Volkswagen: 94 jetta stumbles on accelration, coolant temperature sensor, spark plug wires

Question
94 Jetta 2.0 66K miles manual trans
On warm days 80+, when the engine temp is in the operating range, engine stumbles on accelration. Not always at the same RPM. Runs good in cold weather. Timming belt changed 45K. Changed plugs, cap & rotor, air filter, fuel filter. crankcase pressure regulator, checked all vacuum lines. Add fuel additve for injectors. Have not changed plug wires. I hate being a parts changer. Gets expensive with no results

Answer
Hi Ron,

Check two items (I am presuming you have the Haynes/Bentley/Manual or the like):

1) The MAP (Manifold Absolute Pressure) sensor.  The manual will give you the correct Ohm values to check.

2) The coolant temperature sensor.  Again, the book will provide the correct ohm values.

This, along with damp or cracked spark plug wires are the chief causes of hesitation in the Jetta.