1989 240 wagon dl idle problems

[89jordan240]

This car is a 5 speed and i love it. Any help would be appreciated

I have just fixed the MAF and that helped tremendously. But..

When I am idling, especially after turning on the car, and sometimes at stop signs...the car just stalls. (It is a 5 speed). Also, when driving highway, if I stay in neutral for too long, it stalls. I never had this problem for the last few months with the bad MAF, although the car ran way too rich (+/-12mpg). The idle wanders way up and then drops down and stalls. What to do? ...codes I get back are 113, and 131...but I feel this may be beyond a throttle switch problem due to it not doing this with the bad MAF installed. Also, unfortunately, there is an odd hissing sound coming from beneath the car when I let it idle.

-Jordan