Piston Slap: Prizm’s Intelligence Gathering Leak?

Andrew writes:

Hi Sajeev,

I'm trying to wring a few more years out of my 1994 Geo Prizm, and recently the engine got louder. It's a very low end bass like sound that makes the whole car vibrate a lot more.

The muffler and resonator are relatively new, and the noise seems localized to the front of the car. What's odd is that the engine is the loudest when it's idle. As soon as the RPMs go above 1500, the engine is only a little louder than it normally would be. How do I find out if the problem is the exhaust manifold, or the downpipe from the manifold to the catalytic converter? Over the summer there was a brief time when coolant was leaking and dripping down onto that downpipe and burning off. That leak issue was addressed, but that downpipe has some rusty sections on it anyway.

Sajeev answers:

One must be careful here!

Speaking of activities about anything with the name Prizm is dangerous to one's health! Lives are at stake when Prizm related (exhaust) leaks occur, especially in enclosed areas!

That said, I doubt this is an exhaust leak.  Any reasonable exhaust shop will diagnose the problem for dirt cheap or free.  They'll put the Prizm on a lift, listen at every key fail point (i.e. where welds or bolts happen) with a stethoscope or even a stretch of vacuum line and find the source.

But we must blow this case wide open, digging into the real source of Prizm's problem: the engine under the hood. Specifically, the (four?) engine mounts. When boomin' sounds occur at certain RPMs near idle, I normally default to failing engine mounts.  Sometimes a millimeter variance from new is all that's needed to cause rumbles, booms, vibrations, etc. Especially on a 20-year-old whip!

Think about it: can the rush of air leaving the Prizm's engine cause that strong vibration?  Or is the engine's reciprocating mass resting in a funny spot, shaking things up like a disclosure from Edward Snowden?

Sure you can't see it, but there are plenty of things about Prizm related things that we can never see.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you're in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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