Rough idle in cold weather


Asked by Jan 30, 2014 at 04:12 PM about the 2000 GMC Sierra 1500 SL Standard Cab SB

Question type: Maintenance & Repair

In cold weather my 2000 sierra starts, reves up idles back down to rough idle and continues to do so for about a minute. Once it warms up runs great and does not do this in warm weather. Have had several sensors replaced (even tho they checked good) and just had a tune up done on it.  Mechanic can't figure it out. Someone told him maybe the computed needed to be upgraded. Has anyone else had this problem. Help please. I really love this truck and do not want to get rid of it.

9 Answers


Is there any modifications to the intake, engine or exhaust. If not I would check which ever sensors were not fixed. If you have lots of mods than you may need some things changed in the PCM, but it sounds like an issue under the hood to me. Something small like this will be hard to find becasue it doesn't sound like a hardware failure just a bad reading during the engines warm up. The fastest way to find and repair it will be with a scan tool that can read live data from your motor. Otherwise it's just random guessing. Record the values of each sensor/output when the truck is idling rough then start it again later when it's cooled off enough to run right but not cold enough to have the issue. Do you have a list of the sensors you replaced??


I will look and see if I still have all the receipts. It started this 4 years ago and every winter they try something else, but it never made the Service Engine Soon light come on until this week so they were never able to get a code. The codes this week were PO171 and PO507. He said he checked all the sensors and they are checking good. I know sometimes in the mornings after if has done its reving up and down I can smell fuel like it is flooded. I would think if it was a computer update needed it would do it all the time.

1 of 1 people found this helpful.

To answer you on the modifications. I had dual exhaust put on it in 2006 nothing else has been changed on it


Sorry, the exhaust was put on in 2004 and it has 118,000 miles on it so it isn't really high milage.


Codes only tell you when there has been a failure or something is way out with the way your engine is running. Live data is where you can pin point sensor computer issues. It would also tell you if you need a re-flash. Quick run down on the efi system. Your engine uses the input from the sensors to control the outputs. For the fuel injectors the PCM matches the input values (sensors) to the applicable table(s) and delivers the required fuel for the current engine condition, called open loop. For the most part these are set at the factory or changed with a programmer or by a tuner. Once the engine warms up it switches to closed loop were the oxygen sensors can add a trim to your fuel calculation to adjust the air/fuel ratio and make it run better. For the most part the tables are fine with the stock values and it is usually a failure somewhere that causes the issues, but if you make drastic changes to the engine and it's components then you need to modify. I doubt the dual exhaust is causin the issue and it is most likely a failure causing you to run rich until you warm up and hit "Closed loop" I'd check the engine coolant sensor first?

1 of 1 people found this helpful.

Thank You I will have it checked

Hi all, I also experience this same issue with my 2001 GMC 1500 and I am going to try fuel injector cleaner. I could be way off and wasting my $2.99 but my theory is the "gunk" build up on the injectors. I'm thinking the particles in the build up are freezing and creating crazy spray patterns to the heads. I could be crazy but its worth a try!


If you live in a cold winter area you may need to switch to a winter weight oil. I agree with the above poster - try some fuel system cleaner - you have nothing to lose.


It turned out to be bad manifold intake gaskets. Had it fixed last January and has been running fine since then

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