engine might stall, it depends on computer software and timing.
The computer may think engine is being shut off when MAF data stops, so cuts fuel and spark, but most computers recover in time and engine will stay running.
In any case just restart engine with MAF unhooked, and see how it drives.

Computer can run the engine without MAF data, it will run richer and be less responsive but it will be drivable.

What was the original code that caused you to replace MAF sensor the first time?

Thinking would be that you fix a symptom of the original problem by replacing MAF sensor but didn't fix the actual problem.
Could be a wiring issue for the MAF sensor, could be air leak in the big tube connecting MAF to intake, or vacuum leak in intake manifold.
I would clean old MAF sensor if you still have it and try it.