RPMS go up and down while breaking.
 

Hello, i have a 2006 Chevy HHR automatic transmission. The car runs beautiful even with 130,000 miles on it now. The problem im having is when i am slowing down. The rpms go down from 750-1000 to 200-350 and it causes the lights to dim and it feels as if the car is going to stall out. It has actually stalled 3 times now in the past few weeks but i just put it back into park in it starts back up and off i go. As im breaking the car feels like it wants to lurch forward and i have to put a little more oomph on the break pedal to stop. Again this is strictly happening only when im slowing down or breaking to come to a stop. Someone told me it could be the speed control sensor but im just not sure. My check engine light is on by the way but the code reader comes back as a problem with my thermostat which i need to replace but could that cause this? Any help is greatly appreciated as i am financially challenged and want to know for sure before i spend any money on trying to fix this.

Welcome to the site, tell us, what are the actual codes the code reader read?
Just the codes please, and we’ll go from there, otherwise we’ll be dancing in the dark, and you’ll be spending money you might not have to.
Also how old is the battery in your HHR?

I do not have the actual code, i had a coworker who had one the night it started happening. It came back as the car wasnt coming up to optimal temperature which i could tell you its running at 185 or so with a 190 thermostat. The battery is brand new.

Have these problems started since the new battery was installed?

Should have a 180 degree thermostat.

P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature
If it's this code, it translates to the engine not reaching the correct temperature quick enough. Mostly caused by a bad thermostat letting coolant pass through which in turn makes the engine take too long to reach operating temperature.

A fully stuck open thermostat may even cause your temp readout to show a reading of "----" on the display.

x2 what FMG said - and make sure it is a Genuine GM or AC Delco one - ACDelco 131-158 or GM P/N 12622410

T-stat 190f IS TOO HIGH. 180F is correct.

The reason for near stalling on braking could be trans problems, A/C problems, alternator problem or the brake booster vacuum; among other things. The A/C runs in defrost mode.

So it’s not possible to go to parts palace and get codes pulled? Or the guy from work?
And yes a 190 degree thermostat is wrong, the computer system won’t see the correct parameters and cause issues, like you are experiencing.
As stated above AC Delco 131-158 180 degree thermostat
Go cheap, get towed home!

Ok yes the code that is coming back is p0128. I will be replacing the thermostat this weekend. I was really just wondering if this particular code could cause the problem im having or if its not even related.

I will post my results after i install it either tomorrow or the next day. And i do not know if this problem started since the battery was replaced because i was given this car as a gift around 7 months ago. I do remember however in the summertime when i would have to run the A/C that it would get warm when coming to a stop as well. Like as if the RPMS werent high enough to turn the a/c fast enough to produce cold air. But as soon as i accelerated even if it was just to 10 mph it would get cold again.

No, the thermostat should not have anything to do with the braking/idling/RPM issues.

However, you stated "As im breaking the car feels like it wants to lurch forward and i have to put a little more oomph on the break pedal to stop." - if it is happening as you describe (ie: dangerous braking) - you need to have the car looked at by a reputable mechanic, not try and diagnose it over the internet.

Re: the battery - you said it was brand new. Are you now saying it is at least 7 months old?????