2006 XC90 2.5T, 150K, changed belts at 100K, all engine mounts replaced, full PCV replacement at 120K. Background: The car has always had an annoying idle and while you’re sitting at idle it will periodically shudder and then correct itself for no apparent reason. It’s not associated with AC compressor cycling.

The Problem: Driving routinely about a week ago and the car began whistling when in Drive and accelerating. I inspected the turbo and all associated lines, nothing found.

Out of a bit of frustration this evening I gave it full power and it went up to 5K rpm at about 25 mph. Very little power. The tone of the noise has now changed and instead of high frequency whistling it is more like a kazoo.

Video at:
LINK

You’ll note that I could make this noise while in Park and tapping the gas quickly up to 3K rpm. When I had the previous whistling you could only get the noise while in Drive.

Could this be the MAF meter or something else? Thank you.

 

does the whistle go away when you pop out the dip stick? If so, you probably have a clogged PCV

 

Thanks, great question! No, I tried that right before I recorded that video. What else could it be?

 

ok next thing I'd look for is an intake air leak - say between the turbo / intercooler and intercooler/MAF. Have you scanned for any stored fault codes?

 

Thanks, I'll inspect and check the codes. I drove the car to work this morning and the noise varies even at a constant throttle setting. Could it be the MAF itself? When I inspect the turbo/intercooler and intercooler/MAF, are there any specific checks for the MAF?

 

The MAF is just a sensor - its a wire mesh that uses a heated element to compare to a non-heated element to use the air's cooling effect to measure the airflow. If it gets dirty (say due to oil from a failed PCV, turbo seal, dirty air filter etc) it may produce a wrong value. Generally this results in a fuel mix issue - particularly when the O2 sensor can't correct enough during driving or at idle. The whistle could be the sound of boosted air leaking out, could be the turbocharger whining, could be an accessory (AC compressor, alternator, PS pump, vacuum assist pump....

 

I drove the car to work again this morning and I now have an oil leak associated with the turbo. At first, the kazoo sound was replaced by the previous whistling noise. As I exited the highway, I noticed that the whistling sound had stopped too...and there' was white smoke behind the car. It reappeared whenever I accelerated and was pretty smoky and a big cloud! I knew of a garage in the area I had used before so I drove 1/2 mile at a time, shutting down the engine, until I got there. The mechanic checked out the car right away: we both agreed that the engine itself is fine. Our guess is that oil is leaking inside the turbo, passing through the intercooler and being burnt in the engine, exiting the exhaust. The bottom of the car is dry with no evidence of a leak but we didn't remove the bottom engine panel to inspect. The engine increasingly pinged as I limped it to the garage, which would be explained by the oil vapor ingestion.

My guess is that the oil intake/output to the turbo may have been getting increasingly clogged over a long time, back pressure potentially built up, and the oil is coming out of the turbine bearing? Does that make sense? Would it have explained the previous whistling?

At this point, I may as well replace the turbo which I'm fine with. Background: I consistently replaced the synthetic oil every 7500 miles...maybe that's too long and it's caused some clogging.

 

Okay, here's an update. Bottom line: the whistling/kazoo sound was likely coming from the vacuum lines because I discovered that the vacuum lines were swapped on the boost pressure solenoid valve! The turbo was replaced, along with the oil lines and the intercooler and associated hoses. However, the turbo wastegate was not operating correctly after I test drove the car. I bought a replacement boost pressure solenoid valve and as I was installing it noticed that the vacuum lines were mismatched. Test drove the car... lots of power.

There is a slight misfire that I'm reasonably confident is caused by fouled spark plugs/injectors since I limped the car to a garage with an oil leak in the turbocharger. I'll clean them and use an oil additive to remove contaminants plus a fuel system cleaner and should be good to go.