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98 c280: my rpms dont drop below 1200..Check engine light…

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I have a 98 c280...
I have a 98 c280 and my rpms dont drop below 1200. Check engine light up after some driving. What can it be?
Submitted: 8 years ago.Category: Mercedes
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Answered in 4 hours by:
6/27/2010
Mercedes Mechanic: Michael Warfield, Factory Mercedes-Benz Technician replied 8 years ago
Michael Warfield
Michael Warfield, Factory Mercedes-Benz Technician
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Satisfied Customers: 3,143
Experience: Mercedes-Benz Master Certified technician
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Why were you replacing all the gaskets in the first place? Was it because of the high rpm or something else?

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Customer reply replied 8 years ago
No. I had some oil leaks from valve covers and I decided to take out the intake manifold to clean it. Before the repairs everything was fine. I did have loss of horse power so I replaced spark plugs and wires. The car has 175k and I never replaced the wires and plugs. I also replaced some of the vacuum hoses due to some wear on them. When I assembled everything and started the car, the rpms just stayed at 1200. I thought that it will go down eventually. I drove the car for about 15 miles and the check engine light came on. I reset the light by disconnecting the battery. Now the light stays off but I didn't drive the car yet.
Mercedes Mechanic: Michael Warfield, Factory Mercedes-Benz Technician replied 8 years ago
I would try spraying some brake and carburetor cleaner around the back of the intake with the air cleaner on and engine running. If there is a vacuum leak, which I highly suspect there is, the idle will change and the engine will start to run rough. I am almost positive there is some line on the throttle body or valve covers that is torn or leaking. Check the PCV hoses on the valve cover breathers for being torn. There is so many miles on this car, it is common for these hose to become brittle and crack when you remove the valve covers.
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Mercedes Mechanic: Michael Warfield, Factory Mercedes-Benz Technician replied 8 years ago
Have you gotten anywhere with the engine?
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Customer reply replied 8 years ago
Hi, I replaced all the hoses with original ones but i will try to spray the cleaner inside to see what happens. Does egr valve play a big role? Because when i was removing it, I broke the bottom of it that sticks inside the intake manifold. I went to a dealer and that thing was over $200 so i glued it with the a & j weld. But there is a gasket so i don't think it could leak. Sorry it took me a bit to respond. The problem went away and now its back. I took the car for a drive and after driving for 40 miles the RPM went to 2000 while the car is in park position. And check engine light came on again. I don't know, its crazy. Thanks.
Mercedes Mechanic: Michael Warfield, Factory Mercedes-Benz Technician replied 8 years ago
Yes the EGR does play a vital role. If it is leaking it will cause a major vacuum leak. Try spraying the car or brake clean around the egr valve to check it.
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Customer reply replied 8 years ago
When i will spray what am i going to see? I will do it right now.
Customer reply replied 8 years ago
I just sprayed the brake cleaner on the egr and also in the engine while it was running. The idling was rough but i didn't see any leaks.
Mercedes Mechanic: Michael Warfield, Factory Mercedes-Benz Technician replied 8 years ago

If it changed the idle then you have a leak. You may not hear the leak over the engine noise.

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Customer reply replied 8 years ago

Is there anyway to see where the leak is coming from? Will the obd2 code reader let me know?

Mercedes Mechanic: Michael Warfield, Factory Mercedes-Benz Technician replied 8 years ago

Eventually the check engine light will come on and code up for self-adaptation at idle. Vacuum leaks are hard to see, but if the idle did get rough or the engine stumbled you definitely have a leak. So I would advise taking the EGR and tube back off and looking at that.

Michael Warfield
Michael Warfield, Factory Mercedes-Benz Technician
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Customer reply replied 8 years ago
Ok. Thanks.
Mercedes Mechanic: Michael Warfield, Factory Mercedes-Benz Technician replied 8 years ago
let me know how it goes.
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Customer reply replied 8 years ago
Well. I took water with soap and started spraying everywhere. And I noticed oil leak from the valve cover plug. Not the main one but the second one. So i think that could be the problem. I used a sealer on it so will see. Other then that i so no leaks. Thanks.
Mercedes Mechanic: Michael Warfield, Factory Mercedes-Benz Technician replied 8 years ago

The point of using the brake or carb clean is the vacuum leak will suck it in creating the rough idle or stumble. If you notice the engine starts running rough when spraying the brake clean in a certain area, thats where I would look first such as your EGR valve.

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Customer reply replied 8 years ago

Oh, ok. I will do that tomorrow. I will let you know. Thanks a lot.

Mercedes Mechanic: Michael Warfield, Factory Mercedes-Benz Technician replied 8 years ago

Ok let me know.

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Customer reply replied 8 years ago
Hi, I just found the codes for the check engine light and it says, PO170 and PO173 fuel trim malfunction bank 1 & 2. Also i started spraying the brake cleaner everywhere and everything was fine until i sprayed on the throttle body the engine started idle rough. I took it apart and there is no gasket between the throttle and intake manifold. Is there supposed to be one? Thanks a lot for your help.
Mercedes Mechanic: Michael Warfield, Factory Mercedes-Benz Technician replied 8 years ago

Those codes mean you definitely have a vacuum leak or a bad MAF sensor. I believe there is suppose to be a gasket, but also check to make sure the rubber vacuum lines that connect to the throttle body aren't torn or missing. When you spray the brake clean, make sure intake pipe is on the throttle motor.

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Customer reply replied 8 years ago
Ok thanks. I called a dealer and they said that there is a gasket so i will go pick it up. Will see what happens. Thanks again.
Customer reply replied 8 years ago
Hi. I installed a missing seal on the throttle body and now the car runs great no more problems. Thanks a lot.
Mercedes Mechanic: Michael Warfield, Factory Mercedes-Benz Technician replied 8 years ago
Your welcome
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