[CFBoost1]

Today the Service Engine Soon light came on the dash of my 2008 e90 335I and I'm looking for opinions on what could be the culprit. To help I'll provide quick explanation of my ownership.

Bought the car in about 9 months ago in March 2016 with around 43,500 miles. The day after I bought the car the SES light came on. I took it to the dealer mechanic the next day and the SES light turned off as I was pulling into the parking lot. The mechanic hooked up a diagnostic tool and indicated there were a few insignificant codes. Seeing as the light went off on it's own, I trusted him and hoped it wouldn't come back on.

I've since put about 6,500 miles on the car as it hit 50,000 miles today. the car isn't my daily and usually gets driven every couple days but sometimes not for a week.

Two weeks ago I was driving when the Low Coolant indicator came on. The car remained parked for two weeks until today when I was able to get BMW coolant to fill up. It took about a half gallon of 50/50 mix to bring the the float indicator to the very top.

I then drove the car a few miles to run some errands and on the way home the SES light came on. It should be noted that the car seemed to be running fine.

A few questions:

Does needing a half gallon refill indicate for sure a cooling system leak? I checked under the car after it sat in the garage for a few hours and the only small puddle was water, completely clear.

Is it possible I over filled?

Could an SES light be triggered by a weak battery? The car starts fine but usually the time, trip, mpg, etc are cleared if I don't drive the car for a few days. This makes me think the battery isn't 100%.

Should I drive the car to see if the SES light turns itself off as it did the first time?

Sorry for the long post and thanks for all the help!

[bloppitybloop]

You should pull the codes.

[F22_Raptor]

^ pull the codes to see what they are. This way you know what your car is trying to tell you rather than wondering what is wrong.
As for the coolant, 1/2 gallon is a lot for just a top up. To check for leaks you can't just look under the car because of the plastic shield on the bottom of the car.
To my knowledge a weak battery won't throw a code.

[catcher22]

The only way to know what the light is about is to pull the codes and look em up. Without doing that it's a complete guessing game.

I've never just topped off my coolant, always done the entire flush procedure when needed. Hopefully your waterpump and thermostat arent on their way out (mine died at 59k and 74k). Yes I'm on my third waterpump and tstat thank u bmw

[gutshot]

If you are losing coolant, it could be a cracked expansion tank. I just replaced mine last week on a 2011 335xi with 67,000 miles. The water pump went out about a 1000 miles before.

[kkasson]

The coolant level should be checked when the engine is completely cold, and the correct level is when the top of the float is even with the neck of the tank, or slightly above it.

Sometimes the float will get a little stuck, so I like to add a little bit and push the float down and wait for it to pop up. If it's still low I'll keep adding it.

1/2 gallon is a lot, but not necessarily a leak. I've had to add 1/2 to 1 quart from normal evaporation. Now that it's full, just keep monitoring the level. If it's not dropping then I wouldn't worry about it.

A low battery can cause the resetting issue, but another common cause is a bad battery cable/IBS sensor. The battery is easily accessible in the trunk...sometimes there's a date marked on the battery of when it was sold. You could quickly check to see how old it is.

[GoRomeo]

yeah pull the codes. Test your battery. Do a load test if you can. You may have 2 issues, low coolant due to a leak and a weak battery. Especially is you dont drive it often. You battery may be old now and not holding the charge.

[CFBoost1]

Originally Posted by kkasson

The coolant level should be checked when the engine is completely cold, and the correct level is when the top of the float is even with the neck of the tank, or slightly above it.

Sometimes the float will get a little stuck, so I like to add a little bit and push the float down and wait for it to pop up. If it's still low I'll keep adding it.

1/2 gallon is a lot, but not necessarily a leak. I've had to add 1/2 to 1 quart from normal evaporation. Now that it's full, just keep monitoring the level. If it's not dropping then I wouldn't worry about it.

A low battery can cause the resetting issue, but another common cause is a bad battery cable/IBS sensor. The battery is easily accessible in the trunk...sometimes there's a date marked on the battery of when it was sold. You could quickly check to see how old it is.

Great. Thanks for the information.

[CFBoost1]

Originally Posted by autoart

Their was a recall for the battery cable, did that get done?
Here is a big tip, If you take the battery out DON'T CLOSE THE TRUCK!!

I'm not sure if the battery cable recall has been done. Is there a way to find out?

And thanks for the tip on the not closing the trunk!

[robthewrench]

Originally Posted by CFBoost1

Does needing a half gallon refill indicate for sure a cooling system leak? I checked under the car after it sat in the garage for a few hours and the only small puddle was water, completely clear.

If this is your first time filling up the coolant overflow tank, you may not have a leak. If system goes low again in the next 3-6 months, you may have a small leak.

You don't say when the last time the coolant was topped off or the cooling system was serviced. It is quite possible it was not full earlier and you only lost ~a quart over a long period of time.

Wait and see.

[CFBoost1]

Originally Posted by robthewrench

If this is your first time filling up the coolant overflow tank, you may not have a leak. If system goes low again in the next 3-6 months, you may have a small leak.

You don't say when the last time the coolant was topped off or the cooling system was serviced. It is quite possible it was not full earlier and you only lost ~a quart over a long period of time.

Wait and see.

That's a good point and I am not sure when the last time the coolant was topped off as I purchased the car in March 2016 and have only put about 6,500 miles on the car.

As you mentioned, I've filled it up and will wait and see.