All Things Mk5 > How to Guides / Troubleshooting

MK5 GTI down on power

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MartinNorhave:
Hi there. I just bought a mk5 gti. It has a full milltek exhaust with downpipe aswell as an k&n cold air intake. I therefore suspect that it is also remaped.
However the previous owner doesn't know if it is or not.

I feel like it doesn't have the power it should. It doesn't even feel like the 200hp that it came with from the factory.
It had a P0299 underboost code which made me change the diverter valve (It was in fact broken) but I can't feel much of a difference. It doesn't even wheel hop in first gear.
It doesn't have any other fault codes. Any ideas as to where my problem could be?

MKVASMR:
I had this issue with two of mine mk5’s.
I would start with looking at the boost hoses if there is oil then you should change the orings. Clean the MAF sensor, look if your dipstick is broken. Also the pcv. You should do some logs. If the difference between the actual and specified boost is under 300mbar I would rule out the turbo. Last thing do a smoke test to find if you have some vacuum leaks. But you can hear those with the engine running (a hissing 🐍 sound). The n75 valve is also important. You can monitor if it works with vcds or obdeleven.

MartinNorhave:

--- Quote from: MKVASMR on June 20, 2024, 03:07:28 pm ---I had this issue with two of mine mk5’s.
I would start with looking at the boost hoses if there is oil then you should change the orings. Clean the MAF sensor, look if your dipstick is broken. Also the pcv. You should do some logs. If the difference between the actual and specified boost is under 300mbar I would rule out the turbo. Last thing do a smoke test to find if you have some vacuum leaks. But you can hear those with the engine running (a hissing 🐍 sound). The n75 valve is also important. You can monitor if it works with vcds or obdeleven.

--- End quote ---

I just did some quick looking around at home. When I pull the oil cap there is a lot of vacuum and the engine runs poorly. Same with the dipstick. Is this the pcv valve thats bad? And if so could it cause the loss of power?

MKVASMR:
That usually means that it’s ok 👌. However I would change it anyway since the diaphragm can get lazy over the time.

pudding:
Yep, removing the oil cap should cause rough running as it creates a massive air leak.  If no change, the PCV is shot.

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