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    Thread: Less power in closed loop

    1. 07-19-2020 11:14 PM #1

      I have a 90 GTI with a swaped 98 ABA with very low milage.

      The car run very well until it switch in closed Loop. When in closed Loop, it's like the timming is pulled, or the car run too rich, the car become slower.

      If I turn off the ignition and restart it, the car run very smooth (and powerfull) for 30/45 sec and the "loss" of power come back.

      I change the following thing :

      Spark plugs, rotor/cap rotor, ignition coil, front 02 sensor, coolant sensor, intake air temperature sensor, and I just clean my MAF.

      No i cannot read my ECU codes, because I dont have the diagnostic port in my car. It's the next thing I will do to weld in place a diagnostic port.

      So, do you guys have any ideas what's happening with my car?

      What's frustrating, it run so well in open loop...

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    3. 07-22-2020 01:04 PM #2
      More info...

      When I unplug my O2 sensor, the car run great. My O2 is a brand new Bosch OEM one.

    4. Member
      Join Date
      Apr 19th, 2019
      2015 Jetta S
      07-22-2020 09:21 PM #3
      Not the right O2 sensor for the ECU? I think the O2 sensor you choose should be the one for the ECU, not necessarily the engine if that has been swapped.

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    6. 07-23-2020 11:10 AM #4
      Test the O2 sensor signal.

      O2 sensors usually fail by shorting out. The computer interprets this as a lean mixture so it will add more fuel. Fairly common issue.

    7. 07-23-2020 09:27 PM #5
      It is an O2 sensor for the ABA engine. I run the full ABA ECU and harness.

      I will test the signal, maybe my new O2 is bad??? The old one was crappy, I tested it.

    8. 07-25-2020 09:35 AM #6
      The sensor should be plugged in when tested. Report back with your test results. 'Fine' is not a test spec.

    9. 07-27-2020 01:26 PM #7
      Wouldn't the shorted 02 sensor throw a cel and code ? How do u test an 02 to see if it is shorted or failed ? I'm planning on running a flow test on my injectors see of they clogged on my 2.0 golf. It could be that the 02 failed and is making the computer throw more fuel ?making the idle rock the car and engine back n forth. Something is going on with my golf and I can not pinpoint it. New maf , new plugs new coil new plug wires and even a couple vacuum lines. All authentic VW parts except the vaccum hoses. Cant figure it out . Hopefully the fuel injectors when. I test them will show probable cause and fix the issue .

    10. 07-28-2020 10:43 AM #8
      Yes, it should throw a code, but I do not know if a 90 car has that capability.

      How would you test it? There are a couple ways. The quick was is to disconnect the O2 sensor and check the signal voltage. You will have to rev the engine to see the voltage change. With that test, check the bias voltage from the control module. Same wire, but the other side of the connector. It normally has about .45v.

      You should test it plugged in but sometimes it's harder because you really should not poke a hole in the insulation of the wire to hook up the volt meter. That reading should vary from .1-.9v. The older the sensor, the closer those values will be. Such as it may only read .3-.7v.

    11. 07-31-2020 12:22 AM #9
      OK, I tested the O2 sensor, and the signal bounce withing the limits.

      Tonight, I decided to change my PCV valve and the crankcase breather hose (the plastic one). I suspected the PCV to be blown and the hose was "leaking" oil. When I removed the old hoose, I see that it was cracked at two different places.

      So I replaced everything and went for a ride. The car ran perfectly but it seems to be slower. I said to myself, maybe I had a vacuum leak and now that everything is "tight" the car runs richer. So I decided to plug back the O2 sensor. WOW, what a difference...my GOD!!! The engine even sounds more aggressive/mean now.

      All this cause by a cracked breather hose. I really enjoy my car now with the added "free" HP!!!

    12. Member
      Join Date
      Apr 19th, 2019
      2015 Jetta S
      07-31-2020 07:10 AM #10
      Good to hear you found the problem! Now we can all add the breather hose to the list of things to check for an apparent vacuum leak. I remove my breather hose every spring to clean it out and inspect it. It tends to accumulate condensation during the winter and can get plugged up as well.

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