1988 ford bronco 302 idles irratically and stalls when it is put into park. Changed the throttle by-pass valve and it has not improved. Any other ideas?

How does it idle in park , does it run ruff or smooth? Are any lights lit on the dash , check engine etc. ? Could have a partially plugged IAC air passage .idle air control (IAC) valve ! The engine computer uses the IAC to control idle , when under a load such as when put into gear the computer raises the idle ! It does this by allowing more air to by pass the closed throttle into the engine raising the idle ! If these air passages clog they restrict the air flow an cause a idle problem ! Now if it runs ruff at idle in park you could have vacuum leak ! Rough Idle How to Fix Ford Taurus Engine Stalls at Stop Signs

Fuel filter. Low fuel no power. Catalytic converter. no air out, no air in. loss of power.

This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve but always run a OBD2 fault code as well as the cleaning procedure. Buy a can of throttle valve cleaner (do not use carburetor spray cleaner!) from NAPA or Carquest (made by CRC chemicals) and spray it into the air intake while the engine is running, use up about 1/2 the can, engine will try to stall hold the speed up, shut it down and let it soak for 30 minutes, restart and blow out the remaining fluid, shut it down and disconnect the negative battery cable for 5 Min's to reset the base idle control. If the above cleaning doesn't cure your issues you need to have the computer controlled automatic idle speed control system diagnosed and repaired by a dealer or electronic engine control repair shop.

This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve but always run a OBD2 fault code as well as the cleaning procedure. Buy a can of throttle valve cleaner (do not use carburetor spray cleaner!) from NAPA or Carquest (made by CRC chemicals) and spray it into the air intake while the engine is running, use up about 1/2 the can, engine will try to stall hold the speed up, shut it down and let it soak for 30 minutes, restart and blow out the remaining fluid, shut it down and disconnect the negative battery cable for 5 Min's to reset the base idle control

Usually a sign of a faulty idle air control valve or a dirty throttle body.

throttle position sensor maybe.

A number of things could be the problem here:

1. Bad plugs - fouled plugs would cause erratic idling
2. Bad wires - also would cause erratic idling
3. Bad or worn distributor cap
4. Bad MAF (Mass Air Flow) sensor
5. Bad TPS (Throttle Position Sensor - this controls the idling speed under all conditions. A bad TPS would idle fine under certain conditions ie engine cold, engine warm with AC on etc but could be bad when truck is warm and at a stop.

Check engine light? Have you checked fuel pressure?

is the idle speed correct? do you shift into second before trying to engage reverse?, is the clutch ok, this is most likely not the trans at all but one of the above issues, the clutch is my guess.

try cleaning your throttle plate and iac valve