Diagnosing engine noises

Jan. 1, 2020
If only diagnosing a noise from the engine area was as simple as quickly listening to the vehicle and then ordering a new part. Unfortunately, there?s often more to it than that.
The pulleys on this Yaris line up correctly, and the belt shows no signs of wear along the edges.

If only diagnosing a noise from the engine area was as simple as quickly listening to the vehicle and then ordering a new part. Unfortunately, there’s often more to it than that.

Anyone who has ever replaced an air pump – or any other accessory – on a vehicle, hoping to eliminate a noise only to discover that the noise is still there when the new part is installed knows that engine noises can be a real challenge to pinpoint and fix.

The reality is that not only do you have to be sure that a noise is even coming from the engine, and not from an accessory or pulley, but you also have to be sure that whatever you do to the vehicle actually will make the noise go away, and that your customers will be happy and trust your judgement. And you need to be able to do that quickly. Yikes.

Actually, it’s not that bad.

Deciding if the noise is from an accessory, drive bearing, or actually from the engine itself does take practise. But there are, however, a few ways to find out just where that noise is coming from and how to fix the problem successfully.

Here’s how.

Location, Location, Location
First, be sure that you’re actually chasing a noise from the right place. Even if the customer swears the noise is from the engine, take a second to verify that that’s correct. The Internet often is full of bad advice, and customers frequently check it to solve their problems, so do your own diagnosis and be sure. Also, remember that transmission concerns, tires, even noisy radiator and condenser fans all can be mistaken for engine noises. Taking a moment and be sure you’re diagnosing the correct component is time well spent.

Carefully using a long screwdriver or stethoscope to listen for noises from suspect components can speed up diagnosis.

Engine noises usually are present in each gear range (including reverse), whether the clutch is depressed or not, while transmission noises usually aren’t. Engine noises usually are still there when the vehicle stops moving – tire noises aren’t. And an engine noise usually is present no matter if the fans are turning or not, but faulty or damaged fans usually make noise when they turn on. Taking a minute to make sure that you’re looking in the right area for the problem can save headaches and diagnostic time later on.

And here’s a tip: After you’re sure the noise is related to the engine area, do a quick check of fluids, fluid levels and TSBs. The reason for this is simple: If the noise is because of a component that has run out of oil, you don’t want it to finally seize up and “die” while it’s in your bay. Because most vehicles need at least a few minutes for the oil and fluid to drain down a bit before they’re checked, a quick TSB search is a productive way to spend those few minutes until you can inspect the fluid levels and possibly prevent a catastrophic failure while the vehicle is in your care.

Once you’ve verified the levels and you’re confident the vehicle can be run safely for a few minutes during diagnosis, you’re ready to check into the noise a bit further.

Eliminating the Drive Belt
Drive belts, especially serpentine ones, are a common source of noises coming from the engine area. These can be especially tricky to pinpoint on vehicles with multiple drive belts. Serpentine belt noises can sound like squealing, screeching, clicking or popping that changes with engine rpm. It’s not uncommon for a belt to be the cause of an engine noise at start-up.

Clean oil, no fillings, no coolant or mixture problems, correct level ... nothing here to indicate an internal engine problem that would cause a noise.

Fortunately, troublesome belts often can be spotted quickly and the noise repaired successfully without too much bother, and you’ll look like a hero to your customer.

Inspect the belt for missing chunks or sections in the ribs and also across the back of the belt, high spots that might be catching on something and making a noise, or separated pieces of the belt hanging from components (it happens). Also look for chunks of debris embedded into the belt on the front and back. If you find any of these things, take a careful look at the pulleys involved. The pulleys should be in-line with each other and clean and free of oil, debris and damage. In other words, figure out if the problem was caused by a worn belt, or if a pulley or component damaged the poor belt and will do the same thing to the next belt that comes along – something to avoid.

It’s usually easy to slip the belt off and then run the engine to check if the noise goes away. But it’s not always the best method because engine noises sometimes will only happen the engine is loaded up. Still, it can be a cheap and effective test, especially if the noise is indeed still present in the engine with the belt off, likely indicating an engine problem. Conversely, if the noise goes away when the belt is off and the components aren’t turning, the noise is likely to be coming from a belt, a component or both rather than the engine.

But before trying this, just make sure that you’ll remember how the belt goes back on afterward. There’s no shame in drawing a diagram or snapping a quick picture before the belt comes off. Make sure the belt goes back on the vehicle so that it rotates in the same direction (don’t flip it around), because this might cause a new noise.

Even though this belt looks only slightly glazed, it was indeed making a noise. Replacing the belt and cleaning the pulleys fixed the problem.

Also, if you do decide to replace the belt, check the condition of the spring-loaded tensioner, not just for tension but also for side to side travel. Move the pulley around a bit to make sure it’s staying in line and providing tension. If it’s not, it may throw the new belt off – not a good thing!

If the belt is the cause of the noise, the best way to ensure a high-quality, lasting repair is to install a new belt and clean off the pulleys, so the new belt isn’t contaminated.

Eliminating the Pulley Bearings
Even if the belt is OK, a component with a noisy bearing might be the cause of that “engine noise.”

Noise from a pulley bearing is usually present (or changes) whenever that particular component is turned on or placed under load (for example, the noise from an air conditioning compressor will often change or go away when the air conditioning is turned on or off). The noise can sound like a growl or rattle that changes with engine rpm. One tech described the noise a faulty alternator bearing made as, “The alternator’s trying to eat its own bearing.”

The two best ways I’ve found to check bearings are slipping the belt off and spinning and wiggling each pulley individually, and to listening to the pulleys using a long screwdriver or stethoscope while the engine is running and the belt’s in place.

True, this belt is pretty badly cracked, but it's still running quietly. Note how easy it is to slip the belt off to see if the noise is still present when teh engine runs without it.

By slipping the belt off and rotating the pulleys by hand, you often can locate the noisy one, and it’s easy to do. Spin each pulley, feel for smooth operation and listen for a noise. Check for free play in the pulley bearing, both up and down or side to side movement. If any of the bearings are loose, noisy or don’t spin smoothly, you’ve likely found the problem. Just remember that certain components don’t rotate as freely as others just by the nature of the systems, and that certain alternators have one-way clutches in them (to reduce belt tension and improve fuel economy).

The screwdriver or stethoscope method is also really effective – so effective that many technicians start their diagnosis with this step. Placing the tip of a stethoscope (or long screwdriver) on the suspect component and listening for the faulty component can quickly locate a noisy bearing (front or rear bearing). Note that since all components will make some noise, the trick is listening for the noise that “matches” the complaint. Raising the rpm and cycling components on and off helps to quickly locate the problem.

One thing to note, though. If it’s a pulley bearing that’s making the noise, make sure the replacement component comes with a new bearing. Sometimes the manufacturer or rebuilder expects you to re-use the old one, which won’t solve anything.

Eliminating the Components
Sometimes you just can’t tell where a noise is coming from. If the belt, bearings and pulleys all seem OK and the noise is definitely there, well, that’s when you need some real diagnostic skill.

Many circuits are operated by a control module via a relay. Current flow can tell you where to start looking when the circuit fails.

Always start by checking TSBs and calibration updates to see if there’s a known fix. You might find out that reflashing the computer is the only way to fix an engine noise.

If there are no TSBs, a good way to tackle the problem is by eliminating possible components and systems until you find the problem. Turn the air conditioning on to see if that changes anything. Move the steering wheel from side to side and listen for the noise to change. If possible, command the suspect component on and off with the scan tool. Put the vehicle in 4WD, depress the clutch, you get the idea. Systematically check and eliminate each system. This type of problem can be a challenge, but with a systematic approach you’ll be just fine.

One quick note about intermittent noises that occur only on start up. Since these can be really challenging to duplicate, let alone diagnose, check for stored DTCs that might help steer you in the right direction. This can be particularly helpful on engines with Variable Valve Timing (VVT-i) if you suspect that either the actuator or the mechanism is faulty. A stored code for the system is usually a hint that the noise is coming from the engine (there are a few TSBs for this issue).

Confirming that it’s the Engine
If you’ve eliminated the pulleys, bearings, accessories and the belt, the noise could indeed be a problem with the engine itself. And just to make absolutely sure that this (very expensive) diagnosis is correct, there are a few quick tricks to confirm this.

Corrosion is just one source of unwanted resistance that voltage drop testing will help you find.

The first way is to check for DTCs related to engine faults or timing problems, such as camshaft signals or engine overheating codes. Actually, this step also can be performed early in the diagnostic process to confirm your suspicions if you think it’s an engine fault as soon as you hear the noise. Stored codes can help confirm your suspicions that there’s an engine problem.

Another quick trick is pulling the out the engine oil dipstick and looking for anything that shouldn’t be there such as debris in the oil, globs of goo, discoloration or coolant in the engine oil (you get the idea). For example, if the engine has a growling noise and there are filings in the oil on the dipstick, the problem is likely in the engine itself. If there are overheating codes stored and coolant in the engine oil on the dipstick and the crankcase is overfilled, you’re also likely dealing with an internal engine problem. Dipstick checks can confirm your suspicions that the engine has a problem.

Also, if the vehicle has enough miles in it and the noise sounds like a stretched or damaged timing belt or chain, checking for timing and timing marks – and obvious damage and wear – might solve the mystery. More than a few weird problems have been traced to a belt that’s off by a tooth. If that’s the case with the noisy vehicle, the noise is indeed quite likely from the engine itself and hopefully nothing was damaged by the incorrect timing.

And, of course, using the stethoscope or screwdriver to listen to the oil pan, block or cylinder head can pinpoint the source of the noise. It might not be the fanciest diagnostic trick, but it sure does work. Just remember to make sure that the noise from the stethoscope matches the noise the customer is complaining about.

Engine noises can be challenging to diagnose and repair since there are so many different components that can transmit sound and fool you into thinking they’re coming from some place they’re not. However, by thoroughly and systematically checking the vehicle you can usually find the problem pretty quickly and repair the vehicle.

Inspecting the belt, bearings, pulleys and accessories, and using a few diagnostic tricks to verify your suspicions will make sure that your diagnosis is correct and your troubles are minimal. And most importantly, your customers will think you’re some kind of genius when in reality, you just need to be very thorough.

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