Volkswagen Repair: 1994 2.0L induction problem, mass air flow sensor, throttle position sensor

Question
I have a 94 Jetta that has 230 000 km's on it with a 5 spd manual/2.0 litre NA gas engine.  until the car is warmed up, the check engine light comes on everytime the rpm's peak (2500 or sometimes lower just on the way up) while i'm accelerating/shifting up.  it goes back off once the rpm's drop.  it doesn't do it when the car is in neutral and revved up and it doesn't do it once the engine is warm.

the car feels like it's bogging down and not getting enough air at high rpm.  the engine doesn't stall and it idles fine.  i've checked every vacuum line and air hose but haven't noticed any cracks or leaks.  could it be the oxygen sensor?  what do you think?

Answer
Darcy, Thank you for your question.  This is truly a brain teaser. Oxygen sensors can do that but I would look elsewhere first.  Since the problem occurs before warm-up, you should take the car to a shop or auto parts store that has a computer checker.  The EGR valve could be sticking until after warm-up.  Usually, this problem is a vacuum leak.  One thing to do is to take a can of carburetor cleaner and spray it on each of the lines.  You know you have a leak if the engine speeds up suddenly.  There are electrical tests on the O2 sensor.  The actual value is different for every model, best to check in the Bentley manual for 1994 Jettas.  Other things are faulty fuel regulator, throttle position sensor, and possibly a MAF (mass air flow) sensor. That is a few things I can think of that can be causing the problem.  Taking it to a repair shop that has the computer diag system is a quicker and lower cost way of finding the cause of the problem.
Hope that helps.