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Marty
Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 49406
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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1985 380sl: intermittent..it quits..high idle problem..control unit

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i have a 1985 380sl, intermittent stalling when running after an hour, now stalling all the time. idle oscillates up and down before it quits. previously fixed high idle problem with new idle control unit, idle control valve. it also has a new distributor._xtra_<jarootja>optional information: <br> 1985 mercedes-benz 380sl v8<br><br></jarootja>
Submitted: 5 years ago.
Category: Mercedes
Expert:  Marty replied 5 years ago.

HiCustomer I see from your previous posts that you've been battling this problem for awhile. It's absolutely necessary that a CIS fuel pressure gauge be connected to the fuel system to monitor all three of the system pressures in order to diagnose this problem. The CIS fuel injection is highly dependent on fuel pressure. Loss of fuel pressure or incorrect upper and lower chamber pressures can result in loss of power on acceleration. With the gauge connected you can see if you're getting the proper acceleration enrichment from the warm up regulator. It's also possible that the overvoltage protection relay is acting up. This would be noticeable by the anti-lock warning light coming on in the instrument cluster. You could also try disconnecting the CIS control unit and resetting your fuel mixture with it diconnected and see if it still bogs on acceleration with the control unit unhooked. We're focusing on the fuel system right now because from your description that's what it sounds like. Let me know if you have any questions.

 

Marty

Customer: replied 5 years ago.
the anti-lock light does come on once in awhile but is it suppose to come on when there is no power on acceleration? if your answer is yes it could come on, where is overvoltage protection relay located?
Expert:  Marty replied 5 years ago.

The anti lock light should never come on while the car is running. If it does then the relay may be bad. The relay is located above the fuse box. I've attached a diagram and picture of the relay for you.

 

graphic

graphic

Customer: replied 5 years ago.
changed out relay, but it still no power. i am going to try to cis control. where is it located?
Expert:  Marty replied 5 years ago.

The CIS or Lambda control unit is located in the passengers footwell underneath the carpeting. There will be a plastic panel in the footwell area. The control unit is located behind the panel.

 

Marty

 

graphic

Customer: replied 5 years ago.
<p>when you mention about resetting the cis control unit, couldn't this be done just by disconnecting the negative side of the battery? getting behind the plastic panel proves very difficult. i have removed three retaining clips for the panel but something is still holding the plastic panel down. </p>
Expert:  Marty replied 5 years ago.
Actually what I was referring to was disconnecting the unit and making your adjustments with it disconnected. We can accomplish the same thing by disconnecting the frequency valve under the hood. Do you know where that is?
Customer: replied 5 years ago.
the frequency valve is much easier to get to i will try it tomorrow,
Expert:  Marty replied 5 years ago.

Ok, when you disconnect it you will need to readjust your mixture. I really think a fuel pressure test is necessary however. We need to know what the fuel pressure is doing when it runs bad. I'm thinking it's also possible that our idle control valve is shutting down for some reason. We can disconnect that next and drive it with it unhooked. We'll need to choke down the air through the valve so it doesn't idle too high. Let's do one thing at a time though.

 

Marty

Customer: replied 5 years ago.
it is so difficult to re-enact the problem and measure the fuel pressure, by the time you get it to a mechanic it stops doing it. a mercedes mechanic did measure the the fuel pressure because he did catch it stalling and corrected the fuel pressure. but he was unable to recreate the now power when stepping on the pedal. could it be the frequency valve or the lambda unit? i will go ahead and accept after your answer.
Expert:  Marty replied 5 years ago.

HiCustomer you can drive the car with the frequency valve disconnected. You'll just need to readjust your fuel mixture. By doing so you will rule out the lambda control system as the problem. I'd be suspicious of the warm up regulator but you'll need to test the fuel pressure to diagnose it. Intermittent problems are the worst to try and figure out. It takes patience and persistance. Please let me know if I can be of further assistance.

 

Marty

Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 49406
Experience: Bosch Certified Master Tech. Over 35 years experience.
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