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1983 380SL
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Discussion Starter #1
Hello everyone! I have been enjoying driving my 107 a good bit during the covid lockdown. Up until now, I drove it so infrequently, I did not pay attention to fuel consumption. My 380SL seemingly runs fine. Idle is smooth. Power is OK. However, my gas mileage is terrible. Typically less than 12 MPG. It does not seem to be blowing any black suit out of the tailpipe.

When initially restoring it, I worked to clear up most of the vacuum leaks. Replaced the injectors, holders, rubber air hoses and most of the fuel system besides the distributor.

From reading different posts, I think I should check the fuel pressures before I try anything else. I purchased a CIS pressure gauge years ago when I was first trying to resurrect my 107, but I never mastered how to use it. I found in startek.info the ideal pressures. Is there a video or set of instructions that not only shows where to connect everything but also how to perform the testing?
 

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1983 380SL
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Discussion Starter #3
Thanks. I think I saw this last night, but I was too tired to comprehend it was what I was looking for.
 

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1983 380SL
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Discussion Starter #4
I am having a hard time deciphering what this means. I think I followed the thread properly and this is what I found.

At startup -> 2.5
After about 3 minutes ->3.5
After about 5 minutes-> 3.65

When closing the valve for system pressure ->3.2
 

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1989 560SL
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144 Posts
That varying pressure is the control pressure which changes based on engine temperature (more precisely, the warm-up regulator temperature) which provides a richer mixture at cold start and leans it out as it warms up. The closed valve shows system pressure which is what the main supply from the pump to the fuel distributor is running at. If these pressures are correct or within a close range, then the fuel mixture needs to be checked in open loop (oxygen sensor disconnected) and closed loop (to be sure the oxygen sensor is functioning properly).


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1983 380SL
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Discussion Starter #6
I pulled a couple of my plugs and I found them to be somewhat sooty, but not wet or oily. I believe that is an indication that the engine is running rich. After reading the EGV on Oxygen Sensor grounding, I decided to try that. I filled up last night when I got done, so I will drive it a bit this week and see if my MPG changes.
 

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1984 380SL
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Bet you find your sensor has failed. That will cause a rich condition. I used to get ~25 mpg although I drive like your grandmother.
 

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1983 380SL
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Discussion Starter #8
Bet you find your sensor has failed. That will cause a rich condition. I used to get ~25 mpg although I drive like your grandmother.
I replaced the sensor as part of the sorting process when I first got it. A few years back, but less than 3K miles. Think it is hosed from running rich?
 

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1989 560SL
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I replaced the sensor as part of the sorting process when I first got it. A few years back, but less than 3K miles. Think it is hosed from running rich?
Try simply disconnecting the sensor for a tankful and see if it changes. I have seen them fail causing an excessively rich mixture.
Ideally, using an exhaust gas analyzer is the best way to go as you can immediately confirm proper O2 sensor function as well as make sure your mixture setting is correct.


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1983 380SL
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Discussion Starter #10
Try simply disconnecting the sensor for a tankful and see if it changes. I have seen them fail causing an excessively rich mixture.
Ideally, using an exhaust gas analyzer is the best way to go as you can immediately confirm proper O2 sensor function as well as make sure your mixture setting is correct.


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Yeah, I guess I am sort of running the reverse test right now. I just grounded it Saturday night. I need to drive it a bit this week. If my mileage improves, then the problem is solved. I will probably also pull a couple plugs and see if the carbon soot is still there.

If not, then I should probably just disconnect it and see if it gets worse. They are not that expensive.

Replacing it last time was a PITA because it was stuck in the exhaust pipe with rust and heat for who knows how many years. This go round it should not be as difficult if I need to do it.
 

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It’s easy to check the sensor operation. Just hooking a voltmeter up to the black lead you should see the voltage swing from .1-.7Ish volts. Anything over .5 is richer than lambda and under is lean. You should also hear the frequency valve buzzing or feel it.
 

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1983 380SL
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Discussion Starter #12
It’s easy to check the sensor operation. Just hooking a voltmeter up to the black lead you should see the voltage swing from .1-.7Ish volts. Anything over .5 is richer than lambda and under is lean. You should also hear the frequency valve buzzing or feel it.
Can you be a bit more specific, where can I connect the voltmeter to the black lead? Somewhere not directly under the car?
 

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Trace the wire from the O2 sensor to its plug. If it’s a heated sensor there will be 2 white wires and one black wire. Volt meter goes on the black wire. Ground lead to a ground point like the battery negative or engine block. Engine is considered to be in closed loop when the O2 sensor is reporting a changing voltage around the .5 volt center point. it’s best to run the engine to operating temp and hold it at 2000 rpm’s for 30 seconds to wake up a weak sensor. engine remains in closed loop from idle to just before the full throttle switch. If the engine is not in closed loop at idle after warming the sensor but goes into closed loop above idle it could be the mixture screw has been fooled with to compensate for a vacuum leak.
 

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1983 380SL
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Discussion Starter #14
Trace the wire from the O2 sensor to its plug. If it’s a heated sensor there will be 2 white wires and one black wire. Volt meter goes on the black wire. Ground lead to a ground point like the battery negative or engine block. Engine is considered to be in closed loop when the O2 sensor is reporting a changing voltage around the .5 volt center point. it’s best to run the engine to operating temp and hold it at 2000 rpm’s for 30 seconds to wake up a weak sensor. engine remains in closed loop from idle to just before the full throttle switch. If the engine is not in closed loop at idle after warming the sensor but goes into closed loop above idle it could be the mixture screw has been fooled with to compensate for a vacuum leak.
I have the single wire type. I had to splice it together with the old wire. Is there a place inside the cabin or under the hood where I can test the voltage? I read something about the passenger side footwell.
 

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Maybe someone can chime in. I looked at my service info and they do not list plug location. I might also be on a diagnostic plug. I know K-jet but have not worked on that model 107.
 

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1985 380 SL
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Can confirm on the 380 the single wire O2 sensor plug is under the passenger footwell, beneath the plastic cover. The lambda control unit is located there as well.
 

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1983 380 SL
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If your gas mileage is that bad, then looking at the O2 sensor and the Lambda control is kind of a waist of time. The O2 sensor and the Lambda control make fine adjustments to the fuel mixture (via the joke called the frequency valve) which itself was a Rube Goldberg band-aide for 380's to pass emissions tests in the US. I'm not saying a bad O2 sensor/Lambda control can't cause poor gas mileage... just not as poor as you're experiencing.

If your system pressure is in the ballpark and your control pressure is in the ballpark and you have verified that the cold start injector is not leaking, but you're getting horrible gas mileage and black smoke with carbon fouled plugs, you probably have a FD issue.

The CIS system (Continuous Injection System) routes fuel to the injectors 100% of the time (the injectors do not turn on and off). Even if the injectors are bad it will not increase the amount of fuel the FD routes to the injectors... that is 100% under the control of the fuel plunger and fuel metering slots inside the FD.

If I were forced to guess I'd say you have an issue inside the FD that is allowing excessive system pressure (bottom chamber in the FD) to get past the diaphragm and flood the upper chamber(s) with fuel. That will cause excessive flow to the injector(s) which then causes incomplete combustion (not enough O2 for the amount of fuel) which results in black smoke, carbon fouled plugs and horrible gas mileage.

Good luck.
 
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