Thermostat bad but no code

[shawnfrompa]

Hey all my mpg is just horrible about 12 mpg average when 2/3 of my driving is highway. So I read around that the thermostat could play into this I got a scanner and after 15-20 miles of driving my temp is at 189-190 I read its supposed to be 199-207. My problem is that I have no code so when I took to my dealer for warranty work they would not fix it because because there is no codes? So what can I say to tell them to fix it? Thanks

 

[The_Roadie]

Welcome!

That's not unusual for the scan tool temp data. Is your dash temp gauge straight up exactly on 210, or lower? That variation is normal and the dealer is right. Your horrible mileage is most likely from other causes.

Mileage, and any other maintenance? Have you used the scan tool to check long and short-term fuel trims? Condition of the air filter and plugs?

 

[shawnfrompa]

Welcome!

That's not unusual for the scan tool temp data. Is your dash temp gauge straight up exactly on 210, or lower? That variation is normal and the dealer is right. Your horrible mileage is most likely from other causes.

Mileage, and any other maintenance? Have you used the scan tool to check long and short-term fuel trims? Condition of the air filter and plugs?

My dash temp is 1 1/2 tick to the left. My trailblazer only has 60k on it. Also checked plugs they are good and filter is brand new. Also I have used that techron fuel system cleaner twice .

 

[The_Roadie]

That's wicked low on the gauge then. The dealer is just following their rules to save GM money. Sooner or later it will throw a code, or you can just swap it yourself for $30-35 if you're into DIY repairs as you will need to be after it's out of warranty. Depending on how much you drive, you might gain that back in a couple of weeks if it's the cause of the low fuel economy.

I'd also check those LTFT and STFT numbers and see if they have a clue to something like a vacuum leak.

 

[shawnfrompa]

That's wicked low on the gauge then. The dealer is just following their rules to save GM money. Sooner or later it will throw a code, or you can just swap it yourself for $30-35 if you're into DIY repairs as you will need to be after it's out of warranty. Depending on how much you drive, you might gain that back in a couple of weeks if it's the cause of the low fuel economy.

I'd also check those LTFT and STFT numbers and see if they have a clue to something like a vacuum leak.

Thanks for the info man its just a bummer spending 100 bucks on fuel every week. I will go out hook up my scanner and get that info

 

[The_Roadie]

Depending on if it's factory or aftermarket warranty, your scamful "diagnostic charge" or deductible could be more than the new thermostat would run you. Is there any reason to not DIY? Or work with a buddy?

 

[bobdec]

To OP you do not have vehicle info included.. the code(s) setting varies a little by year.. The code (DTC P0128) is set when coolant is below 176* F (80* C) and engine conditions (run tine , ETC, miles driven) are met. +1 on roadie's post #2. I have a '07 and while moving on the road I stay at 185-190 using just the fan friction rotation and air flow. When in stop and go traffic it goes up to 200-210 till the fan activation takes over cooling responsibilities (lower temps if A/C is on). In the Atlanta climate I run 20-22 MPG on Highway and 15-17 MPG around town. In PA your temps will probably lower you mileage a bit, short hops in cold weather are bad for mileage. Follow roadie's advice looks like you have another problem.. Again what are you LT and ST trims averaging ?

 

[shawnfrompa]

Depending on if it's factory or aftermarket warranty, your scamful "diagnostic charge" or deductible could be more than the new thermostat would run you. Is there any reason to not DIY? Or work with a buddy?

My deducible is 100 bucks. Also since I was in a motorcycle accident iam not very flexible right now so I cant do myself wish I could. I could just see how much it might be for my dad friends shop to do it. Also the short term and long term ft on my scanner are jumping around a good bit. Ranging from 5 to -4

 

[qsp01]

I recently had the P0128 code and so i changed the thermostat. I noticed that now the coolant temperature gets to 210 probably within 5 mins and 1 mile. Here is what I found on the p0128 so maybe you can determine why the error code is not getting set.

- Doug

DTC P0128

CIRCUIT DESCRIPTION
An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the powertrain control module (PCM) for engine control and as an enabling criteria for some diagnostics.

The air flow coming into the engine is accumulated and used to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the regulating temperature of the thermostat, diagnostics that use ECT as enabling criteria may not run when expected.

This DTC will only run once per ignition cycle within the enabling condition. If the PCM detects the calibrated amount of air flow and engine run time have been met, and the ECT has not met the minimum thermostat regulating temperature, DTC P0128 sets.

DTC DESCRIPTOR
This diagnostic procedure supports the following DTC:
DTC P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

CONDITIONS FOR RUNNING THE DTC

* DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0171, P0172, P0201, P0202, P0203, P0204, P0205, P0206, P0300, P0335, P0336, P0351-P0356, P0442, P0446, P0452, P0453, P0455, P0480, P0496, P0502 and P0503 for automatic transmission only, P0601, P0602, P0604, P0606, P0621, P1133 are not set.
* The ECT is less than 75°C (167°F) , but less than 70°C (158°F) .
* The intake air temperature is more than -7°C (+19°F) .
* The engine is running between 30 seconds and 30 minutes .
* The vehicle has traveled more than 2.4 km (1.5 mi) at more than 40 km/h (25 mph) .
* The mass air flow (MAF) calculated is more than 15 g/s .
* This DTC runs once per ignition cycle when the above conditions are met.

CONDITIONS FOR SETTING THE DTC

* The calibrated amount of engine run time has been met
* The calibrated amount of engine air flow has been met
* The calibrated vehicle speed and distance have been met.
* The calibrated ECT of 80°C (176°F) has not been met

ACTION TAKEN WHEN THE DTC SETS

* The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
* The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC

* The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
* A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
* A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
* Clear the MIL and the DTC with a scan tool.

 

[gmcman]

Shawn, what particular vehicle do you have? Can you fill out your profile?

Something that comes up occasionally is a vacuum leak from this port on the pass side of the intake manifold. There is a port where on the SWB models it's capped off and sometimes doesn't even pull vacuum, on LWB models it's used and has a hose attached to it. Just something to check.

 

[triz]

How many miles on your vehicle right now.

 

[Bill05EnvoySLT]

Thanks for the info man its just a bummer spending 100 bucks on fuel every week. I will go out hook up my scanner and get that info

If your interested, im In Bristol PA and can offer some assistance for you , the main thing is the thermostat and then Sensor..
I found removing the altenator first and then the thermostat seconds gives plenty of room to remove the sensor... either one of these can cause the code.. i through personal experiance recommend replacing both at the same time...

 

[shawnfrompa]

How many miles on your vehicle right now.

I have 60k miles

 

[BuckeyeEvan]

Last year around 60k, my needle indicated things were on the cold side. I suspect mine is slightly stuck open but has not thrown a code.
I plan to get my thermostat replaced sometime this summer hoping the needle will go back to sitting on 210 and warm up after 1.5 miles vs 7 miles of driving down the highway.

 

[The_Roadie]

Last year around 60k, my needle indicated things were on the cold side. I suspect mine is slightly stuck open but has not thrown a code.
I plan to get my thermostat replaced sometime this summer hoping the needle will go back to sitting on 210 and warm up after 1.5 miles vs 7 miles of driving down the highway.

I wouldn't defer that because you're wasting fuel and that's costing you with 1000 small cuts every fillup. The risk to the cat is another motivation.

 

[IllogicTC]

Shawn, what particular vehicle do you have? Can you fill out your profile?

Something that comes up occasionally is a vacuum leak from this port on the pass side of the intake manifold. There is a port where on the SWB models it's capped off and sometimes doesn't even pull vacuum, on LWB models it's used and has a hose attached to it. Just something to check.

On an interesting note, if you have one of the ones that's capped (or hooked up) this is a beautiful place to hook a vacuum dial apparently. Rolling through my Haynes and it shows all sorts of info on reading a vacuum gauge (just happened to be hooked to that nipple) to detect things from bad valves to intake manifold gasket leaks. Good stuff.

And considering the plethora of sites such as GMPartsDirect and the like that offer Genuine GM and ACDelco parts for nice prices, it's especially not worth holding off on.