SQ8 Performance Drop: Boost Control Fault Symptoms

Had a very similar issue with my 2018 SQ7 last summer. Based on some experience with turbo systems, those symptoms immediately pointed to boost control problems. The main issue was a defective vacuum switching valve causing irregular boost pressure. This created noticeable turbo lag and inconsistent power delivery, especially during acceleration. The boost controller wasnt receiving proper signals, leading to compressor surge under certain conditions. The fix involved replacing the vacuum switching valve and performing a thorough check of the intake manifold system. Total cost was 140 Euro including parts and labor. The repair completely resolved the power delivery issues and returned fuel consumption to normal levels. Before suggesting specific fixes for your case, could you share: Current mileage, Any modifications to the intake or exhaust, When the CEL first appeared, Any unusual sounds, especially near the turbo, Whether the issues started gradually or suddenly This information would help determine if your SQ8 is facing the same underlying problem. Once the boost solenoid and vacuum system are checked, worth having them inspect the entire intake tract for potential leaks that could affect boost pressure.

Thanks for the feedback on the SQ7 issue. My SQ8 just had its latest service at 85000 KM, and now showing 90899 KM on the odometer. The car is completely stock with no modifications. The CEL appeared suddenly during highway driving, accompanied by immediate power loss. No unusual sounds from the turbo area, but the boost pressure seems inconsistent based on the scanner data. The boost controller definitely seems to be misbehaving, causing significant turbo lag during acceleration. Your experience with the vacuum switching valve is particularly interesting, as the symptoms match almost exactly. The bypass valve also makes a different sound than usual when letting off the throttle. Given the similar symptoms and the reasonable repair cost you mentioned, I will have the shop check these components first. Hopefully its just the vacuum system and not a more serious intake manifold issue.

Thanks for providing those details about your SQ8. After dealing with my SQ7s vacuum switching valve issue, I can say the symptoms match closely. The bypass valve sound difference you mentioned is particularly telling, this was also present in my case and directly related to the inconsistent boost pressure. The sudden onset during highway driving, combined with the scanner data showing boost pressure irregularities, strongly suggests a vacuum system problem rather than an intake manifold issue. In my case, the vacuum leak was affecting both the boost controller and bypass valve operation. Since your car has similar mileage to when my issue occurred (I was at 88000 KM), its likely the same component degradation. The good news is that the fix should be straightforward, especially since theres no aftermarket modifications to complicate diagnosis. A thorough pressure test of the entire vacuum system would be wise, as smaller leaks can develop alongside the main valve failure. The engine performance should return to normal once the vacuum switching valve is replaced and any additional vacuum leaks are addressed. Would recommend having them check the bypass valve operation while theyre at it, sometimes these can stick partially open when subjected to irregular boost pressure for extended periods.

Great discussion thread, but I need to end it from the perspective of the SQ8 owner. Here's the closing response: Really appreciate the detailed insights about your similar experience. Will schedule an appointment tomorrow and have them focus on the vacuum switching valve and related components first. The fact that your repair resolved identical symptoms gives me confidence its not a major engine performance issue. The matching mileage and similar fault progression is particularly convincing. Having them test the entire vacuum system and bypass valve operation makes sense too, better to catch any developing issues while theyre already checking the boost controller system. Very helpful to know the repair was straightforward and reasonably priced. Will update the thread once I get it diagnosed and fixed. The matching symptoms and similar vehicle history really helped narrow down the likely cause. Knowing its probably not a serious intake manifold problem is a relief.