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Camshaft solenoids, sensors, and timing cover leaks
I recently purchased a 2017 E400 wagon with ~28k miles on it. I've been reading about some common issues and saw many mentions of the cam solenoid and sensors leaking, so I figured I'd check mine. I found oil residue under some of the solenoids and around the base of one of the sensors, but did not find any oil in any of the connectors. I also found oil on top of the valve covers, circled in red, but difficult to see, in my attached picture. I have a few questions regarding these items:
1) Is there any reason I can't just replace the o-rings that seal the solenoids and sensors, rather than replace the solenoids and sensors? What is the advantage of replacing the solenoids and sensors?
2) I saw mentioned that the route of oil into the harness is migration through the plastic bodies of the solenoids and sensors; has this been confirmed? Does this mean that these items are essentially wear items that will migrate oil no matter what? Or is oil seeping past the o-ring and wicking around the plastic bodies?
3) I am having no trouble finding the solenoid harness extensions, but the only sensor harness extensions I have found are some random ones on ebay for the m278. I assume these should be fine, but wanted to know if anyone had a recent verified source of them before risking questionable cables.
4) What source could the oil on top of the valve covers be from? I don't see how any of my solenoid or sensor leaks could get back there, and while I do have a timing cover leak, it does not appear that it could find it's way to that location.
You can buy the kit that includes 2 pig tails or four (if you have four solenoids). You will want to replace the solenoids if they're leaking. You cannot replace the O Rings on the Camshaft position sensors but you can replace the o rings on the camshaft magnet solenoids. However, the issue is that the oil wicks in through the magnets and go through the wire. So replacing the O Ring won't do you any good unless you're leaking oil from the sides of the solenoids.
Valve cover leak: need to reseal it. I just did mine after 160,000 miles. You'll want to fix the solenoids and get the pigtails before it ruins your ECU and wiring harness. That will be a $4,000+ job to fix later on.
Are the replacement sensors and solenoids any different, or will they just continue to have the same oil wicking issue?
Since I don't have any oil in my harness or connectors yet, I'm assuming my oil leaks are strictly from o-rings at the moment. They are a cheap try, so I will be giving it a go.
Why do you say you can't replace the o-rings on the position sensors? I've already removed the o-ring and replacements are a standard size. Is there something else I am missing?
Sounds like all of these oil leaks are more age related than mileage related. This does not bode well for the quality and condition of the seals that are far more difficult to access. Not impressed with the MB quality so far.
Are the replacement sensors and solenoids any different, or will they just continue to have the same oil wicking issue?
They'll leak one way or another. I've replaced one of my solenoids twice in 160,000 miles. The other solenoid was fine and hasn't been replaced yet.
Originally Posted by tdwpgtp
Why do you say you can't replace the o-rings on the position sensors? I've already removed the o-ring and replacements are a standard size. Is there something else I am missing?
I mean you technically can. But MB doesn't sell the O Rings for the position sensors. Only for the solenoids. You can however go to Harbor Freight or a hardware store and buy o rings for them. Will they be like OEM? prob not. Will they seal okay? probably. Would I risk it? No. I'd get new sensors.
Originally Posted by tdwpgtp
Sounds like all of these oil leaks are more age related than mileage related. This does not bode well for the quality and condition of the seals that are far more difficult to access. Not impressed with the MB quality so far.
Welcome to the modern day and age of owning a car my friend. They all suck when it comes to QA and materials.
Contact FCP Euro parts support with your VIN and see if they can identify an oring for your camshaft position sensors. I was able to change the orings on Audi engine sensors and avoided replacing the solenoid which was working fine. From what I understand from the TSB, the harness wicking issue occurs when oil begins to pass through the sensor or magnet, not around the external oring seal.
If you bought the Permatex for the valve cover leaks, you have to remove the covers, clean them, then apply a bead of sealant on the valve cover flange surfaces before remounting to the heads. The labor time guide is 8.3 hrs for one side and 12.4 hrs to do both sides because there is a lot of component removal required to access the covers.
Contact FCP Euro parts support with your VIN and see if they can identify an oring for your camshaft position sensors. I was able to change the orings on Audi engine sensors and avoided replacing the solenoid which was working fine. From what I understand from the TSB, the harness wicking issue occurs when oil begins to pass through the sensor or magnet, not around the external oring seal.
yes exactly: the CPS leak is internal not the O-ring.
Last edited by CaliBenzDriver; May 5, 2026 at 03:47 PM.
I've had very good results with Permatex on German cars. I believe Elring supplies the MB silicone sealant and you can buy the direct Elring Dirko much cheaper than the dealer option.
I've had very good results with Permatex on German cars. I believe Elring supplies the MB silicone sealant and you can buy the direct Elring Dirko much cheaper than the dealer option.
Yes, I believe that is the OE equivalent. I always check the product info on the manufacturer website when in doubt. The spec sheets will also include information on set time before final torque, cure time before operating engine, etc.
When using mb (loctite) make sure the product is less than six months old. It has a date on it. It won’t cure if old. I also like reinzosil / reinzoplast products..
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