My 1985 Jetta with 1.8 gas engine quite suddenly started to run very rough with little power. I changed the fuel and air filters as well as the plugs,distributor cap and rotor and the wires. Any chance the timing belt slipped? The belt hasn't been changed in MANY miles and years. How do I check the timing to see if it is off? I can't find any timing references on-line.

Either not enough air or fuel. Check the air filter and gas filter. Also could be the exhaust system Catalytic converter clogged. No air out, no air in.

Change your air filter and fuel filters.It's either not getting enough air or gas,or possibly both,but JUST enough to run.

When checking for Vacuum leaks, did you use a water spray bottle on seams and vacuum lines and EGR pod?(engine running)

Check for air leaks in exhaust before O2 sensors. O2 thinks its running lean from air leak in exhaust header and enrichens fuel charge.

Fuel injectors dirty, not closing tight. Switch injectors to other bank and see if Error Code moves to relocated injectors.

Bad spark plugs, check if you have fuel runners in your intake.

signs of a bad fuel pressure regulator. the vehicle runs rough, the engine will not start usually in the mornings. many times engine starts but stalls shortly after starting. a bad regulator can result in fouled plugs. black soot on plugs, blake smoke from tailpipe, running rough, stalling. you could have residue in fuel tank and need to change fuel filter again. have a very good day !!

Sounds like the warm start choke. It positions the carb butterflies when the car is warm and adjusts the fuel mixture.

make sure your plugs and wires are in good condition.also fuel and air filter.the engine has to be well tuned up.because air conditioner compressor put a little load on engine.if engine is not well tuned.the AC will pull it down.cause it to lose power and run rough.

Check the fuel filter, and i would suggest replaceing the fuel in the tank. It only takes about a month for gas to start to deterioate and if it sat that ling there is probally some junk in the tank. witch probally caused the fuel filter to plug up.

Top candidates for this problem are:
Clogged Fuel filter
Faulty Fuel pump
Clogged fuel injectors
Dirty / worn spark plugs

Of these, only the clogged fuel injectors are likely to respond to a gas tank additive, and even then you need to get the right kind of additive.
If your fuel filter has not been changed in a while (couple years), start with this. It is fairly easy to change. The fuel filter is under the car near the the driver's side rear door.
If this problem started "suddenly," then the fuel pump might be the culprit.
Fuel injector / spark plug issues are not likely to stop the engine right away, but would tend to cause it to miss cylinders, idle rough, or start rough.

First change fuel filter and then change air filter. A turbo requires good air flow to spool-up correctly

there is a cold start fuel heater in the inlet maifold try to see if thers power to it first if so just replace it should sort your prob
andy