'02 VW Gti 1.8t - Overheating, High Intake Vacuum, Check Engine Light

Hi All,

First of all, I am a complete beginner, so I apologize in advance.

I have a 2002 VW Gti 1.8t (337 Edition). There are a bunch of issues with this car, but I am extremely broke (hence trying to fix it myself) and I am just trying to get the thing up and running.

Initial Problems:

  • Coolant Leak (Fixed) - slow coolant leak, found crack, replaced piece no problems for past 2 months since fix.

  • Check Engine Light - has been on, codes read P0420 (Catalyst System Efficiency Below Threshold) and p0411 (Secondary Air Injection System Incorrect Flow Detected)

  • Have noticed some decreased power, idles high to start and then drops once warm.

I replaced the downstream O2 sensor, upstream too difficult to remove. The codes and drop in power makes me think this is a bad catalytic converter. However

New Problems:

  • Overheats after maybe 5-10 minutes of driving.
  • When I open the coolant reservoir, I can hear the coolant start to bubble and the level rises as the cap is opened.

Ok, being a beginner, I figured this was bad catalytic converter -> can’t exhaust -> heat & pressure build up. So, I googled a bit and saw I could test it with a vacuum gauge.

I hooked the vacuum gauge up to the port just behind the throttle body before the intake manifold (see pictures here). I was fully expecting to see the level slowly drop indicating an exhaust restriction. However, even at idle the pressure is on the high end (though I do notice it’s not at zero even when not hooked up). Then when I bring the RPMs up to ~2000 the vacuum increases and stays constant at about 25 inHg.

So I am officially stumped. Could I be hooking up the vacuum gauge incorrectly? Any ideas would be appreciated.

Final note: I do also see the needle vibrating quickly indicating worn valves. But I really am not looking for performance, just for it to last maybe another few thousand miles before I can either replace it or get it fixed properly.

tl;dr - Check engine codes imply bad catalytic converter. Car now overheating. Vacuum gauge shows an increase in vacuum when revved, but no slow decrease.

Bad turbo seals?

If the vacuum is too high, than the pistons are trying to pull air past a restriction. Have you checked your induction system?

What is the vacuum reading at idle? Ideally it should be around 20 inches rock steady. If you blip the throttle quickly the needle should drop to zero instantly and then rebound back to the original reading.

I’ve found exhaust clogs to cause the needle to rebound higher than the original normal at-idle reading and then sink slowly back to the original norm.

As for overheating what about thermostat and fan operation?