Piston Slap: Frozen Solid, Yet Self Aware Edition
I have a question regarding cold weather starting my 2001 GMC Sierra P/U. When the temperatures dip to -40 C (-40 F) wind chill is there an advantage to starting my truck and leaving the transmission in neutral (with the parking brake on)? It has 235,000 kms and is the S/L version (strippo) with the 4.3 V6 and 4 speed auto. The transmission is original and I had the fluid changed at 215,000 kms. I bought it with 206 on the clock, not sure what was done before me, but I check the fluid regularly, and it has remained the same colour and viscosity as it was originally.
The problem is that when I start to drive after the vehicle has been warming up for 5 minutes, it takes another 10 more minutes or so for the transmission to shift up into 4th gear. This adversely affects my fuel economy, and I’m worried that long term this may not be good for the transmission. I haven’t noticed too much of a difference this week warming the truck in neutral, but it’s been so cold that I don’t know if it’s helping at all. My father, who is in his 80’s, always advised to warm the transmission in neutral in winter conditions, but I’m concerned that he may be using a trick from the 40’s or 50’s for manual transmissions, and it may not help with the automatic version. And yes, I use a block heater once the nighttime lows exceed -15C.
Sajeev answers:
So the concerns presented here are fuel efficiency and transmission durability. Luckily, nothing is wrong with the truck. Perhaps you’d like to join me down in the balmy Gulf Coast? Because the truck would be oh-so-much happier down here, and let me tell you why: vehicle computers prevent overdrive engagement until the transmission fluid (or engine coolant) reaches a certain temperature. Or until a certain speed is crossed: my car shifts into OD at 55mph in near-freezing temperatures, even with no warm up time from my parking garage at work to the adjacent freeway taking me home.
As to idling in neutral versus park: there’s no benefit, if you let the truck warm up before you drive. The transmission doesn’t care either way, and neither should you.
Speaking of caring, let’s get back to why your truck wants to move down south: fuel economy. As we all know, idling is a huge waste of fuel. What’s less apparent is just how much fuel is dumped in a cold motor to keep the motor running properly. Modern EFI systems completely ignore the O2 sensors until they reach a certain temperature, making an engine run like a richly tuned carburetor. But this is a necessary evil, if you live in cold climates.
Obviously I’m kidding about moving due south. We have a little problem with fuel economy with the A/C running at full tilt. So what’s the moral of the story? Enjoy your truck, because it’s doing everything to keep you motoring safely for years to come.
(Send your queries to mehta@ttac.com)
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I've lived in the Black Hills of South Dakota my whole life. It gets incredibly cold here on a regular basis, and stays that way for nearly 9 months. Lots of people, especially those with remote start, will idle their car, but that's mostly so it's warm inside and it defrosts the windows. Until recently, I was not blessed with such technology, so I just started, let it run long enough to scrape the windows off and took off. Don't rev the nuts off it when it's cold, as the cold oil will offer you little protection. I kept it below 3000, your rev range will vary. Just did this until the heat gage got past the C level and then I drove normally (Which is to say, I drove as if I was being pursued by Satan and all the chariots of hades at all times) As far as the trans goes, it's pretty much the same thing. Mine actually liked it better when it was cold. It had a bit of a hesitation and a clunk in the warm weather when shifting into drive, but when the fluid was nice and viscous in the cold weather, it worked a lot better.
In my 94 Dodge pickup, the computer will not let it shift into overdrive til the trans temp reaches 60F and will not lock-up the torque converter til 70F, to prevent damage to the transmission. Your truck probably behaves the same and is operating properly. The Chrysler 727 automatic does not pump fluid in park but it does in neutral (the valvebody can be modified to change this behaviour). The trans in my truck is still based on 727 design, but I don't know if it still behaves this way. Probably other automatics do or did act the same. That is probably where the advice to idle the vehicle in neutral when cold came from.