I have a 2004 Volvo XC90 AWD T6 and when the engine is cold, there is a high-pitched whistle for about the first half-hour of driving. If I take my foot off the accelerator to let the car coast, I hear the high-pitched whistle. If I accelerate, the sound either disappears or it’s very faint. Within the first half-hour (when the engine warms up), the sound disappears. I took it to the dealership but it hasn’t been able to come up with an answer. This all started about one year ago.
Ken Anderson, Victoria, B.C.
I know your shop is having a tough time with this Ken, but this is easier than it sounds. The symptoms you describe have diagnosed the problem for us.
This is a classic case of an intake system “vacuum” leak. The tip-off is the warm-up period combined with the throttle position. Vacuum, or more accurately, low pressure, is created inside the intake system of spark ignition engine. The reason is the throttle plate. With the throttle closed, air trying to enter the intake ducting to the cylinders, is restricted. This restriction creates a very low pressure (or vacuum in lay speak) which provides a means to accept fuel from the fuel injectors and take this mixture into the combustion chamber.
We need to understand that pressure differentials work in the following manner: no matter what the medium – air, water, oil, etc. – the substance will always move from an area of high pressure to low pressure. That is, the “vacuum” created inside the intake system of your Volvo is not actually pulling outside air in, the outside air pressure (atmospheric pressure) is pushing air into the intake system. In a properly functioning induction system, which seals out all extraneous air, there will be no noise or whistling. The sound you are hearing is air leaking across a damaged gasket or a warped sealing surface.
With the throttle closed, or in deceleration mode, the manifold pressure is very low and if there is a leak anywhere in the induction system that should normally be sealed, you will hear the (very common) whistle. Opening the throttle to accelerate increases the manifold pressure bringing this pressure closer to atmospheric pressure. If this pressure differential gets close to being equal (it will never be completely equal), the whistle will stop or lessen.
The change to the noise once the engine warms up suggests that a gasket surface is not mated properly. The heat of the engine expands the metals and plastics that make up the intake system, thereby changing the relationship between the offending engine parts, which reduces or eliminates the whistle altogether. This could be a damaged gasket or metal surface and as the common metal used in induction systems is aluminum, it is easily damaged if careless work is performed in this area.
This brings me to a couple of points:
- You mention that this has only happened within the last year. Have you had engine work done in this time frame? If work has been done on or around the intake system, it’s possible that something could be damaged.
- Is the dipstick fully in its installed position? Fuel injected engines measure every little bit of air entering the engine and the Positive Crankcase Ventilation (PCV) system exposes the crankcase to this intake manifold pressure. The dipstick tube is essentially a hole leading to the crankcase. If the dipstick isn’t fully installed, air leakage past the stick can sometimes cause this whistling effect.
Have your shop try this the next time you take the car in, Ken. Leave it overnight. When the technician starts the engine the next morning, have him or her squirt water at every gasket seam and connection point in the intake system. If the whistle is caused by an intake leak, the water will temporarily seal the leak and the noise will stop for that instant. This is a cheap and quick method to check for “vacuum” leaks, but unfortunately this is followed by the expensive part – the repair.
However, if the shop has performed a repair this past year, you should be able to strike a “goodwill repair” with the shop – if it wants to keep you as a customer.
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