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Matt
Matt, Engineer
Category: BMW
Satisfied Customers: 21250
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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2001 BMW 325i, with the DTC code P0128. The Thermostat has

Customer Question

2001 BMW 325i, with the DTC code P0128. The Thermostat has been changed twice, latest one from BMW. It takes the engine temperature a long time to reach up to 170 deg. Fahrenheit. It is like the thermostat is somehow open. The engine coolant temp. and
the radiator temp senor PIDs are within 10 deg. of each other. The IAT sensor seems to be OK. Please answer if you have experience with this condition/problem and have specification/values for the tests that ECM is going to perform to come to this code . Please
no generic answer about P0128. Changing the Thermostat is not the answer to this problem. I have the PID readings for Eng. Coolant temp, Radiator Coolant outlet temp and Intake Air Temp. from cold start for every 5 minutes as follows: Eng. Coolant temp Radiator
Coolant outlet temp Intake Air Temp Cold 66 72 73 5 min. 116 103 73 10 min. 139 130 80 15 min. 162 153 88 20 min 195 159 111 Before start the code were cleared. After 10 min the pending code P0128 is set.
Submitted: 1 year ago.
Category: BMW
Expert:  Matt replied 1 year ago.

hello

the 1st thing I'd suggest is to use a thermal probe to check what temperature you're actually getting on the cylinder head or radiator top hose? as I suspect the coolant temp sensor may be faulty or have damaged wiring

if this is OK then I'd suspect that the coolant bypass connection just under the thermostat ( which is normally closed off as the thermostat opens) is not being closed off and coolant is therefore bypassing the thermostat all the time

if you look at the thermostat the lower smaller disc should push into a second hole and its possible that the this hole in the housing is distorted or corroded and so preventing a seal

so try removing the thermostat again and cleaning out this seating area

Customer: replied 1 year ago.
Thanks for the reply . Did you look at the pictures that I loaded for the PID readings? Do you find any suspicious numbers when you compare them to each other? Would they give you any reason to think that there is wiring problem or malfunctioning sensor?
I have already used a thermal probe to measure the temperature at different location of the engine and radiator. As I recall the measurement where appropriate to the points compare to the PID reading of the engine temp sensor. I didn't write them down tough. I will remeasure and will provide you with readings.
Expert:  Matt replied 1 year ago.

hello

I viewed the images and to be honest they all look believable so I think the faulty thermostat seating is the most likely

Customer: replied 1 year ago.
As I mentioned before this is the second thermostat in there. And it is from BMW. I will take it out again and check.
Expert:  Matt replied 1 year ago.

hello

please note I don't think the thermostat itself is the issue

but rather the plumbing downstream from the stat?

so there should be a smaller hole at the bottom of the thermostat socket that is the bypass circuit and this is blocked off as the thermostat opens

I suspect that this circuit is not being sealed off as the thermostat opens

Customer: replied 1 year ago.
Is it possible to have some pictures regarding this "smaller hole".
Thanks
Expert:  Matt replied 1 year ago.

Hello

this diagram describes what I mean as the bypass circuit

Customer: replied 1 year ago.
Here are some pictures from the thermostat and the thermostat opening from this BMW. It is a heated thermostat. It is a little different from the diagram you send me.
Expert:  Matt replied 1 year ago.

hello

thanks for these, if you look on image 1 at the bottom of the hole there's some kind of casting fault so the bottom bypass disc won't be able to seal properly as it just a metal on metal seal so it may be worth trying to fit some sort of rubber seal to the bottom disc or drop a flat rubber washer into the bottom of the hole and secure it with some silicone sealent

also as its a heated thermostat these apply power to the heating element according to the ECU as this normally opens at 100 degrees C with the ECU applying power to the element if the engien is under load to lower the coolant temp to 85 degrees C

so I suspect that the heater may be on all the time ? so forcing the thermostat fully open all the time? this would mean that there's 12v on the plug all the time

which would either mean a fault with the ECU or with the wiring ,loom

Customer: replied 1 year ago.
Thanks for the reply. I had checked the wiring before and the 12v of the heater element is always there but I assume that the heater is controlled by the ECM though ground wire. Let me double check to see if the heater element is commanded on by the ECU.
Expert:  Matt replied 1 year ago.

understood

Customer: replied 1 year ago.
Dear Matt,.
I put the thermostat back and disconnected the heater element's connector. Started the engine cold and the result was a code for the heater element in addition to P0128. It is telling me that the heater element is not the cause of slow engine warm up. Am I wrong?
Expert:  Matt replied 1 year ago.

Hello

I suppose so

what I'd suggest is to try adding an additional ground wire to the thermostat heater that way you can be sure its energised

then see if this cures the slow warm up

Customer: replied 1 year ago.
Hi Matt,
I can do that. But as I understand the purpose of the map controlled heater element is to provide external heat and open the thermostat when it is needed, IE Load signal> 5.87 ms, Engine temperature > 113 degrees C, Intake air temperature > 52 degrees C or Veh. speed > 110 mph. If I ground the heater element to energize it than the thermostat will open and cause a slow warm up. Am I wrong?
Expert:  Matt replied 1 year ago.

Hello

your're correct, I had the operation the wrong way round

does the engine warm up faster with the heater element disconnected?

Customer: replied 1 year ago.
Hi Matt,
As I mentioned previously, the answer is no. It didn't make any difference. It still warms up slowly. I am going to do some temp. modifications and take the radiator out of the circulation to see if it warms up quickly or not. You have any other suggestions.
Expert:  Matt replied 1 year ago.

hello

I still its possible that the thermostat bypass is open all of the time so I'd say look into adding some sort of seal to cover the bypass circuit

Customer: replied 1 year ago.
Hi Matt,
I don't understand your logic that "the by pass valve is stuck open". Correct me if I am wrong: When the thermostat is closed (engine is cold) the by pass valve is open and vice versa. In my understanding if the by pass valve is stuck open than the engine would over heat. Also remember that this is the second thermostat and it is from BMW.
Customer: replied 1 year ago.
With temporary modification I mean replacing the thermostat with a plate that will cover the place of thermostat.
Expert:  Matt replied 1 year ago.

Hello

if you blank the thermostat off entirely then this is an interesting test as in theory the engine should heat up very quickly

let me know how you get on

Customer: replied 1 year ago.
Hi Matt,
I am sorry that it took a while before I can get back to you. Any way, I tested with the thermostat out of the loop and it worked just fine and reached the working temp. normally. I did another test in which with thermostat installed i disconnected the return hose from the radiator to the thermostat and started the car when cold. The thermostat was closed how ever a small amount of coolant stream would come out thorough the by pass valve. As long as the return hose was disconnected the radiator temp. would not change and the engine temp. would raise normally but and as soon as i connected the hose the radiator temp would go up and would cause the engine temp. to raise slowly. It was like the stream from the by pass valve would cause to keep the engine temp. not to raise quickly. Is there a possibility that you could get a second opinion from an advanced BMW mechanic in this regard.
Sincerely
Expert:  Matt replied 1 year ago.

Hello

sounds like I originally surmised is that your bypass flow isn't being blocked off correctly ? I would suggest that either the thermostat housing needs replacing or just fit a thin rubber washer at the bottom of the housing to provide a seal

if you would like someone else then let me know

Customer: replied 1 year ago.
Dear Matt,
This is (as it has been mentioned before) the second new thermostat and it is from BMW. It is a little difficult for me to come to the conclusion that both new thermostats are bad. How ever I called the BMW dealer that I bought it from explaining the case and suggested that the thermostat is bad. They said they only will replace the part if their own mechanic would say the part is bad. So I will try your suggestion and will get back to you soon.
Expert:  Matt replied 1 year ago.

hello

when I say the housing I mean the the part / casting that the thermostat plugs into

Customer: replied 1 year ago.
There is nothing wrong with the casting. As you remember the "Thermostat by pass test" resulted in the engine heating up normally. What I meant with the by pass valve is the hole/passage/opening that allows a small amount of coolant to circulate trough the thermostat and the radiator no matter of the state it is in. How ever I will do what you are suggesting and fit a rubber to make sure it is sealed.
Expert:  Matt replied 1 year ago.

understood