Limp mode when cold - resolved with restart when warm

[davidbxc90]

My 2009 XC90 with the D544T4 motor and 185k kms is in limp mode every morning when the engine is cold. On decelerating, there's a whooshing/whining side that seems to come through the air box. Some people suggest that sounds like over-boosting.

It stays that way until it warms a bit and I turn the engine off and wait a few minutes before restarting. Then it's fine, running well without whooshing sound.

My mechanic has replaced the swirl flaps and removed about half a shoebox of carbon from intake pipes and the EGR. It didn't fix the limp mode problem. He now suspects a faulty EGR motor but isn't certain. Replacement will be pricey of course.

His scan before the decarb and swirl flap replacement showed the following:

6662 Glow relay control. Faulty signal
18A0 Swirl actuator control. Offset fault
3000 Camshaft position (CMP) Sensor. Faulty signal
1517 Turbo control. Pressure too high
1540 Turbo control. Pressure too high
1600 Boost pressure sensor. Faulty signal

Very grateful if anyone has seen a similar problem and can offer any advice.

Cheers,
Dave

[Tannaton]

Firstly I would get the codes re-read to see which persist.

It shouldn’t overboost on deceleration, if you lift off completely and you are at a speed where the torque converter is locked and hence the road speed will drive the engine at RPMs above tickover, no fuel is being injected and hence it’s not possible to generate boost.

EGR issues can generate peculiar symptoms as it can cause a leak between the inlet and exhaust but usually this leaks boost rather than causes overboost - hence I would suspect the turbo actuator (there is another similar thread on this recently).

You may also have more than one fault interacting with each other - hence any codes that persist need investigating and resolving, the codes relating to boost are symptomatic of an issue but the cam sensor for example is very specific.

[davidbxc90]

Will do, thanks for the thoughts. Is it possible that turning the engine off for a few minutes usually fixes the problem suggests there is an issue in the ECU?

[SwissXC90]

Originally Posted by davidbxc90

Will do, thanks for the thoughts. Is it possible that turning the engine off for a few minutes usually fixes the problem suggests there is an issue in the ECU?

Unlikely. Out of all the components which could be faulty, the ECU would be the absolute last thing on the list to replace.

[Tannaton]

Originally Posted by davidbxc90

Will do, thanks for the thoughts. Is it possible that turning the engine off for a few minutes usually fixes the problem suggests there is an issue in the ECU?

Unlikely - I suspect the turbo actuator is sticking but freeing itself when warm or after repeated actuations. However if the ECU cannot regulate boost it will go into limp mode until the engine is restarted regardless of whether it starts working again or not.