Vacuum and misfire issues

Time for an update. I got the engine running a little a few weeks back and at first it looked like it was going well. The engine started up as it should and idled normally. I ran the engine at idle for a little while just to warm it up and listen for unusual noises. All temperatures were in the green so I decided to go for a test run. I ran the car lightly at first just to make sure things were running smoothly before I did a slight pull when running up a hill. The turbo started charging and everything looked good, but then, misfires. A lot of misfires. I pulled over to a parking lot to do a quick visual inspection (the engine was still running) but couldn’t see anything. As quickly as the misfires came, they disappeared. I did another light pull and this time it behaved normally. I decided it was a one-time thing and went for a longer trip while still driving pretty lightly. It ran fine for about 10 to 15minutes before I went for a pull in second gear. Like before, it started to misfire, this time more severe. I pulled over to see if any of the pressure hoses had come loose, but they were all fine. I had my VCDS running and it was showing random misfires on all cylinders as well as the lambda O2 header element being bad. After a restart of the engine, the misfires were gone. I drove home to do a more thorough inspection of the engine compartment and found the crankcase breather pipe (the plastic 90degree one) was completely broken.

I did some research and found quite a lot of reasons for why the engine might misfire randomly on all cylinders:

  • Bad gas (water etc.)
  • Bad coils
  • Bad spark plugs
  • Bad MAF
  • Vacuum leak
  • Low fuel pressure
  • Bad crank position sensor
  • Bad O2 sensor
  • And several others.

I quickly ruled out coils and plugs since I find it strange that they should all fail at the same time. They are also less than a year old. The misfires should also be there (to some degree) all of the time, not intermittently like the ones I had.

A vacuum leak is pretty common for these engines, and since the crankcase breather pipe was broken, I decided to get a new one. I also ordered a new O2 sensor, the PCV valve, a few hoses and a crank position sensor. When removing the intake manifold, I saw a few vacuum hoses being worn down so I decided to replace them as well.

After replacing the O2 sensor, the crankcase breather pipe, the PCV valve, the crank position sensor, a few vacuum hoses as well as the intake manifold gasket and the throttle body gasket I has high hopes that the issue was resolved. To my disappointment, it was not. When going out for a test-run it ran fine at first, but the misfires all came back when I did a light pull. I managed to get back home and did a quick check as well as changing the spark plugs. No immediate change with new spark plugs, which was expected, but as it is a relatively cheap fix I decided to just get new ones. Using VCDS I saw that the short term fuel trim was fluctuating between -2 and 2% and the long term fuel trim was stable at about 3.2%. I read that the long term fuel trim should normally be a little lower so I cleared DTCs and saw the short term values being very high at more that 25%. After a few minutes, the long term increased to about 3.2% and the short term stabilised between -2 and 2% like before. I deduced that it had to be a vacuum leak somewhere, but was not able to find any so I went in to grab some food and do some research.

As I couldn’t find a obvious vacuum leak, I decided to replace the PCV relief valve (pancake valve on the TIP) as it’s now about 16 years old and is likely to be passed its prime.. When removing the intake piping where I found some water having gathered there. That is most certainly not good since it could be sucked up by the turbo when doing a pull, getting mixed with the fuel and wrecking havoc with the fuel mixture… I decided that I have to rebuild the intake to move the air filter up into the engine compartment. The problem with this is that it’s a very tight squeeze getting a 3in hose passed the headlight…  

That’s it for now – I’ll update once I have some news 🙂

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