I have a 91 f150 with a 4.9l. The truck will start and idle fine but when I start increasing rpms the tach will drop to nothing and it dies. When the tach starts to quit I can hold the key in the start postion and the tach will start working again and it will stay running. I feel like when the tach starts acting funny it tells the computer that the truck aint running, but I dont know. This all started after i went through some water. Please help me I have to start working on it in the morning. Thank you.

 

I'm no expert but it sounds like a hydrolock issue, or a pre-hydrolock issue if you are lucky.
Try changing the oil and filter and pulling the plugs and crank the motor and see if any water comes out, I have heard of people putting rags or paper towels over the spark plug holes to catch water to see if there was actually any inside the motor.
Hopefully someone else chimes in though cause I could be completely wrong.

 

How deep was this water, and did it get inside the cab? How long ago was it?

The first thing to check is the ignition system; especially the ignition module & coil. The Haynes manual has an excellent diagnostic procedure that only requires a digital multimeter ($10-50). Look for an old blue-cover Haynes (not the current edition).

 

The water wasnt real deep, but I did go through it pretty fast. I was going to pick up a haynes but couldn't find one. Any other ideas? Thank you