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Check Engine Light P0299 Turbo Underboost 2015 C300

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Check Engine Light P0299 Turbo Underboost 2015 C300

 
Old 10-09-2017, 10:14 PM
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Check Engine Light P0299 Turbo Underboost 2015 C300

I got a check engine light a week ago. The code is P0299 Turbo Underboost. I took the car to Mercedes Service they said:
1. I need a new pressure valve part number 000-153-18-59 ($250)
2. I need a new water pump ($916)

I'm not sure how the water pump is related to the turbo boost. Can anyone help me out by explaining how they are related. Dealer says that tested it and it's coming back as failing and that its leaking vacuum from the turbo. It wasn't leaking coolant nor did the car ever overheat. I had them change the pressure valve first. I drove the car home and check engine light came back on.

The car is a 2015 C300 with 75k miles. I have had several Mercedes vehicles over the years and I never had to change a water pump. I had an ML350 with 196k miles before I sold it not 1 major issue. I guess my luck with Mercedes has run out.
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Old 10-16-2017, 05:27 PM
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I spoke with the technician who diagnosed my car. He indicated that the water pump is actuated by the car's vacuum. The water pump actuator on my car is leaking vacuum and as a result the turbo waste gate which is also actuated by the car's vacuum is not operating properly hence the check engine light.

Attached is a pic of the water pump. The actuator comes with the pump as one part. It looks like a it's pain to get to under the hood. I'm not sure why it's designed this way. I thought water pumps were always on. I saw another post on this forum about the C300's overheating. This is probably the cause.


W205 Coolant Pump
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Old 10-16-2017, 05:35 PM
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Here is a YouTube video of the pump being actuated:
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Old 10-17-2017, 08:21 PM
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Originally Posted by hiawan View Post
I spoke with the technician who diagnosed my car. He indicated that the water pump is actuated by the car's vacuum. The water pump actuator on my car is leaking vacuum and as a result the turbo waste gate which is also actuated by the car's vacuum is not operating properly hence the check engine light.

Attached is a pic of the water pump. The actuator comes with the pump as one part. It looks like a it's pain to get to under the hood. I'm not sure why it's designed this way. I thought water pumps were always on. I saw another post on this forum about the C300's overheating. This is probably the cause.
Good to know, thanks for posting this info.
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Old 11-16-2017, 05:17 PM
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I wanted to give everyone an update. I had the dealer replace the pressure valve (part number 000-153-18-59 $250) only. As expected the check engine light came back on my way home.

I removed the the vacuum line from the water pump and plugged it temporarily to isolate what the dealer said was the problem. This did not fix the issue. The check engine light p0299 comes on after hard acceleration and the car goes into limp mode where there is no power to the wheels after 4k RPM. This only happens when the car is hot.
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Old 11-16-2017, 07:50 PM
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Are you out of warranty coverage?
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Old 12-11-2017, 11:33 AM
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Originally Posted by RunwayOneSix View Post
Are you out of warranty coverage?
yes, I have 80k miles
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Old 12-11-2017, 03:35 PM
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The vacuum line is to the computer controlled thermostat. It uses vacuum to actuate an ice cream scoop looking flapper which speeds up or slows down the amount of coolant that goes in and out of the motor and to the radiator. Have you ever noticed how fast the car warms up and then will drop like 1/2 way down and then come back up? I don't think this is linked to the underboost issues. The underboost issue could be the wastegate flapper getting stuck open or there could actually be a boost leak in the piping system somewhere. Have them do a boost pressure test. If you have a leak, it'll show up. If no leak, I would look at the actuator. If nothing there, try unplugging the ECU and allowing it to reset 100% to the base mapping without any of the learned strategy and see if that fixes it.
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Old 09-10-2018, 09:53 PM
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the real issue here is that mercedes is JUNK and that they design their cars to FAIL. I work at a private shop and work a lot on benzes and i drive a benz too, but the newer stuff is just garbage. All plastic housings, prone to leak, prone to break, plastic vacuum lines brittle as ****, We had a gla 250 today with 35,000km with check engine and needed a thermostat. Well guess what? The intake manifold has to come out, to do the thermostat housing. I can't blame mercedes fully, but most new cars are designed overly complex, with no quality parts that warranty the complexity of the situation. High pressure pumps failing, turbos leaking, and all your money spent that you supposedly saved at the pump + more now. Buy a tesla , the gas industry is a conspiracy in itself.
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Old 09-12-2018, 08:27 AM
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Originally Posted by hiawan View Post
I'm not sure how the water pump is related to the turbo boost. Can anyone help me out by explaining how they are related.
The turbo outlet air is cooled by an air-to-water intercooler. If you're water pump isn't operating correctly then your manifold inlet air temperatures will be too high and the computer is likely cutting turbo boost and going into limp mode since it is out of the design specification. Hence why this happens when the car is hot vs. cold.
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Old 01-12-2019, 09:02 PM
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Originally Posted by TurboKits.com View Post
The vacuum line is to the computer controlled thermostat. It uses vacuum to actuate an ice cream scoop looking flapper which speeds up or slows down the amount of coolant that goes in and out of the motor and to the radiator. Have you ever noticed how fast the car warms up and then will drop like 1/2 way down and then come back up? I don't think this is linked to the underboost issues. The underboost issue could be the wastegate flapper getting stuck open or there could actually be a boost leak in the piping system somewhere. Have them do a boost pressure test. If you have a leak, it'll show up. If no leak, I would look at the actuator. If nothing there, try unplugging the ECU and allowing it to reset 100% to the base mapping without any of the learned strategy and see if that fixes it.
how do I unplug the ECU
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Old 01-14-2019, 10:17 AM
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Originally Posted by Joeydistio View Post

how do I unplug the ECU
Easiest way is to pull the negative battery cable for about 30 seconds.
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Old 01-15-2019, 09:16 AM
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Originally Posted by TurboKits.com View Post
The vacuum line is to the computer controlled thermostat. It uses vacuum to actuate an ice cream scoop looking flapper which speeds up or slows down the amount of coolant that goes in and out of the motor and to the radiator. Have you ever noticed how fast the car warms up and then will drop like 1/2 way down and then come back up? I don't think this is linked to the underboost issues. The underboost issue could be the wastegate flapper getting stuck open or there could actually be a boost leak in the piping system somewhere. Have them do a boost pressure test. If you have a leak, it'll show up. If no leak, I would look at the actuator. If nothing there, try unplugging the ECU and allowing it to reset 100% to the base mapping without any of the learned strategy and see if that fixes it.
Strictly subjective, it seems like my '16 C300 takes an inordinately long time to completely warm up. I drive 3 miles to the Interstate and then another 3 miles at interstate speed before the car reaches full operating temperature. Dealer says it's normal. I don't see the fast warm up, drop down, and then back up you describe. Is there anything specific that can be checked to see if the computer controlled thermostat system is functioning properly?

Last edited by Popoff; 01-15-2019 at 09:24 AM.
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Old 01-15-2019, 12:08 PM
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Originally Posted by Popoff View Post
Strictly subjective, it seems like my '16 C300 takes an inordinately long time to completely warm up. I drive 3 miles to the Interstate and then another 3 miles at interstate speed before the car reaches full operating temperature. Dealer says it's normal. I don't see the fast warm up, drop down, and then back up you describe. Is there anything specific that can be checked to see if the computer controlled thermostat system is functioning properly?
Monitor the actual coolant temp via OBDII. I was giving my experience on the electronically controller thermostat.

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Old 02-07-2019, 02:16 AM
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Hi there, just wanted to leave my experience with this issue: last week I was driving on the mountains when suddenly I decided to lower a gear or 2 to reduce the downhill speed... well I guess I reved the engine too much, almost hit the red bars. I was able to go all the way to the bottom of the mountains with no problems until when I tried to pass a car I realized the engine had no power, the limp mode kicked in. I thought it was the transmission but moments later the check engine came out, this actually happen after I turned off the engine to get rid of the limp mode, I decided to plug an CEL Reader and the P0299 came. Then I loked under the hood and guess what? Burst pipe... see pictures. Since I needed to return home and drive thru the mountains I decided to place a bandaid... clean the crack with alcohol, fill it with ABS cement, then place a fiberglass matrix over the abs and then more abs around... I placed a total of 3 layers of abs, and let it dry for the night, the car has been working fine but, again this is a band aid while the replacement part arrives. I got the original pipe for 100 bucks from the Mercedes Benz of Miami (online) shipping included and no tax.


Air charge pipe damaged (due to excessive rev/pressure)

Air charge pipe (repaired)

Repaired pipe installed

View from under the car

Last edited by Orlando Torres; 02-07-2019 at 02:18 AM.
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