If you’re planning on owning something old or crappy (or both!), you’re going to need to develop some audio diagnostic skills. The ability to discern between something that’s no big deal, needs attention in the near or distant future and OMFGSHUTITDOWN! goes a long way toward keeping your hooptie on the road with minimal (unintentional) wasted time and money.
Today we’ll help you graduate from “it’s making a funny noise” to “sounds like the #3 exhaust lifter”.
The first step is to establish a baseline. Turn off your damn dubstep and listen to your car while it’s functioning properly…or as close to properly as it gets. My Falcon’s suspension and chassis squeak, my Wagoneer’s leaf spring shackles pop and my Wrangler’s mud tires howl. Good to know before I go chasing noises that are never going away.
The first thing to check is whether a noise is engine or wheel speed dependent. If you’re standing in the driveway with the hood open or coasting to the shoulder with a dead motor, we can cross this off. Otherwise, rev the motor while stopped and/or put it in neutral while rolling and see what happens. Sometimes engine-speed noises only happen under load, so a bit of a brake stand may be in order.
If the noise is wheel-speed dependent, start hunting from the wheels in. Wheels, wheel bearings, axle, driveshaft then transmission. Things get funky once you get to the tranny, as one end spins with the motor and the other with the wheels (that’s kinda the point). We’ll get to that one in a second. A failing wheel bearing is typically accompanied by steering loosness, odd brake noises or a crunch/tick noise at low speeds that goes away if you put the car in the air. Typically you can feel wheel/tire or driveshaft issues via some kind of vibration. If the vibration comes and goes with load (get on the gas, then off, then back on), it’s most likely a driveshaft. Check the u-joints/CVs/guibos as well as the slip-splines. Due to a lack of space to wiggle, differentials don’t vibrate, but will tend to howl as you get on or off the gas.
Forward (or sideways) to the transmission, there are a number of ways to figure out what’s what…provided you have a stick. Press on the clutch pedal and rev the motor. Next, rev it in neutral and with the clutch engaged. If you’ve got noise with the clutch pedal in, look to the clutch, throwout bearing or flywheel. If you’ve got noise with it out, it’s more likely on the tranny input shaft. Another trick for sensing what’s going in in the cog box is to use the shifter as a stethoscope. Rest your knuckle or fingernail on the shifter and feel for changes in vibration as you go through the gears or get on and off the throttle.
Onward to the engine.
Here’s where another baseline comes in handy. The “engine noise” you hear from the cockpit is actually a chorus of components, each clicking, popping and wooshing in their own key. Poking around while you debug, you’ll hear each component’s distinct noise, so don’t necessarily panic because your BMW M30’s injectors are making a racket. For the purposes of this article, your engine has three sides: front, top and bottom. We’ll start with the most likely (and cheapest) suspects and move on from there.
Engine accessories are the most common source of noise. Belt squeal is typically due to a loose belt or unhappy power steering pump. Assuming no squeal, mystery noises can be probed with a long screwdriver or pry-bar with the handle against your ear. A failing component will be obvious when you hear it.
If trying to distinguish between a belt-driven accessory and something on the front of the motor itself, don’t be afraid to briefly run the motor with the belt(s) disconnected. No noise? Blame the accessories. Still there? Think timing chain or mechanical fuel pump.
Moving up top, we’re into cam and/or valve issues. Typically you’ll hear a tick caused by excessive play between a valve and rocker or rocker to pushrod or cam. The mechanical stethoscope is your friend here as well. If a noise is confirmed (or even if it’s not), yank a valve cover and take a look. Obvious broken or worn parts will be obvious, but in their absence it’s crucial to know the specifics of your vehicle’s valvetrain. Depending on design and whether or not you’ve got hydraulic lifters, a perfectly in-spec valvetrain might seem “loose” at rest. Consult your manual or forum of choice.
While not specifically in the head, an exhaust leak can sound a lot like a valve tick. The trick is to listen closely for softer “edges” on the noise, then run an ungloved hand close to (but not on!) the entire manifold manifold as well as the manifold-to-head seal. At idle, a puff of hot air will be your sign.
An important way to distinguish head problems from bottom-end problems (I’m sure there’s a joke there somewhere), is that everything in the head happens at 1/2 speed from the crank. At 600 RPM, that’s 10x per second on the bottom end and 5x at in the head. They also tend to have more of a “clonk” sound than any kind of tick. Once you know the problem’s in the bottom end, everything else is moot. It’s time to start shopping for an engine hoist…which just so happens to be the topic of next week’s Wrenching Tips!
Leave a Reply