Piston Slap: Shake, Shivers…so Roll!

TTAC commentator Kitzler writes:

Hi Sajeev,

Quick question everybody ignores: I personally do not like racing a cold engine. My last two cars, a Dodge and a Lexus, both had automatic transmissions. When the engine was cold, Summer or Winter, worse in Winter, you had to rev the engine to 3000. before it would shift properly. Worse, the automatic would not shift into top gear until the engine was lukewarm, a couple of miles. Now here is the clincher, as the cars got older, the couple miles became three. What gives?

Appreciate an answer about racing a cold engine and why the damn automatic won’t shift properly, thanks.

Sajeev answers:

First off, this problem is multiplied when you are crazy enough to install a high-stall torque converter in your Lincoln Mark VIII. It’s bad enough with a stock stall speed unit, but add the looseness of a drag-racing worthy fanbox in your gearbox and you need even more throttle and more wait time before the car behaves normally. But you didn’t come here to read that, did ya?

This is normal for darn near any car with electronic fuel injection and an electronically controlled gearbox. Both are needed, as transmission behavior depends on the reading from the engine’s coolant temperature sensor. Yup, engine coolant is a fine bellwether to the future of your engine and transmission’s programming parameters. Let’s face it, a cold engine is not ideal for any driving condition. You want the motor to warm up ASAP.

Cold fluids are bad for performance, tailpipe emissions, and longevity of your car’s power train. A vehicle needs to be loaded up to get the fluids up to operating temperature ASAP, so the computer does just that. And once the coolant temperature looks peachy, the engine/transmission oil follows suit. Revving the engine to 3000rpm is a good idea, as opposed to stewing in “friction-filled” cold fluid at idle or trying to circulate this thick/cold stuff at 6000 rpm.

And here’s my point: you are NOT racing a cold engine from what you stated, you are doing exactly what is needed.

As to why this behavior is worse as cars age, well, that’s harder to say. Perhaps the coolant temperature sensor isn’t reading accurately: its internal resistance from cold to warm (to hot) is no longer linear. Perhaps old fluid has a viscosity problem that throws a monkey wrench into the system. Or perhaps it gets harder to overcome “the shivers” as we all get older. Which is why we all gotta roll the moment we wake up in the morning!

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