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Bimmer ****ie
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Discussion Starter #1 (Edited)
I got code PO128 about a month ago(coolant thermostat below thermostat regulating temp). I cleared it to see if it would come back and it has not. On a 400 mile roadtrip this weekend I noticed the temp gauge fluxuating from the 12 O'clock position back down to almost the first mark when I would come down a long mountain pass. Looks to me like the thermostat is stuck open. It never overheated and is running fine and the SES light has not come back on. Just looking for some input before I order a new thermostat. Will do the waterpump while I'm in there too. Anything else I should address while the fan and shroud are out? Belts are new.
 

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BMW forever
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When you coast down there is no fuel being burned in the cylinders and therfore no heat generated, if the coast down in long enogh the temp will drop.even thou the thermostat is in good condition.
 

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Bimmer ****ie
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Discussion Starter #3
When you coast down there is no fuel being burned in the cylinders and therfore no heat generated, if the coast down in long enogh the temp will drop.even thou the thermostat is in good condition.
Agreed, but the gauge has never moved once it came up to temp. Uphill or down. Once it came up to temp (needle straight up) it stayed there. This is a new symptom since I got the PO128 code so something isn't right. Thanks for the input:)
 

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Your diagnosis sounds correct. I believe there is a way to confirm actual water temp on your instrument panel. The water temp on a properly operating cooling system will never drop below 180's (once it has achieved that level) which is not enough drop to show on the gauge.
 

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Bimmer ****ie
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Discussion Starter #5
Your diagnosis sounds correct. I believe there is a way to confirm actual water temp on your instrument panel. The water temp on a properly operating cooling system will never drop below 180's (once it has achieved that level) which is not enough drop to show on the gauge.
I thought there was a way to get the instrument display to show actual temp while driving but I'm not sure how to pull that up. It is definitely dropping noticeably as I head down a long grade. Comes back up to normal when I get back on the gas.
 

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E39 On Board Diagnostics

I thought there was a way to get the instrument display to show actual temp while driving but I'm not sure how to pull that up. It is definitely dropping noticeably as I head down a long grade. Comes back up to normal when I get back on the gas.
Google E39 (OBD) Hidden On Board
Diagnostics. You want to learn how to use this menu. You want Test #7.
Real time coolant temperature.
Very handy to know.
 

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It's very likely that you have a
(Lazy) Themostat that is not closing
all the way when it should. Just
replaced mine. My 528i was running
10 degrees Celsius low, 85-86. Now
I'm back to 95-96. That's a 50 degree farenheight difference. That's a lot.
 

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Bimmer ****ie
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60 Posts
Discussion Starter #10
It's very likely that you have a
(Lazy) Themostat that is not closing
all the way when it should. Just
replaced mine. My 528i was running
10 degrees Celsius low, 85-86. Now
I'm back to 95-96. That's a 50 degree farenheight difference. That's a lot.
That is what mine is doing I'm sure. I have ordered the new Mahle/Behr thermostat and a OEM BMW waterpump. I just serviced the cooling system a couple months ago with new BMW blue coolant. Guess I should have put a new stat and waterpump in then. Oh well, I will just drain it and reuse it.:thumbup:
 

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2000 528i bad thermostat

http://www.ideafile.com/bmw/540-obc.html

Works for all high clusters.

However, it sure seems like a bad 'stat.
By the way Edjack, you were right
In regards to my t-stat. I guess I
didn't want to believe I had another
t-stat failure. It's my fourth. This
last one which was a Behr lasted 2 years. Got a lifetime warranty so in
went another last Wednesday. Gotta
love FCP Euro for that warranty. Thanks for the good advise as usual.
 

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Glad I could help.
 
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