- Aug 25, 2012
- 955
Okay guys, now before we get started I know this has been discussed several different areas on here but no real diagnostic steps were noted so thats why I wanted to start this in hopes of pinning down what can be bad.
On that note... I have an 04 GMC Envoy, 4.2l I6 that threw a p0128 code today. It last threw one of these a year and a half ago and the t-stat and ECT sensor were replaced as well as the lower radiator hose and while my mechanic was at it I had him flush out all the coolant and put in fresh since he had it opened up. Well now the code is back and I figured I would do a few things to try and figure out if it is the thermostat, temperature sensor or both that are bad. So I reset the CEL with the scan tool my dad has and it is still in learning for the catalyst mon but everything else is cleared out but I know that with the catalyst still learning that it will not throw a CEL for the temperature being too low. But anyway I went for a ride both in the city and on the thruway with the scan tool hooked up so I could watch the temperature graph while driving around. What I noticed was a steady and fast increase in the temperature from 60 degrees up to 183. It never got any hotter than that which I know is wrong. Anyway I was on the thruway and it just started to reach 170 and then it jumped at 170-173 for 2 seconds. And when I say jumping I mean like a very quick flutter from 171, 173, 172, 171 in that order for the 2 seconds it did that. Then I drove some more and it jumped to 174 and then did a very slow climb to 180 and stayed there the whole time on the thruway. I had got off the thruway and then in the city it went up to 181 but again it did another flutter from 180, 181, 180, 181 and then it stayed at 181. Then when I pulled into my driveway and stopped driving but had the engine idling it went from 181 to 183 and stayed there. Now I know the thermostat on the truck normally starts opening at 180 am I right? But that flutter I experienced can't possibly be the thermostat opening and closing. I know they do that but not in 2 seconds. lol.
Some things to note:
1.) I was driving at 65mph on the thruway for 10 minutes before getting off
2.) I was driving at 30mph in the city for 12 minutes before sitting in my driveway
3.) The outside temperature during the drive was 40 degrees fahrenheit and until I pulled into my driveway the heat in the truck was completely turned off.
Here are 2 pictures, one of the temperature gauge in the dash and another of the temperature reading on my scan tool. Both were taken at the same temperature reading.
View attachment 34265
View attachment 34266
On that note... I have an 04 GMC Envoy, 4.2l I6 that threw a p0128 code today. It last threw one of these a year and a half ago and the t-stat and ECT sensor were replaced as well as the lower radiator hose and while my mechanic was at it I had him flush out all the coolant and put in fresh since he had it opened up. Well now the code is back and I figured I would do a few things to try and figure out if it is the thermostat, temperature sensor or both that are bad. So I reset the CEL with the scan tool my dad has and it is still in learning for the catalyst mon but everything else is cleared out but I know that with the catalyst still learning that it will not throw a CEL for the temperature being too low. But anyway I went for a ride both in the city and on the thruway with the scan tool hooked up so I could watch the temperature graph while driving around. What I noticed was a steady and fast increase in the temperature from 60 degrees up to 183. It never got any hotter than that which I know is wrong. Anyway I was on the thruway and it just started to reach 170 and then it jumped at 170-173 for 2 seconds. And when I say jumping I mean like a very quick flutter from 171, 173, 172, 171 in that order for the 2 seconds it did that. Then I drove some more and it jumped to 174 and then did a very slow climb to 180 and stayed there the whole time on the thruway. I had got off the thruway and then in the city it went up to 181 but again it did another flutter from 180, 181, 180, 181 and then it stayed at 181. Then when I pulled into my driveway and stopped driving but had the engine idling it went from 181 to 183 and stayed there. Now I know the thermostat on the truck normally starts opening at 180 am I right? But that flutter I experienced can't possibly be the thermostat opening and closing. I know they do that but not in 2 seconds. lol.
Some things to note:
1.) I was driving at 65mph on the thruway for 10 minutes before getting off
2.) I was driving at 30mph in the city for 12 minutes before sitting in my driveway
3.) The outside temperature during the drive was 40 degrees fahrenheit and until I pulled into my driveway the heat in the truck was completely turned off.
Here are 2 pictures, one of the temperature gauge in the dash and another of the temperature reading on my scan tool. Both were taken at the same temperature reading.
View attachment 34265
View attachment 34266