P0128 - not like the other threads on here.

[motc777]

I have a 2004 Mercury Mountaineer, V6. A bit of history.

In November, I had a problem with a leaking Coolant thermostat housing. I decided to replace the whole thing and the entire assembly came with a thermostat. Up until this past two weeks, the temperature needle on the cluster read right between C and H like it always had. NOTE: I did not replace the ECT sensor, just the O-ring on that item. Ran fine after that.

Next, I got a service engine soon light. Pulled out my code reader, P0401. I fixed that with a new EGR valve. I drove it around all weekend with no codes and even went on a long road trip of about 170 miles without issue.

Yesterday, the Check Engine light came on, but this time I'm getting a P0128 code. I looked it up, and some of the threads on here and the consensus is usually that you need a new thermostat, but I just replaced mine less than 45 days ago. So the question is, do I just have a bad ECT sensor, or did I get a bad thermostat with my housing unit? Thanks for the help guys. THe Haynes Manual is not helping.

 

[Number4]

From enginecodes.com

Possible causes
- Leaking or stuck open thermostat
- Low engine coolant level
- Insufficient warm up time
- Faulty engine coolant temperature sensor
- Engine coolant temperature sensor harness is open or shorted
- Engine coolant temperature sensor circuit poor electrical connection


Once the needle reaches the midway point, your engine temp should be 195-212, even up to about 230.

At this point go squeeze the upper hose, if coolant is flowing it should be "firm" and hot.
If the therm isn't opening, then the hose closer to the radiator should be cool.

Now, the code sounds more like it's stuck open. In this situation the coolant would reach optimal temp while driving, but at startup it wouldn't and the PCM will cry.

Start with the wiring harness connection, then I'd do the sensor (if you can't diagnose it, I don't know its resistance values.)

 

[motc777]

Thanks for responding. When you say start with the wiring harness connection what exactly are you suggesting that I do?

Just a quick note, my engine coolant level is right within the cold fill line area and it is fresh coolant because when I replace that water inlet valve I had to drain my coolant refill it with fresh. So I don't think it's the low coolant level my only concern is if it winds up being the thermostat the thermostat came as part of one housing with that valve but I don't want to have to takethat whole thing off

 

[Number4]

Remove the connector and make sure the contacts look good, also, make sure the connector is fully seated.

Another issue could be an air pocket in the cooling system. With the car cold, start to remove the sensor. Either air or coolant will start to come out. You could repeat with the engine warm, just don't do it with the engine at operating temperature. Anything above 133 can cause burns.

 

[motc777]

Remove the connector and make sure the contacts look good, also, make sure the connector is fully seated.

Another issue could be an air pocket in the cooling system. With the car cold, start to remove the sensor. Either air or coolant will start to come out. You could repeat with the engine warm, just don't do it with the engine at operating temperature. Anything above 133 can cause burns.

Will check it out. Thanks for the tip.

 

[motc777]

O.k. I took a close look at the sensor contacts. They look pretty good. However, I decided to take a step back on the coolant level.

The level is there, that's not the issue. I got a little ball tester for antifreeze. There are 5 balls in there. I only floated two, which means that I'm protected down to 10 degrees F. That's not very good.

Antifreeze I used? Supertech 50/50 from WalMart.

Question, did I just get a bad batch of antifreeze or does Supertech suck that bad? I bought two jugs of the stuff at the same time to fill it up after the water inlet valve and t-stat replacement.

Thoughts? Could this be the reason why my vehicle is running so low on the temp gauge?

Edit - I have never performed a radiator flush on this vehicle. The fluid has always remained clear, but is it time to do so you think?

 

[motc777]

Bumping this just curious as to what people think of changing out the radiator fluid might help

 

[motc777]

Okay I did a radiator flush and while I was doing that I was monitoring the air temperature of the vehicle with my scan tool. I noticed that the temperature steadily increased and again I have my heater on full blast for 10 minutes just at idle and my temperature never got above 125°. However in the middle of the test I pulled the connector off the The engine control temperature sensor and I noticed that the reading went to negative numbers on my scan tool. So I think that the sensor is working okay but when I read the codes for this it also says that the intake air temperature sensor might be bad. I am unable to find the sensor anywhere on my 2004 Mercury Mountaineer. Is it incorporated with the mass airflow sensor perhaps? Also does the intake air temperature sensor have anything to do with the temperature reading on the cluster of my vehicle

 

[motc777]

I did the hose test and it's firm and hot with the engine idling. The IAT temperature reading right now is 53°F and that I don't the ECT temperature is 127.

 

[motc777]

Checked the ECT sensor. It was all black and fouled. I scraped some of the stuff off but I'm thinking I found my problem. I also checked the thermostat and it doesn't seem stuck. Spring worked good. Thoughts?

Edit:

Disregard what I stated here. I think you are right about my thermostat being stuck open.

I found something online about how to test a thermostat. With my engine off I took a reading on my scan tool of the temperature of the engine. Outside the ambient temp was 52 degrees, and stone cold before starting up, the scan tool said it was 87 degrees in my engine. I decided to fire it up and after 10 minutes, never got above 135 degrees, and when I felt the upper hose, it was hot.

So I think it's the t-stat like you said, but just making sure that it's not the sensor still. Would appreciate your feedback, or anyone else's for that matter.

 

[motc777]

[MENTION=226817]Number4[/MENTION]

Just letting you know that it was indeed the t-stat. Thanks for your help.

Went to Oreilly's this morning, told them my t-stat came with my water inlet valve. They told me that I would have to purchase another complete assembly and could not swap out the T-stat because they don't sell a separate one.

So I said, to heck with it. Got a Murray t-stat, popped it in and boom, gauge immediately started to rise. Also, I notice that the rough idle has all but disappeared.

Thanks for the help again. Sorry I didn't go that route like you suggested, but I guess sometimes, we have to learn the hard way. So what I accomplished this weekend:

1. Radiator flush and fill.
2. Replace t-stat.

:exp:

 

[skpyle]

Hey, you did good. Maybe started off on the wrong path, but you stuck with it and eventually found the problem.

Keep on keepin' on...

Seth K. Pyle