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BigCo540i 03-11-2011 05:21 AM

Bad electric thermostat causing rough or fluctuating idle-
I had a shop do a diagnostic on my rough idle issue and it threw a fault code for a faulty electric thermostat. The tech said the Tstat is telling the car that the temp is either hotter or colder than it actually is so the car is either adding too much or too little fuel which is causing the fluctuating idle.

They did a "smoke test" and found no vacuum leaks anywhere on or around the engine including CCV so vacuum leaks were ruled out.

Has anyone heard of faulty electric thermostat causing fluctuating idle issues?

Nittenyadhav 03-11-2011 06:58 AM

This could be your MAF sensor. Mine has the same problem chnaged the MAF and all is fit and fine.

bluebee 03-11-2011 09:12 AM

Dunno if it helps, but there are thermostat-related technical documents here:
- What is the temperature of the coolant & when the thermostat opens under normal conditions (1)

edjack 03-11-2011 09:36 AM

The two issues are not related. The code suggests that the heating element in the t'stat is burned out, or there is a fault in the wiring. Get it sorted out before the heater shorts out and damages the DME.

Was this a BMW specialist? If not, best to find a good one.

BigCo540i 03-11-2011 09:52 AM

Ok... I'm installing the t stat tomorrow morning. If it still has fluctuating idle then maybe maf?? How do you test it?

Yes BMW spec.

bluebee 03-11-2011 10:05 AM


Originally Posted by BigCo540i (Post 5910543)
maybe maf?? How do you test it?

Dunno, but these might help:
- Mass air flow sensor (MAF) and idle control valve (ICV) require periodic cleaning (1) (2) & buying a replacement MAF cheaper than the BMW MAF (RangeRover MAF) (VW MAF) (Hyundai MAF)

BigCo540i 03-11-2011 11:17 AM

Where are all of these best links located?

BigCo540i 03-13-2011 06:58 PM


Ed you are correct that the two items are not related.

Good news- I replaced the thermostat earlier today and the code for faulty electric thermostat did not reappear. Took me about an hour and a half, I did not remove the aluminum throttle elbow but if you are going to do it this way you need a good swivel socket set. A hook tool is also handy for prying the lower hose off the tstat. I couldn't find mine so I used a small Allen wrench and vise grips to hook the hose and pull it loose.

Bad news - fluctuating idle still there. I'm going to replace the CCV next.

BigCo540i 03-23-2011 12:30 PM

Well it turns out that I have a leak in the intake gaskets that's causing all of my rough idle issues.

I was quoted $2100 for the shop to replace spark plugs, Intake gaskets, Valve cover gaskets, Valley Pan gaskets, Rear ball joints, rear brake lines, bleed brake system, and CCV.

I know this is all DIY type stuff but does this sound like a fair price?

bobdmac 03-23-2011 02:35 PM

For all the stuff that you mentioned, I think that's a fair price, providing it's done correctly. The valley pan gasket and rear ball joints in particular are time consuming. The CCV, spark plugs and intake and valve cover gaskets can be done in concert with the valley pan gasket. Others can speak to the issue of brake lines, but I doubt they're a large component of the costs.

BigCo540i 03-23-2011 03:35 PM

I just met with the owner and we settled on $1600. One benefit of a local owned business and cash.

bluebee 03-23-2011 03:41 PM


Originally Posted by BigCo540i (Post 5910793)
Where are all of these best links located?

This screenshot was taken at roughly 110K views; now there are about 130K views ... so a lot of people must be using 'em.

BigCo540i 03-23-2011 07:23 PM


Originally Posted by bluebee (Post 5941724)
This screenshot was taken at roughly 110K views; now there are about 130K views ... so a lot of people must be using 'em.

5 views are mine.

540indiana 03-23-2011 07:51 PM

I think a man of great wisdom or he may be full of chit mentioned that to you sometime last week about the intake gasket??? Congrats man, I'm glad you figured out the issue.

BigCo540i 03-24-2011 05:06 AM

Your mention of intake gasket leak confirmed by spraying carb cleaner around the intake seals and getting fluctuating values on the GT1. :thumbup:

I found out that spraying carb cleaner to find leaks doesn't necessarily raise the idle to a point that you can notice without the proper equipment.

BigCo540i 03-28-2011 04:52 PM

Rough idle solved. Just got her back from the shop and everything is running great. They put florescent dye in the oil and coolant that will light up with a certain type of light if any leaks were to appear. 2 year 24 month warranty on the work.

The inside of my engine was really clean with no sludge and hardly any wear on the cam lobes and chain guides. Synthetic has done its job for 150k mi.

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