Mustang MonthlyHow To Engine
Diagnose Underhood Engine Noises
Is it a death rattle or just a bad alternator bearing you need to replace?
You're tooling along on your way home from a cruise night or a car show when you pull up to a stop sign. As the engine drops down to an idle, you hear a strange sound from under the hood. It's not a good sound either. Your heart sinks as you realize the sound most likely means expensive internal engine problems and you're going to have to take your Mustang off the road, not to mention spending considerable cash to make it all right under the hood so you can enjoy your Mustang once more.
Hold on though, all may not be lost. The sound you're hearing could simply be a bad alternator bearing--a simple fix that'll get you back on the road in an afternoon. However, that sound could also be coming from deep within your engine and mean you're about to get elbow deep in an engine swap or rebuild. So how do you tell the difference? That's what we're going walk you through in this story.
We recently had an engine noise, a metallic chirping sound, rear its ugly head under the hood of our '90 Fox-body. Intermittent at first (the worst kind of noise is the kind that doesn't happen all the time!) and hard to pinpoint, the noise came and went over the course of several weeks, finally reappearing and staying with us for three solid days. It was at that point we knew we had to get under the hood and delve deeper.
Using a mechanic's stethoscope, the noise was pinpointed to the No. 2 cylinder area. The sound was also determined to be at the top of the engine and based on rpm (we'll get into how we did that shortly). After breaking out the tools and pulling the valve covers, we found excessive wear on the roller rockers. Mind you, this is a 347 stroker with over 120,000 hard-driven miles. We bolted up a fresh set of rocker arms, but upon restarting the engine, the noise was still prevalent. Going back in, and deeper this time, we found failed lifters and damaged cam lobes, including a bad cam bearing. Our day was done and the engine would have to be pulled for a rebuild or replacement.
Hopefully, your engine noise won't be as grave, but if so, you'll at least know how to pinpoint it with these instructions. Feel free to share your engine horror stories with us on our forums at www.mustangmonthly.com!
Identifying the Noise
First and foremost in engine noise diagnostics is identifying the noise. Is it a metallic sound or perhaps a ticking sound? Maybe it is a knocking sound. Whatever the sound, you have to determine its location, operating condition (is it based on rpm, change under load, and such), and at what rate the sound is happening. We can start narrowing the noise down by two primary categories/locations. Generally, an upper engine noise issue will be a ticking sound, while a lower engine noise will be associated with a knocking sound.
Upper Engine Noise
For upper engine noises related to valvetrain, the noise will happen at camshaft speed, which is half the speed of the crankshaft or the rpm indicated on a tachometer. For example, if your engine is idling at 600 rpm, the camshaft is rotating at 300 rpm. As such, a valvetrain noise will occur five times every second at this rpm. Valvetrain parts that can be sources of such noise include the camshaft, lifters, pushrods, rocker arms, springs, guides, and the timing chain. Noises associated with the upper engine and valvetrain will increase in frequency with rpm, but will still be half of the engine (or crankshaft) rpm.
Lower Engine Noise
Lower (or bottom end) noises, generally appear as a knocking sound, deeper in tone than something from the top of the engine. The bottom end noise will occur once per engine revolution. If your engine is idling at 600 rpm (and therefore that is the crankshaft rpm), the sound will occur 10 times per second. Lower engine noise sources include the crankshaft, connecting rods, pistons, main and/or rod bearings, flywheel/flexplate, and various excessive clearances such as a mis-bored cylinder causing piston slap. Like upper engine noises, lower engine noises increase in frequency as rpm increases, but they can also be affected by engine load and temperature as well. A knock might be heard in gear under load, but not in park at idle.
Load and Temperature Variables
As we just stated, engine load can vary the intensity of the noise. Sometimes load will make the noise appear, and other situations it will make it go away. An old trick is to pull the spark plug wire off of a cylinder in question to remove the load from that cylinder, lessening the pressure and usually reducing or eliminating the knocking sound during diagnostic testing. Temperature also plays a vital role in engine noise diagnostics. Noises associated with excessive clearances will often go away once the engine is up to temperature and the metal parts have expanded. Sounds based off of the oiling system (bearing clearances, and such) often do not make noise when cold, but will once the oil has warmed and thinned out. These are all diagnostic tips to keep in mind while checking out your engine for noise issues.
Where to Start
We all hope that a noise coming from under the hood is going to be a small and inexpensive repair. That may not be the truth in most cases, but we can keep a positive outlook until we know for sure where the sound is coming from!
Using a simple mechanic's stethoscope (or even a long screwdriver with its handle nestled into your ear), place the tip of the tool on the accessory drive items first. Listen to the alternator case, water pump housing, power steering pump housing, and so forth. For you late-model owners, check the spring-loaded belt tensioner as well. Be mindful of spinning engine parts, but get in there and check everything. You'd be amazed at what a bad water pump shaft bearing sounds like.
Amplifying the sound with a stethoscope or screwdriver allows you to easily pinpoint the location. If need be, have a friend control the throttle to determine sound pitch changes to the offending part/bearing. If your accessory drive checks out okay, then move rearward, checking components on the way. Exhaust leaks can sound like valvetrain ticking, so be sure to listen around collectors, gasket surfaces, and more. Loose sparkplugs can also cause a metal-to-metal clicking sound due to the combustion chamber's pressure rise and fall, causing the loose plugs to rock in their threaded holes. Listen at the valve covers for rocker related noise. You may even have to get under the car (while it's running) to listen at the bellhousing for a loose or cracked flexplate, or damaged clutch. Remember, safety is your number one priority while performing these checks.