Porsche Boxster Case Study

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picowight
TwoWaves
TwoWaves
Posts: 162
Joined: Sat Oct 26, 2019 6:45 pm

Porsche Boxster Case Study

Post by picowight »

Called in to diagnose this 6 cyl 2.7 Porsche Boxster, with misfires on Bank 2 Cyl 4,5,6 - only misfire codes set + an overall engine rich code. It's a GDI engine, garage also reported misfire was worse when hot.
Porsche.jpg
One thing to note about this car is that there is no engine access without 20-30 mins of dismantling and the same to re-assemble with some serious risk of paint damage. Removal of both wheels does get you spark plug access on a lift (only just!). The garage had put it on an exhaust analyser that showed high HC and CO with low O2 and they had swapped over lambda sensors (no change).
Scan tool showed B2 running at -40% LTFT at idle
Basic fuel/spark misfire tests were conducted without conclusion. Scan tool data also showed the up/down stream Cat temperatures were the same for both banks so that ruled out a blocked cat - confirmed with a IR thermometer.
I did a relative compression test that looked fine.
Screenshot from 2022-05-26 18-16-17.png
I figured whatever was happening it had to be valve/timing related so went in-cylinder on both banks (not an easy option as access was very restricted)
Cranking on both banks: B1 in Green, B2 in gold
Screenshot from 2022-05-26 18-28-13.png
- Identical and no exhaust back pressure confirmed.

However in-cylinder idling on both banks compared, again B1 in Green B2 in gold
Screenshot from 2022-05-23 23-07-05.png
First up, significantly less compression on B2, the exhaust valve opens at about 33degs BBDC for both banks. It is clear that the lower compression is due to the 40degs later closing of the intake valve - marked by verticals - hence the rich running and fuel trims.
As the engine is GDI then intake valve carbon blockage is always a consideration - but the identical cranking waveforms rule that out, it also rules out a standard timing issue.
As the opening of the intake valve is only 10-15degs difference from the other bank - I felt that ruled out a VVT issue on the intake cam so that left the variable valve lift system being faulty and causing the intake valve to close late.
I asked the garage to swap over the valve lift system solenoids and they confirmed that the problem moved bank to bank with the faulty solenoid. 8)
Attachments
20220523-0004 3-6 comparison.psdata
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liviu2004
TwoWaves
TwoWaves
Posts: 472
Joined: Sat Mar 19, 2016 8:23 am
Location: Netherlands

Re: Porsche Boxster Case Study

Post by liviu2004 »

Good one, shows that you need to know what you’re dealing with in terms of valves lift control. Information is key.

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