Piston Slap: A buzz or a thud from the donuts?

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Honda

Matt writes:

Hello Sajeev!

I have a maintenance question for you. I occasionally write for Hagerty as well, and I was wondering how important it is to replace a broken motor mount? I have a 2009 Honda Odyssey with 150,000 miles. In the last month we have put over $1000 into fixing an oil and power steering leak. Then we were told that two of the motor mounts were completely gone, and that would cost another $500.

Is that an immediate-fix kind of situation, or is it safe to drive with broken motor mounts? The van has been great to my family, but I hate putting more money into it at this point. Trying to decide if it’s worth investing in repairs (and keeping it until it is collectible and covered by Hagerty!), or replacing the van. Thanks!

Sajeev answers: 

Thank you for writing to Piston Slap, fellow Hagerty writer!

Basically we are dealing with two levels of failure with bad engine (or transaxle) mounts: issues with the insulation (be it solid rubber or hydraulic), or an issue with the metal part safely holding the engine in the chassis. While driving with a failure on the former is not a horrible idea (just an engineer’s NVH nightmare), a failure on the latter is a serious threat to your wellbeing. Or to your wallet.

First, determine what you’re dealing with: an insulation failure is usually just an annoying vibration at certain engine rpms. (This most often happens at idle, when in gear and on the brakes, like when waiting at a red light.) That’s usually how engine mounts fail at first, unless the vehicle was also in a collision. Either way, at some point the vibration does more than wear out the insulation; it eventually breaks the mount’s metal exoskeleton. That’s when you hear louder, more terrifying sounds like thuds, bangs, and clangs. (These are often heard at full throttle from a standstill.)

Given these symptoms, I’ll let you decide how long you can nurse a minivan with bad engine mounts. Waiting for the vibrations to get worse isn’t a bad plan, as it gives you plenty of time to source replacements online for cheap. Provided your shop will install your parts, but that’s a whole ’nother story.

Bonus! A Piston Slap Nugget of Wisdom

A diagnosis was already made here, but spotting the differences between insulation/metal engine mount problems is pretty easy. Sometimes you get lucky, as hydraulic engine mounts (on EX-L and Touring grade vans) sometimes lose their hydraulic fluid like a misplaced bite into a jelly donut. A solid rubber donut can show signs of stress via cracking and collapsing, but it’s a bit tougher to spot.  A failure in the metal exoskeleton is more obvious, but sometimes a cheap endoscope does the trick.

If the visual inspection fails (or is too hard to see on some FWD applications), open the hood, have someone put their foot on the brake, put the van in gear, torque over the motor with the gas pedal, and look at the motor FROM A SAFE VANTAGE POINT. If the motor rocks back and forth under load more than a newer vehicle (i.e. go see how a new vehicle performs to calibrate your eyeballs), then you know you need to replace your engine mounts … eventually.

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.com, give us as much detail as possible so we can help! If you need an expedited resolution, make a post on the Hagerty Community!

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