Engine Idle Speed Adjustment - Mercedes-Benz Forum
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post #1 of 13 (permalink) Old 05-26-2019, 04:32 PM Thread Starter
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Vehicle: 300SE 1991, 300E 1988 SOLD(240D82, 300SD84, 190E91, 420SEL87
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Engine Idle Speed Adjustment

I have 1991 300 SE, gave it to shop for body work, went back to pick up the car, the body shop indicated that car ran out of Gas, so I filled up and after long cranking it slowly fired up and then ran fine would start normally, but next morning i had to crank the engine same way like I did after gas fill up, Took it to the local Mechanic who removed the Air filter assembly and adjusted something with an allen Key in the throttle body, the car is starting OK but now it is idling at 400RPM and tries to stall whenever I stop at the stop light or slow down, can someone explain how to bring the idle speed to around 700-800RPM level, it also seems that it is now on rich mixture adjustment as I smell a lot of unburnt gas when it is started.
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post #2 of 13 (permalink) Old 05-27-2019, 02:47 AM
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Is you gas tank fuel cap closed tight . If loose you will lose pressure .

This is a retro ride .
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post #3 of 13 (permalink) Old 05-27-2019, 04:27 AM Thread Starter
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Yes! fuel tank cap is tight and shows appropriate pressure buildup when opened.
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post #4 of 13 (permalink) Old 05-27-2019, 05:45 AM
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The mechanic over adjusted the screw used to adjust the EHA duty cycle. Probably to the point of holding the air meter open. This is not an idle adjustment.

You most likely have a vacuum leak or other issue causing your low idle. There is no adjustment for idle speed on these cars.
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1989 JDM 420SEL The new project
2003 Volvo V70 5 Speed -Current Daily

1983 BMW 733i Fixed up and traded for above 420SEL
1997 E290 Turbo Diesel Wagon -traded for the volvo, an amazingly comfortable car to drive, great drivetrain, but the rust, oh my god the rust
1992 BMW 525i -traded in for E290
1990 Silver 300TE -Sold Loved the wagon, hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold
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post #5 of 13 (permalink) Old 05-27-2019, 07:17 AM
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I agree. That is not an idle adjustment. Vacuum leak is quite likely a possible reason.

I've heard that running to empty can spoil the fuel filter. I would change the fuel filter after running to an empty tank. If the filter is clogged with sediment, it can cause the fuel pump to overheat and die, as well. Running to empty also might mean the tank filter has issues with sediment. Filter related issues usually cause lack of power when going up a hill. If the car picks up and goes fine up a steep hill, then it's probably not a critical filter issue.
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post #6 of 13 (permalink) Old 05-28-2019, 08:26 PM
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This will be interesting to follow. Been studying the KE Jet injection system. It seems adjusting the Air flow meter may have solved a starting issue but may have caused two new issues: rich fuel mixture and low idle.

As far as I know this adjustment is the last thing to try. You might want to set it back to where is was.

At least for my 1986, the fuel pumps run for a second or two when you turn the key to run. It could be that your starting issue is lack of fuel after sitting for a while. Try turning the key to run for a couple seconds. Do not turn key to start.. You should hear the pumps run and move fuel. Repeat this two or three times until you no longer hear the pumps move fuel then try starting it. Could be a bad fuel pressure regulator or some other item allowing fuel to drain out while sitting.

If this does not work I I'll return my fees!
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post #7 of 13 (permalink) Old 05-29-2019, 07:11 AM Thread Starter
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Thanks for your inputs, I have turned the screw back to where it was and the RPM now is showing as 800 also replaced the fuel filter and it seems to act much better than before. Shall try it again for a cold start and see how it reacts?
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post #8 of 13 (permalink) Old 05-29-2019, 07:20 AM
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To correctly set that screw you’ll need to set the lambda duty cycle. A quick search will turn up some excellent threads on the subject. Particularly by member HD.

1989 JDM 420SEL The new project
2003 Volvo V70 5 Speed -Current Daily

1983 BMW 733i Fixed up and traded for above 420SEL
1997 E290 Turbo Diesel Wagon -traded for the volvo, an amazingly comfortable car to drive, great drivetrain, but the rust, oh my god the rust
1992 BMW 525i -traded in for E290
1990 Silver 300TE -Sold Loved the wagon, hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold
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post #9 of 13 (permalink) Old 05-29-2019, 08:14 AM Thread Starter
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I started it this morning, took more than normal cranking and kept running at 400RPM for about 2 minutes and finally picked up idle speed at 800RPM.
drove it later after an hour and behaved normal start and running.
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post #10 of 13 (permalink) Old 05-29-2019, 05:13 PM
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You need to hook up a fuel pressure gauge and see whats going on. Pump may be getting weak or bleeding down after sitting. Also take heed of my previous post on properly setting the lamda duty cycle

1989 JDM 420SEL The new project
2003 Volvo V70 5 Speed -Current Daily

1983 BMW 733i Fixed up and traded for above 420SEL
1997 E290 Turbo Diesel Wagon -traded for the volvo, an amazingly comfortable car to drive, great drivetrain, but the rust, oh my god the rust
1992 BMW 525i -traded in for E290
1990 Silver 300TE -Sold Loved the wagon, hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold
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