Two cars, the same incredibly annoying problem
Here are two completely different vehicles with a very similar problem.
Question: I have a 2010 Chevy Cobalt with just under 80,000 kilometres on it. It has a humming noise coming from the front when I get to 30-40 km/h, noise that gets louder as I continue to accelerate but does not change when it shifts gears or if I turn left or right. I had my tires rebalanced and rotated with no improvement.
R.S. Question: We own an 2006 allwheel-drive V-6 Cadillac SRX that has a YUN, YUN, YUN sound I can feel in the driver’s seat. In preparation for a road trip, I told my 30-year mechanic we needed to get this fixed. He discovered a broken pad on the left front and replaced both sides. Still have the YUN YUN. Next he raised the vehicle, got both front wheels turning and determined the left front bearing was not bad. Replaced right and left side bearings but still have the YUN YUN. Next, replaced both front rotors, still YUN YUN YUN. Throughout these steps, I have been raising the issue of CV joints but he said the undercarriage was clean and would have had extraneous lubricant if the boots were damaged. The tires are high-end and guaranteed and only have 20K on them. I had the fronts swapped side to side and balanced. Still YUN YUN. Next, replaced the rear rotors, still YUN YUN YUN.
As you can guess, it’s driving me crazy and I hope you have some wisdom that will help.
P.C. Answer: So the Yun Yun is no fun, no pun intended! One of the problems with being a somewhat anal and caring car owner — and this is from personal experience — is becoming overly concerned and frustrated over an issue that is as much annoying as problematic. When this occurs, one tends to develop tunnel vision and focus on just one potential cause.
Looking at both cars, if the harmonic or rhythmical noise/ sound is related to wheel speed, the likeliest suspects are tires, wheels, wheel hub/bearings, drive axles and brakes. Even something such as torque converter clutch slippage could be a possibility.
The tires are still a strong possibility, so I’d suggest checking all the wheels and tires for run-out and out-of-round, then have them road force balanced. Make sure the shop checks the wheels themselves for these issues. Although the Cadillac AWD vehicle comes equipped with different size tires front and rear, check to make sure all four have the same rolling diameter. Mismatched tires may well be the problem.
One potential cause that may not have been checked is a dragging brake pad caused by a sticking caliper slider or piston. The clue that may point to this is the noise developing after driving a few kilometres. You might be able to pinpoint this by driving 20 to 25 km in urban stoplight conditions, then quickly pull over safely and sense the temperature of the each wheel as close to the hub as possible.
The front drive shafts/CV joints certainly could be the cause due to wear (they don’t have to leak grease because of a failed rubber boot to develop enough wear to create an issue). With the car on a hoist, a strong arm can push/pull in/out and up/down to identify excess play in the shaft or joints.
Q: I see you recommend using a professional scanner to see what’s wrong with a car. Can you explain the difference between these and the one I’ve bought at the auto parts store?
Z.P. A: Practically the entire car is now connected in a network that can be accessed using a scan tool. These tools have become an essential part of diagnosing engine, transmission, brake, air conditioning, lighting, accessory systems, just to name a few.
The inexpensive tool you purchased is likely an OBD-II generic scan tool. These are capable of obtaining and clearing emissions related diagnostic trouble codes (generic and manufacturer specified), checking system readiness (completion of onboard diagnostic tests for various systems— important for emissions certification), and viewing a freeze frame (data captured when a code sets). Some tools may indicate diagnostic trou- ble code definitions and other helpful features. OBD-II generic data is very limited in scope and only scratches the surface of the diverse systems and components used by vehicle manufacturers under the hood and elsewhere.
A professional grade (after- market) or manufacturer-specific scan tool goes far beyond the OBD-II system, allowing access to the manufacturer’s in-depth information, tests, and calibration functions for engine systems such as variable valve timing, electronic throttle control, and intake manifold tuning, turbocharger systems. These systems can vary by manufacturer, so specific access is needed. Examples of capabilities beyond simply looking at data includes performing an evaporative system leak check, a crank sensor angle learn procedure, running a cylinder balance test, and commanding the camshaft actua- tor to verify phasing accuracy.
A top-notch aftermarket or manufacturer-specific scan tool also allows access to transmission and all-wheel-drive systems, braking, stability control, traction control, and other under-car systems.
An OBD-II generic scan tool is a great addition to anyone’s toolbox.
Paul Brand is an automotive troubleshooter and former race-car driver. Readers may write to him at paulbrand@startribune.com. Please explain the problem in detail and include a daytime phone number.