MK5 Golf GTI
All Things Mk5 => How to Guides / Troubleshooting => Topic started by: GVK on July 29, 2026, 10:35:47 pm
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I know it's not a mk5 Golf GTI but it's a TFSI , different badge , same sh1t
Mates got a 2008 Audi S3 8P. (BHZ) It's meant to be a stage 2 car but when you log the airflow g/s and do the calculation to the bhp, it comes out about 260bhp (it should be well North of 300bhp)
It's had a fair bit of maintenance in the time he's had it - carbon clean with manifold off, injectors serviced, timing checked both ends. Fuel filter new. Cam follower bucket that drives the high pressure fuel pump was changed Intercooler was leaking out of those stupid spring clip fittings so he's fitted an aftermarket APR intercooler to fix that.
All that's been done and it's no more powerful Sad if you log 115 boost pressure, it spikes to 2300mbar and holds just about a bar. (No wonder it's showing stock power) all the way through him owning it i've wondered if it had a weak turbo actuator.
Next thing was try another map so he's been in touch with Chris at Tuning Technics in London who has sent him a few maps to try with varying differences in boost pressure etc. Chris asked him to log fuel pressure and sure enough block 230 shows the ECU demanding 120bar and when on hard accel. the actual pressure drops right down to 35bar in some cases! When it does that the lambda control tries to add fuel but it can only try to add 25% when it drops to that low pressure it hiccups and goes into limp mode momentarily.
The low pressure readings seem to stay around 5 bar. So, we are wondering if it's a high pressure fuel pump failing or the low pressure in tank pump not being able to keep up with demand. Any ideas to this rambling?
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Semi fixed, he's put VIS hpfp internals in to raise the high pressure, the old piston was worn on the pump - still needs a bit of work on the map we think as it hits what I think is a boost cut and momentarily cuts power.
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It could be the LPFP also as mine used to feel a little laggy when pushed hard. I changed it out for a 2012 scirocco R version that had the newer version of the pump and very low milage. That sorted my problem out.
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Thanks I think the high pressure pump internals were worn causing the issue. Worth keeping an eye on the low pressure though.
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With the Low Pressure side sitting at 5 BAR, that is a little on the low side. The fuel filter is the regulator and that is set to 6.6BAR
Worth replacing the fuel filter if its not been done recently
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Fuel filter is new 6.6bar one. Still having issues every so often on hard use, the low pressure measuring block 231 shows 5 ish bar not quite the 6.6 bar it should at idle so next port of call is a low pressure pump and controller if needed.