For a 1994 F150 5.0 2WD
Diagnose the following problems, if you can can.

1. Whenever the heat fan or defrost is used, engine fumes are being blown into the cab. This does not happen with AC, and there is a small exhaust leak near the passenger side manifold.

2. Idle speed often drops well below normal when in Drive or Reverse, but not in Park or Neutral. Drops to about 300-400 rpm, jumps back up, repeats the process, then eventually stalls out. Only happens when vehicle is NOT in motion.

3. Sometimes, almost always in wet conditions, transmission will not shift gears during acceleration until the pedal is pushed down to the floor, kicking tranny into passing gear.

4. Extreme changes in RPM during mostly cold weather startups, often causing engine to stall. Jumps between 500 and 1500 RPM in about 4 second intervals.

I sure have no clue what the problems are, other than the fact that I bought a LEMON

 

I would fix the exhaust leak and every thing might settle down. You can't have air leaks in the exhaust or intake. There are sensors at both ends that are trying to determine the proper timing and fuel mixture. You are fooling the sensors by allowing air into the exhaust that shouldn't be there. I wouldn't worry about the defrost or heater vs ac or whatever, you must seal the exhaust, the cab is not airtight and the fumes will get inside.

You may also have gummed up throttle body, or other TB issues. I would look in the 5.0 engine forum and you will probably be able to find some fixes for the surging idle, it is a common problem, even my 87 stang with a 5.0 did that.

 

It would be very helpful to know something about the truck. Model, year, engine, and tranny model would be a good start.

 

I did post the info at the top, I clean my throttle body whenever I do routine maintainence and the exhaust leak shall be fixed by next week.

I will say that I know nothing of transmissions and the wet weather shifting does have me completely stumped.

 

I'd be looking for any broken or damaged wiring that might be causing this type of problem in damp weather. 1994 has an electronic transmission. Small exhaust leaks (especially ones from the A.I.R. tubes) can cause profound idle related issues, as well as exhaust smells in the cab.

 

It sounds to me like this could be a few different problems,or one problem that is causing other symptoms.So this is the first thing I would do if I did not have access to a scanner to get my iac, tps,and ect readings.Tomorrow,if it is a nice day and your truck is dry and not shifting weird,crawl under it with a garden hose and spray water on the left side of the trans,(drivers side).Spray it good,the same way the front tires would throw water on it if it were raining,then go for a test drive.If it starts acting up,then you will know that either the connection to,or the trs switch is going bad.You will see it on the left side of the tranny with a wire loom going to it.It could very well take care of the engine problems,but the engine fumes in the cab is probably from the exhaust leak.