By on January 22, 2014

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 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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6 Comments on “Piston Slap: Prizm’s Intelligence Gathering Leak?...”

  • avatar

    We had a similar problem with a ’92 Corolla (with 160K km on the clock) a few years ago and it turned out to be a motor mount. I think Sajeev has nailed it.

  • avatar

    Friend has a 94 Prism with the 1.6, it’s needed motor mounts, as well as the exhaust manifold replaced 3 times (junk Dorman manifold, OEM fixed it). I’d put money on worn mount(s).

  • avatar

    Diagnosed a friend’s 96 Corolla with the same issue. I’ll put my money one one (or more) bad mounts.

  • avatar

    Doing motor mounts on a Focus this weekend for a friend who started with this same “it’s really loud/shakey at idle” problem.

    Unfortunately, one shop managed to dupe her into buying 4 tires to fix it first…
    (probably needed tires anyway, though)

  • avatar

    If the exhaust downpipe at the engine has an accordion-style flex joint (often covered with braided steel), I would check this area very carefully.

    Also look for other contact between the engine/accessory systems and the body – hard A/C or heater core water lines, for example.

    The exhaust pipe contacting anything on the body can have a similar effect.

  • avatar

    I can remember driving my best friend, Greg’s 1994 Geo Prizm (stick) in the mid to late ’90s. If all cars were that dull and boring to drive, I would have gone back to riding bicycles for fun and transportation. Here is Greg with his Prizm, with me making fun of him:

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