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New to Mercedes with vacuum line question

5936 Views 7 Replies 2 Participants Last post by  bnforehand, 
bnforehand
I recently bought a nice condition 1994 c280. Just replaced the wiring harness with non-biodegradable one because I was getting a CEL for egr and air pump. Found the bad wiring at the egr switch ( kept unraveling tying to find a starting point to re-wrap). Anyway.... got the new harness on and it fired right up (cleared the CEL first). After driving a few times the CEL came back on for egr and air pump. I followed the vacuum lines and tested both valves with suction and they both worked and held a vacuum (the secondary air pump valve was stickier but it did open and close). I traced the other direction to find the source of the vacuum to the switch, and this is where it got strange. The vacuum line leads back from the egr switch on the front of the engine to the throttle body riser tube where it connects via a wye and another line to a fuel diaphragm of some sort at the front of the fuel rail. That wye was connected to a port on the riser tube just above the throttle body, but the port doesn’t breach the riser tube so it could get vacuum, it’s just a dead end. Where does the egr/air pump switch and that fuel pressure diaphragm get there vacuum? Those two vacuum lines go nowhere else, they just dead-end into the riser tube.

Thanks for any help on this. I’ll try to post pic if possible
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bnforehand
Also, no noticeable issues while driving (yet) just the CEL is on. The connection I’m referring to is near #128 (check valve) on the vacuum diagram I saw on google.
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bnforehand
#128 check valve. The wye to the right of that check valve is what I’m assuming plugs into that dead-end port on the riser tube above the throttle body

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128 shouldn't be going to a dead end, it should be going to a source of manifold vacuum. Could you perhaps take a pic of your engine at the connection in question? Try to also get one of the surrounding area.
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bnforehand
Patman1, thanks for the reply. Here are some photos. Is that port on the intake boot where it's supposed to get vacuum? You can see in the last picture that I capped the 'y' where it was plugged in to the port on the intake boot (it was a little loose in there so I put a cap on it). The black line goes to the valve on the fuel rail, the white line goes to the egr/air pump switch on the front of the motor.

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Oh that's absolutely the wrong spot to put that line to. It's the source of all your problems. I'm not sure where exactly the line is supposed to go, I'm not familiar with the layout on the M104 engine, but it will probably terminate at a small nipple near other vacuum lines on the intake manifold. You can follow the vacuum line that goes back to the engine from the vacuum distribution block that's mounted on the passenger fender (under the cover, I believe it's a light gray line). If it's similar to the M111, that lines will terminate at one of 3 nipples on the intake manifold, you will have one of those nipples open (where your open lines should be going to). The rubber connector at the end of the line is supposed to go 'around' a nipple, not 'into' a plug. If the nipples aren't clustered together, at least you will know what they look like, and you can find the one that is within reach of your open lines.
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bnforehand
Thanks again for the reply! I didn't find any open nipples for those lines to connect to. There was a nipple just below on the manifold, but it had a factory looking cap on it. Any idea what that very specific looking port on the intake boot is for? I could've sworn I saw those vacuum lines in question terminating into that port when I got the car and pulled it loose when I was fiddling with taking the intake crossover tube disconnected. Maybe someone with a c280 with the M104 engine can snap a photo or confirm that vacuum line placement? I would think if there was a vacuum nipple open to air that I would have some drive-ability issues, or it would be throwing codes related to a lean condition at the o2 sensor... thanks again for the help!
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bnforehand
Update: I found a great video on youtube about removing the intake manifold. I was able to see in that video that the vacuum lines I'm talking about for the egr and fuel regulator did hook up to that nipple on the throttle body that had the factory looking cap on it. I removed the cap and put the connector on there. I'll update if the CEL returns...

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