Hello,
I'm a new owner of a Mk5 GTI with no previous experience of them. I run a classic car and historic race preparation shop so I'm pretty competent with most cars, less so modern ones.
The car is a 2005 GTI with 108,000miles, manual gearbox. I only run it on 97/98 octane. Usually BP, sometimes Shell.
I have a relatively small and intermittent boost issue that is troubling me and I'm hoping that there might be someone local to me in Brighton who may have experience and maybe VAG-COM to log my requested boost and duty cycle.
The issue is an occasional drop in boost. I have some software on my phone which can log the boost so I have attached some graphs below. I also have a rolling road at work and have measured a 30bhp drop and a huge dive in torque when this occurs.
I feel the occurrence of the drop might be as much as 50% of the time and I only noticed it through the seat of my pants. The car has no error codes or lights on the dash. When making a 3rd gear pull from 3000rpm on a good run the boost will peak at about 21psi (according to my software) and hold above 19psi for 5 seconds before tailing off to about 15psi. On a bad run the boost will spike sharply at about 18/19psi for 1 or 2 seconds and then drop to 11 or 12. You can feel a drop in the rate of acceleration which is illustrated in the graph.
I put a new revision G DV on the car a few weeks ago. The old DV was a revision C and ok. This made no difference to the car.
I have tested the PCV one way valve and this seems to function perfectly. Not yet changed it.
I was going to change the N75 but I tested this yesterday and it measured 24.6 ohms which is spot on.
I have physically checked the actuator and vane motion and both appear to be moving fine.
I've had all of the intercooler pipes apart (there was some oil around them). I cleaned them all and put them back one week ago. I checked them again last night and they are all dry as a bone. No sign of a leak.
There is no noise to report, no wooshing or hissing, and no bearing noise from a knackered turbo.
According to my data my fuel rail pressure is between about 750psi and 1800psi and the A/F is spot on.
I'm at a bit of a loss. I'm not sure why it is sometimes ok and sometimes not. The car is still fast, drives perfectly, no lights on dash, idle is smooth, starts perfectly. Sometimes the acceleration feels flat.
If anyone in my area can log the duty cycle and the requested boost that would be grand.
Otherwise any ideas of anything else to check? Shall I change the N75, seals and PCV anyway? Any ideas why this issue might be intermittent and not all the time? Any thoughts welcome. Cheers,
Daniel
Bad boost - look at how the rate of RPM drops at the same time the boost drops
Good boost - the RPM rate is a straight line in this one.
