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I cannot get rid of the Code P0128 "Coolant Thermostat Coolant temp below thermostat regulating temp". I am on my 4th mechanic. Thermostats replaced 3 times, temp sensor, water pumps (secondary pump done at the dealer) even my radiator. I had it finally at the dealer who said it was the Aux water pump... $1,000 later, SES light on with the same code. has anyone seen this as a persistent code? I have had the car at a local mechanic, 2 independent BMW only shops and I have another follow up at the dealer as they didn't fix the problem. My car has 155k miles on it. Still runs like new and there are no problems with the water temp itself. It did seem to warm slowly a few months ago when the code first appeared but seems normal now. I did have the code for the secondary air pump but I have installed Shark software to ignore that code rather than machine the exhaust manifold. One of the indie mechanics suggested that secondary air pump may trigger this coolant code. I have never heard anyone on here ever mention that and they don't seem to be related other than the fact that the secondary airpump only operates for a short time when starting the car and the coolant temp would only be low when starting the car. Anyone have a solution?
 

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Moving to proper forum. Obvious question first: did anyone clear the codes? Second, what KTMPs are you seeing on OBC?
 

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code has been cleared many times

The code has been cleared many times. Sometimes it comes back in one day other times a few days. The other description I'm not sure about, but I'll check as the code was there this AM but on my way home it actually went away on its own. It may be back tomorrow AM so I'l l scan it and post the full reading.
 

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The code is not connected to the SAP in anyway. It comes when the car is started cold. There is a temp curve in the cpu that must be met or the code is thrown. So if the car started at X degrees after so many seconds it should be this temp. It is a constant check for the first ~7 min IIRC. You seem to understand that. Here is a chart of how it should look.
AAN1d.jpg
If I recall correctly this was a fail because the T-stat opened to soon, circled in red at 72 degrees. So it appears but I don't think it was. I think that is where the seal started leaking. In the blue circle is the cold coolant mixing in for the first time. Then between the blue and orange is the T-stat opening and in the orange is what is left of the cool coolant mixing in and cooling the car temporarily.
Here is the seal and and how it fits on the T-stat, normally it is mounted inside the top of the water pump assembly.
WAHLER 120213.jpg P1010004 (2).JPG
Basically you need to watch and time a cold startup to see what is going on. The chart should have gone straight to ~78 degrees then had a drop or pause in temp. The part in blue should not be there.

Step one start your car and go to test 7 on your OBC and watch the time. There should be a steady rise, with constant time, to 78 degrees. It should be there in aprox 7 minutes depends on somethings. Then there should be a degree or 2 drop in temp as the cold coolant mixes, but the temp should be back to 78 quickly.

While you are waiting to do that go over all the bills and look for a seal or thermostat housing (expensive so it should be listed) on the work orders, I think you can see where I am going. Your test results will tell the story.
 

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Thanks for details. The car is at San Diego BMW. They replaced the "secondary pump" said it was fixed and the next day the same code popped up. So it is back at the dealer and I am giving them this thread as they can't figure it out. The last shop before this kept poor records of the parts installed as I went back to get better records. They are still scratching their heads and want me to forward to them the fix....like a have the time for that.
 

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It is probably the seal causing the leak. Thermostat should be good though since you've replaced it few times.
 

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If the sealing ring around the t/s is leaking or missing, it causes overheating, not overcooling.

It could cause overcooling if it was binding on the t/s "piston" and preventing the spring pulling it completely closed.
 

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Ive sent that info to the shop. If it was leaking, wouldn't coolant leak? Interesting that you say it would overheat, it got a little warm last week before going back to shop when it was 85 degrees out with full A/C. That is normally not a problem but I had to turn A/C off to get water temp to fall back to normal. Coolant level was OK and Aux fan was blowing.
 

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Interesting that you say it would overheat, it got a little warm last week before going back to shop when it was 85 degrees out with full A/C. That is normally not a problem but I had to turn A/C off to get water temp to fall back to normal. Coolant level was OK and Aux fan was blowing.
The code you have, also known as the dreaded 69 code, is brought in by the engine taking too long to warm up to ~78 deg C from a cold start, as sailor said above. This is an effective way to monitor t/s performance as extended heat-up time is a much more obvious indicator of early t/s opening or a t/s stuck slightly open than monitoring the steady-state running temperature, especially in hotter weather. That's why you are getting the code, despite seeing normal temps while driving.

And you have ruled out the normal causes by changing the t/s (3 times!) and the sensor on the t/s housing. The new water pump, rad, SAP, and aux. water pump were a waste of money as they will not cause this code.

You will find monitoring the coolant temp, aka KTMP, as you drive quite useful. It's done by getting access to test # 07 in a secret menu on the pixel message screen on the dash. The first post in the thread below shows how to do this. And sailor has shown above the approximate timing of the heat-up although this will be dependent on the starting temp and the engine load. This time of year in San Diego, I would guess you should see the temp up to 78C in ~5 minutes.

http://www.m5board.com/vbulletin/e39-m5-e52-z8-discussion/316282-m5-obc-secret-menu-post3948514.html#post3948514

One thing to ask the shop to check is the connector on the sensor. My engine ran too cool and after a new t/s and sensor, I found it was a slightly bad pin connection in that connector, which increased the resistance slightly and made the sensor read low. I discovered it by pressing down on the connector and seeing the temp jump 5 deg C.

The running hot you are seeing can be caused by a few things and I would say was not related to your code.
 

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Discussion Starter #11
Coolant too cool

Wow, Thanks for such great detail!! I'll work on getting that temp display up, that is crazy. I'll have the connections checked as well as I'm on the third repair shop and they are clueless. I really appreciate all your time in helping!
 

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Possible fix

I was told today that inside the housing that the thermostat sits was cracked. Not sure how that was missed by several other mechanics. Have to get it back together and see if the code remains off but at least we may have a solution.
 

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Strange. It's a thin-walled casting. It's hard to see how it could be cracked enough to bypass the t/s but not enough to leak externally. A photo of the cracked housing would be great.
 

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I have a pic of the broken part!! The thermostat housing actually has a broken off price of the inner lining. Keep in mind I just got the car back so haven't driven it enough to see if the SES light stays off. I will finish this thread in a week or so to let everyone know if this did fix the coolant low code that has been coming back and un-fixable by multiple mechanics and has cost me $$.


Hopefully the link works. They should set up a direct upload.
 

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Is the broken part shiny? Could the shop have broken it when they tried to change the seal? It is extremely hard to get out. Can't wait to see the pics.
 

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I have a pic of the broken part!! The thermostat housing actually has a broken off price of the inner lining. Keep in mind I just got the car back so haven't driven it enough to see if the SES light stays off. I will finish this thread in a week or so to let everyone know if this did fix the coolant low code that has been coming back and un-fixable by multiple mechanics and has cost me $$.


Hopefully the link works. They should set up a direct upload.
It doesn't work. Can't you just attach a jpg?
 

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sailor may be right. That looks like the recess the t/s outside seal locates in. So enthusiastic use of a two-jaw puller or a pry bar to get the seal out may pull the land out like that.
But as I said above, leakage past that seal (and that broken land may not cause leakage) wouldn't cause overheating. I'm basing that on the schematic below but I admit it hurts my brain figuring out what the effect of that seal leaking is when the t/s is closed.

Coolant circuit sketch with TS closed.JPG
 

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There is something wrong with that diagram, I think anyway. The heads on almost all BMWs are separate from the block when it comes to coolant. That is how they get cabin heat before the engine is warm. I can't say for a fact this engine is that way but I have cabin heat within3 -5 mins, which is the same as every other BMW I have had so I must assume it is the same design.
 

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There is something wrong with that diagram, I think anyway. The heads on almost all BMWs are separate from the block when it comes to coolant. That is how they get cabin heat before the engine is warm. I can't say for a fact this engine is that way but I have cabin heat within3 -5 mins, which is the same as every other BMW I have had so I must assume it is the same design.
The S62 has conventional cooling flow. The pump pumps into the block both sides and the coolant flows over the cylinder bores and up through passages into the heads. It flows back to the t/s housing through the two cast aluminum manifolds bolted to the heads just below the intake ports.

When the t/s is closed, coolant just recirculates around that loop, bypassing the rad and picking up heat quickly. The supply to the heater core comes off the back of each aluminum manifold.
 
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