Hello my 87' 420 SEL has been having some engine problems lately. When it is parked and the engine is on it be idleing at 1500rpm when it should be idling at around 650rpm. I dont know what could possibly be the problem. When I checked the engine compartment the choke remains closed. Please help for any tips and suggestions.


Thankyou

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Ashman

Any other symptoms like the ABS lamp being on??
The overvoltage relay is a common problem not supplying power to the idle control computer.

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
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44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

No there is no light on in the car meaning the abs. Can you please tell me about the ovr switch please? Im not really good when it comes to all the engine tech stuff. thx

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Ashman

Ashman:

You don't have a choke, it's not stuck! You have either a big vac leak (unlikely) or a stuck idle control valve, or a failure in the idle speed control circuit.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Sounds like you have an air leak. The engine management controls idle speed by regulating the airflow - if any hoses are loose, leaky, or ruptured, the engine will suck air that the controls can't shut down. Worse, your mixture will be off, because the airflow meter (what you are calling a 'choke') doesn't see this 'leaked' air and can't mechanically meter the fuel into the range where the electronic controls can compensate.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

Then you're going to have to take it to a qualified tech. for correct analysis/repair.

Disconnect the idle control valve if the idle does not change then you need a new one. When the idle control valve fails the idle will sit at about 1500 rpms at all times.

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Martin Ingram
Colorado Springs
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)

To evaluate your problem you will need to know a number of things. They may start all the way back with how an internal combustion engine works. You might not know or understand that engine speed is controlled by controlling the air not the gas. The "Gas Pedal" is a misnomer. You don't step on the gas; you step on the air and the engine control system adds the appropriate amount of gas.

Once you feel comforatble with general engine performance then the individual MB systems involved are very well explained in these archives. THe system you should become familiar with is the idle air control system.

Understand the theory and the testing concept becomes natural. To understand why it doesn't work one must understand why it does work.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

here is a picture of the idle control valve it is an easy test unplug it.
Steve is right I have learnt the difference on CIS injection it is all about air flow.

Attached Thumbnails

 

__________________
Martin Ingram
Colorado Springs
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)