Mercedes 420 SEL Too Slow

Bill Owner/Technician Jackson, Michigan Posted   Latest   Edited  
1990 Mercedes-Benz 420SEL 4.2L 4-spd (722.3)—WDBCA35EXLA545328
Lack of Power

Hello group I have been asked to take a look at this vintage Mercedes for lack of power issue... It's a real clean vehicle with the same owner for the last 28 years.... I am by no means the right guy for this vehicle/job (no experience on this era of MB or this fuel system) but I suspect I will be able to figure something out with a bit of time and testing ideas...I could use some advice for sure

The engine seems to start and run smoothly, it just has very slow acceleration like the gas pedal is not being pushed all the way down but I am on the floor board. So far I have checked exhaust system back pressure ( less than 1 psi at 2500) and replaced the fuel filter with no improvement (cust said never had one 20+ years) fuel supply pressure is about 55-60psi KOEO. I am curious about the throttle linkages and or the throttle input as it seems when I have the gas pedal pushed all the way down I can still move the linkage underhood by hand further? I don't know if this is normal? It doesn't appear anyone has been messing around with the linkages. 

In this picture of the throttle linkages is that center linkage supposed to extend and retract like that? It is almost like a mini shock? 

Thomas Diagnostician
Saint Petersburg, Florida
Thomas Default

Used to have one of those. Great car! Had to pull the engine for a proper reseal and experienced the same problem. I was baffled at first, but sure enough it was in that linkage. It was years ago, so I forget what I did, but 'adjusting" that linkage fixed it.

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Christopher Owner/Technician
Glendale, Arizona
Christopher Default

Make sure both fuel pumps are working, fuel pressure seems low. I believe it should be around 90psi

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Malcolm Owner/Technician
Rochester, United Kingdom
Malcolm Default

this is a Bosch KE system dose the diagram plate move freely

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Manuel Diagnostician
Newark, New Jersey
Manuel Default

I had this problem on same build turned out to be the eha try this youtu​.​be/XclGM2RgDDo

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Hollis Technician
Boulder, Colorado
Hollis Default

In image #3, the tape around the spark plug wires appears recent... Is the lack of apparent power a recent issue, something that's been addressed by someone else ? That linkage mini-shock, IIRC, is an equivalent to a carburetor Dashpot, dampener of sorts, keeps the doohickey from slamming shut :O) Does this system have a single wire O2 sensor, if so check its output. That air /fuel metering…

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Michal Owner
Houston, Texas
Michal Default

there is a relay on the firewall with fuses on top. If one is blown, you will have no power, and a high idle speed, and likely abs light too . The internal board of the relay can fail as well, so no voltage gets to the pins even if fuses are good.

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Long Owner/Technician
San Jose, California
Long Default

That relay is called Over Voltage Protection Relay and it supplied power to KE/ABS control modules. Make sure your system pressure is good (System pressure should be about 76.9-79.8 psi). Here is the fuel pressure test procedure(see attachment): Hope this helps!

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James Technician
Portola Valley, California
James Default

That electric motor on the front of the engine connected to the throttle is the cruise control. Maybe it is seized or not working, You can disconnect the linkage and see if it improves.

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Aaron Owner/Technician
Madison, Wisconsin
Aaron Default

If you suspect the throttle isn’t opening fully, you can push the airflow meter plate fully open and look at the throttle plate visually when someone is pushing the pedal to the floor. It could need bushings or an adjustment. I recommend reading up on Bosch KE-Jetronic. There is a fantastic book out there, “Bosch Fuel Injection & Engine Management” by Charles Probst. I bought mine from…

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Martha Technician
Santa Cruz, California
Martha Default

First thing I would look at is the "mean" value of oxygen sensor. Disconnect plug, should be under passenger carpet near the center console floor area. Test at single wire plug/white wire/ harness end/ KOEO approx. 450-500mv (bias voltage) You can leave the 2 pin plug alone(heater) for now, but tap into the O2 wire and see what occurs to exhaust value when problem is occurring. Likely it will…

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Paul Technician
Upland, California
Paul Default

Bill, Lube all of the throttle linkage. At the dealer, we all kept a squirt can with ATF in it to lube all the linkage ends. If you can move the throttle more even with someone else pushing the gas pedal to the floor, you need to start with the linkage first. I owned a 1986 420 SEL that had been traded in with a jumped timing chain due to a broken rail in the left cylinder head. I bought the…

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Bill Owner/Technician
Jackson, Michigan
Bill Resolution

Thank you to all who took the time to offer me help on this one!! I have been able to make some noticeable improvements to the acceleration of this vehicle and went for a ride with my customer today who is SUPER happy with the results. I ended up shorting the linkage a bit that i was initially questioning if it should be like a shock and plunge in and out. (twisting in the treads on ball socket…

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