Piston Slap: Solving the Black Box Mystery

Sajeev Mehta
by Sajeev Mehta
piston slap solving the black box mystery

TTAC Commentator Mazder3 writes:

Hey Sajeev,

I have an auto parts question that I’ve been meaning to ask for a while but I keep forgetting about it. Back in February, going to my car after doing some shopping, I spotted a cute college woman acting strangely around her Volvo 850 sedan. She’d walk around it a couple feet then would look underneath it. I angled myself to take a peek and I saw her problem; there was a large black box being dragged next to the left front wheel. After exchanging pleasantries and a failed attempt to get the hood open (the car was an obvious salvage, the whole front end was visibly skewed to the right), I finally just reached under the car and popped the box out, muddy and snowy parking lot be damned. At the time I thought it was a windshield washer reservoir, as the hose that was holding it on was a similar size and had a similar fitting to a windshield washer line, but when I got home I realized it couldn’t be. There was no way of filling it or checking its level.

So what was it? My memory has faded on it a bit but it was made of jet black plastic, was roughly the size of a 124 count tissue box, had a hose at the top and two fittings at the base, there was a sponge inside of it and the whole thing came down from in front of the driver’s side axle line. It had some major road rash so all of the parts might not have been there. The thing that I really remember, though, that it was branded with the GM “Mark of Excellence”!

Sajeev answers:

No matter who you are, everyone goes out of their way to spend a moment with a pretty face. Insert “dreamy sigh” here. I am not a Volvo-savvy individual, so let’s do a step-by-step analysis of how to solve an automotive mystery.

  1. Find said component using aftermarket (NAPA, AutoZone, etc) vendors’ websites, provided you got a good look at the part. A big plus for those of us working in a cubicle, but OEM Dealership systems are a far smarter way to find it. Those dealer parts guys often find the part just by pinpointing it’s location on the vehicle.
  2. Hit up the forums for that make or model. They failed me, as I used the wrong search terms. The next step is to post a “Sorry for being a n00b, but I have a stupid question” with as much modesty and humility as possible.

Running out of time before my deadline for publishing, I cheated: an internal discussion with TTAC’s Volvo guru and all around righteous dude Mr. Alex Dykes. Our man is said, “it’s probably the evaporative emissions canister or a part of the vacuum pump system.”

Is he right? I’m betting on him, and I think it’s the charcoal canister in the EVAP system, as this wicked video shows far better than I can explain. Plus, the location sounds right: they are bulky and better off destroyed in a frontal collision compared to something like an ABS metering block. More to the point, parts with the GM Mark of Excellence will be cheap to certify with the EPA, and cheap to replace in said frontal collision. Because, at the end of the day, automotive real estate is just as precious as a corner lot in Manhattan.

Best and Brightest: your collective knowledge will prove me right or wrong…right?

Bonus! A Piston Slap Nugget of Wisdom:

Did I just say automotive real estate is precious? In this age of minuscule overhangs and excessive (but necessary) accessories for our environment, our well being and our need for ever more perfect automotive transports…no really, I have a point here…we need more space for all this stuff. Maybe one day we’ll give our automotive technicians, shadetree wrenchers and our insurance premiums a little more “room” for error, but for now, deal with the stubby noses and the blocky faces that come with our current crop of front clips.

Repeat after me: not everything needs the snout of a BMW, a little overhang is actually good thing.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry.

Join the conversation
4 of 19 comments
  • USguyinMTL USguyinMTL on Jul 11, 2011

    I don't understand something. When Volvo was building the 850, wasn't GM sticking their Mark of Excellence to a different Swedish model (not the one on the right above)? Just curious.

    • See 1 previous
    • Shaker Shaker on Jul 12, 2011

      I suspected that the blonde had something in common with a commercial airliner. That's why I like cars.

  • Jadnhm Jadnhm on Jul 13, 2011

    front driver's side, black plastic box? Almost definitely the charcoal canister for the evap system. My family has owned ~6 of these cars and I am 99% sure that's what it is. I've never had one out of the car so I don't know if they tend to have GM labels or not. Volvo did use a lot of GM's supplier's parts in those days.

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