Piston Slap: A Stupid Question About Engine Retardation?

Sajeev Mehta
by Sajeev Mehta
piston slap a stupid question about engine retardation

Anonymous writes:

I’ve got somewhat of a stupid question; why does my car roll when it’s parked in gear? My current (135i) and my previous whip (Mazdaspeed3) don’t like staying on any inclines without teetering ever so slowly backwards in a scary, slow, lurching motion. Gears don’t matter as it simply increases the speed that the car wants to roll at. Strangely enough, the oldest car that I had driven was a 98 Integra that could sit on inclines all day without this ever happening. Common wisdom says you’re supposed to park with your PARKING BRAKE engaged and I do but I still get a bit scared and would like that secondary backup of my transmission being able to hold them in place. I’ve researched a bit on the manner, seen NOTHING related to this so I’ve more or less boiled it down to vehicle weight (both the recent cars are pigs compared to the Integra) and engine compression and gear ratio (both significantly higher in the more modern cars). Some insight on the manner?

Another silly question is have you ever heard of a car making a slight leaf crunching noise when engaging into gear? And not just any gear but a specific gear. (2nd) I’ve got this issue currently and it drives me nuts because I’m getting all sorts of dealer nonsense saying it’s a pressure plate issue or they don’t hear anything while forums have told me that it may be the release bearing is loose, or the the release fork isn’t properly greased . Bear in mind that this is all Greek to me. It’s not a very loud sound, something that I’m cursed with for living in a relatively quiet neighborhood when I’m shifting, dreading the 2nd to 3rd gear shift as right when 3rd is engaged and the clutch is released… “crunch”. Or 2nd to neutral. “Crunch”. No other gear! No clutch vibrations or anything. It’s barely audible in the cabin but when the windows are rolled down, I can definitely hear it. One little itsby bitsy “crunch”. I had actually overlooked this issue for quite some time as there were a bunch of leaves during autumn but once the leaves cleared, I realized that my own car was CRUNCHING.

Hopefully it’ll be something you can use, thanks for the reads, your columns are awesome!

Sajeev answers:

Thanks for your letter and the Kudos. No, it’s not a stupid question!

Like you suggested, hill holding prowess depends on the engine braking skills of your motor. The bigger/heavier the car, the more engine braking you’ll need to stop the inertia. And, like you noted, engine braking comes from the compression ratio of said engine’s design. While both the 135i and the Speed3 have high compression ratios (10.2:1 and 9.5:1) for turbocharged motors, perhaps that isn’t enough for the vehicle’s weight, gearing and slope of the incline. And depending on the motor, your Acura had ratios that may or may not prove my point.

Then again, the Integra is much lighter than your last two vehicles…and was geared to compensate for a lack of turbo-torque in the powerband. Especially in first gear. With more gear and less weight, I bet that’s why the Integra fared better. But I’m sure the B&B will pull up gear ratios and prove me wrong. I’m cool with that.

Now about the silly crunching question: that’s tough. The items you discussed could be the problem, but I think the synchros are at fault. That’s because it happens as you release the clutch in a certain gear. Judging by your cars, you might be a hot rodder. A racer. A gear jammer. If so, it might be time to find that next car of your dreams. I’m just gonna go waaaay out on a limb and guess that you don’t keep these hi-performance machines for longer than 5 years. I just love how transmission problems bring out a craziness in a diagnostic blog posting!

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2 of 41 comments
  • Greaseyknight Greaseyknight on Aug 18, 2012

    Interesting problem, the only vehicle that has had a problem that I've seen was a diesel that was left in gear on a steep slope with about 8k of trailer behind it. I wonder if the slop in the drive train is what is the problem.

  • Claytori Claytori on Aug 30, 2012

    I'm glad someone mentioned above that the piston rings (seals) are not capable of holding pressure, but leak at a slow, steady rate. When you use only gear engagement on a hill to hold the car, the wheels are forcing air into and out of the cylinders. The engine will rotate slowly as the cylinders admit air in or out. Your car will move. This reminds me of an incident my wife had with our MT Saturn L-Series. She forgot to do either one when she parked the car in our condo garage. It slowly rolled out into the middle where people drive by. She claimed surprise when she learned there is a bit of a slope. I found it later with a piece of wood behind the wheel and an orange cone behind the bumper. I suspect that the crunching noise is coming from the clutch release mechanism. Do we know if this is a cable or hydraulic release clutch?

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