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Discussion Starter #1
Here's the situation, I drive a 2002 330Ci, 135k miles. A week ago the "Service Engine Soon" light lit up. Went to the dealer (on 6/19) and they replaced the thermostat. Drove it off the lot, and started loosing coolant, took it back and what a surprise, the expansion tank had to be replaced. Expansion tank was replaced, drove it home.

Coolant light came on again, I topped off the coolant with 50/50 OEM/water (on 6/21). Now, the coolant light is being very sporadic. On the 10 mile commute to work this morning (mostly highway), the light lit up then went off 3 or 4 times, on for 30 sec to a few minutes at a time.

Engine temp is normal. When I spoke to the dealer prior to topping off coolant, he said that with the two repairs, there may be air bubbles confusing the sensors. But I'm unsure how long to wait.

When I start the car, the light is off, but after driving it starts going on and off again. No significant moisture under the car after being parked overnight.

Any thoughts/recommendations?
 

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Could be air in the system, could be a slow leak, could be a flaky coolant level sensor.

Check very carefully around the lower radiator hose, there is a temperature sensor in this hose that is for the radiator outlet temperature. These leak all the time when the car is off and cool. If there is any sign of moisture or white staining around the switch or on the lower engine cover, replace the O-ring for he switch.

Very easy to change, no tools required, do it with a cool engine and leave the expansion tank cap on and you may loose only 1 cup of coolant.

See the last line in my signature for the O-ring part number, do not try to get the dealer to look up the part number, they ONLY sell the sensor with the O-ring, they do not have a separate listing for the O-ring. These O-rings need to be changed every 5 years otherwise you will have issues.
 

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2016 340i xD 6-spd
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In a.m., when cool, fill ET to between min and max on the ET's stick...do not fill to maximum, as there is no 'over maximum' indicator, but overfill, you'll have a big problem. It could still just be low coolant being indicated by air bubbles.

Read Mango's cooling thread for bleeding instructions...in it, I posted an alternative method that might work for you too.

Start with bleeding. If that isn't it, you do, as mentioned, probably have another leak letting air in and coolant out...a double problem!

Clean things up and see where, if anywhere, you're loosing coolant. Look on the underskirt too. It could be minimal, but dripping on it and blowing off when you drive so you might not notice it on the garage floor or where you park.

If you EVER begin to overheat, though, pull over, turn car off, and get it towed...or you'll suffer the fate others have here...not infrequently.

hth
 

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Discussion Starter #7
I'll check it in the AM. If anything, I'm worried I may have overfilled the coolant a bit. The stick was below the cap and I added 3 cups of the 50/50 mix. I'll check for leaks in the parking garage after work and tomorrow morning in the driveway.

Is there a way to remove some of the coolant without bleeding the entire system?
 

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I'll check it in the AM. If anything, I'm worried I may have overfilled the coolant a bit. The stick was below the cap and I added 3 cups of the 50/50 mix. I'll check for leaks in the parking garage after work and tomorrow morning in the driveway.

Is there a way to remove some of the coolant without bleeding the entire system?
I personally fill the expansion tank to the brim whilst bleeding, then use the body of a fluid transfer pump to bring the top of the float level with the expansion tank neck threads i.e. minimum when cold. Maybe a touch less if below 20 deg C ambient.
 

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Discussion Starter #9
Bad news this afternoon:

Coolant light lit up after about 15 minutes of driving. There appears to be a leak as well, but level was not low. I parked outside a store for 5 minutes after work and there was already some moisture (felt, smelled like coolant). Popped the hood after the commute this afternoon, and there was coolant bubbling/seeping through the cap. The signature white residue was there.

Also, red oil light flickered two or three times on the 5 minute drive from store to home.

Taking the car to the shop first thing in the AM.
 

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the people’s mod
2004 330Ci 115k miles
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14,435 Posts
The intermittant low coolant light is very common, especially when you have recently worked on the cooling system and lost some coolant, then refilled.

You check the level when cold and it is normal, then drive it for 5-10 minutes and the low coolant light comes on (when the thermostat opens). Then hours later after the engine is cold again you check it and the level is normal and say, "WTF?!"

You cannot, and must never, check the coolant level when hot. Yet everytime you check it when cold the level is normal.

The cause is trapped air in the system. Do a proper bleed and that solves it.
 

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Discussion Starter #13
Coolant and oil were low last night. Added 2-3 cups coolant to bring it to proper level and added about a half liter of oil. Dropped the car off and hitched a ride to the office this morning. Hope to have it back today.
 

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Discussion Starter #14
Dealer said it was a bleeder valve on a radiator hose. Thankfully, as of now, sounds like it's relatively cheap and not serious. No mention of oil light.
 
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