We got the following challenge on our Volvo XC90 AWD 4dr T6 Momentum (US version), 105,000 miles / 170,000 km:
The vehicle experiences difficulty accelerating at speeds above approximately 50 mph (80 km/h). When this occurs, the car exhibits a sputtering acceleration, as if it’s hesitating or jerking, similar to the sensation of someone repeatedly tapping the accelerator every quarter/half-second. Additionally, when the issue arises, the automatic transmission seems to struggle, with shifts feeling erratic—either upshifting or downshifting inappropriately.
Frequency:
The issue is intermittent and difficult to predict. Most of the time, the car accelerates smoothly without any problems. However, on certain occasions—whether on the same day or a different day—the issue reappears unexpectedly.
Additional Symptoms:
- The issue is more likely to occur when accelerating uphill or when the vehicle is carrying more passengers.
- The issue does not appear to be related to engine temperature; it occurs regardless of whether the engine is warm or cold. Same for environmental temperature.
- DTC error code readout displays the following:
P004B(00) – Turbo/Supercharger Boost Control B Circuit Range Performance
P02CB(00) & P02CB(07) – Turbocharger/Supercharger B Underboost Condition
– The behavior seems to appear during the handover from “Turbocharger & Supercharger” stage to the next stage “Turbocharger only” at around 3000 rpm.
Analysis & Research
The T6 engine has both a supercharger and a turbocharger that are used in different stages (see Jason’s excellent explanation on Youtube – search for “Volvo’s Engine Is Supercharged, Turbocharged, And Electric – The Best Engines” by “Engineering Explained”).
Both turbo- and supercharger of them are situated between the engine block and the passenger cabin – hard to reach and replace. Replacing them can easily cost more than $3-5k in parts and work time according to my mechanic. To reach them one has to remove many other parts first (+8 hrs for total job). From what I found online the most common causes for turbo underboost error codes are:
- Vacuum leak
- Failed or damaged turbocharger (damaged sealing, binding, components, etc.)
- Failed or damaged supercharger
- Faulty boost/charge pressure sensor
- Faulty wastegate bypass regulator valve
- Dirty engine air filter
Steps Taken So Far
- Replaced the engine air filter with a new one: no effect
- Performed a smoke test and visual inspection on all visible vacuum hoses: no damage found (but we haven’t checked hard-reachable parts yet)
- Recorded data below during driving when behavior appeared. During this test, it always appeared when I was accelerating and hit around 3000rpm.
Conditions: highway driving 45-70 mph; additional cargo: ±250kg load excl. half-full gas tank, 80°F/27°C, 70%RH.
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Open Questions
Given that any repair at the turbocharger or supercharger is going to be quite expensive, I would rather check anything else affordable before going down the route of checking them.
I’m thinking of replacing the wastegate bypass regulator valve (volvo part 31480559) next.
1) Do you have any suggestions as to what the most likely cause of this issue could be or recommendations on what to replace next and if the wastegate bypass regulator valve is a good idea?
2) Do you know by chance if the wastegate bypass regulator valve I mentioned above is just another name for the TCV? And if not, where I could find the TCV in the engine room and a link to replacement?
Sorry for the lengthy post, just tried to share as much as possible as I learned so much from previous detailed posts in this forum.
Thanks a ton for your help!