Mercedes-Benz Forum banner

Acceleration dips? Dangerous malfunction 420SEL

2K views 14 replies 6 participants last post by  alexstorm 
#1 ·
Whenever I am at a stop and attempt to accelerate, I press the pedal down a little more than halfway down, and the car slightly goes, then suddenly it free-falls and there is no acceleration whatsoever, and i'm forced to wait 3 seconds for it to "catch" again and resume, or I have to press it to the floor in order to actually continue.... I am unsure as to what this particular issue is, but here's a list of parts i've replaced, with OEM. The acceleration pedal is not broken either, it's in excellent condition. I use ONLY premium gas, don't worry.
-Fuel Accumulator
-Fuel Filter
-Ignition Wires
-Distributor Cap + Rotor
-Changed the oil + filter
-Replaced all serp belts
-Alternator voltage regulator brush
-PCV filter
-Both O-rings for EHA valve
-Had a mechanic adjust the fuel-air mixture to proper setting(still didn't fix)
***I lubricated all the throttle linkage and whatnot also***

I understand it could be the fuel distributor itself; though, i don't believe so, as about 95% of the time the acceleration dips and doesn't work from stops, yet about 5% of the time it will work fully and have no issues... Had the fuel pumps diagnosed by a mechanic and he said they were in good working condition and do not need to be replaced.
Please help me fix this issue. Thanks
 
See less See more
#4 ·
If nothing is binding on the linkage (btw ATF is a good lubricator for the links), a sticky throttle body was one of the first things to come to mind.
With air cleaner housing off, gently push the center of throttle plate.
Was the plunger on the bottom of the fuel distributor moving freely when the O- ring was replaced?

Sorry I am all over the place with this. A more systematic approach should be better. Beginning at fuel pump function and volume (shorting out relay) and so on going forward.
Assume you meant filters ?
I am sure the experts will chime in.
 
#5 ·
I had someone else drive the car, and they informed me that he feels like? When youre at a complete stop, you're trying to accelerate in a gear other than first. So, it's like accelerating from second or third gear as opposed to first, which is why it takes forever to build up the momentum to actually accelerate
 
#7 ·
You're a sage, I swear, I think it might be that modulator; was digging deep into the depths of all the forums and FINALLY found it might be the trans modulator.
Just ordered the upgraded one with instructions from Mercedessource, Hopefully that fixes it. Best regards, also --- this is my tranny modulator currently (the green, and old/original one.)
It appears that the vacuum line is in healthy condition; albeit, i'm aware that the plastic may very well crack
 

Attachments

#10 ·
Hopefully you can gather if it is/or not from this photo? I've attempted to diagnose this also, but I can't seem to conclusively say it is or isn't. Everything is tightened(to my knowledge) and lubricated thoroughly.

PS- Did you use "insert image" ? I'm fairly new to this site and am trying to master all the commands and whatnot
 

Attachments

#15 ·
Make sure the timing is set correctly. I set mine to 14 advanced. Also pull the idle control valve, spray the interior with carb cleaner, let it sit for 30 minutes, pour it out, let it dry and put it back. This valve often gets dirty. A replacement part is too expensive. Be careful that all vacuum lines and hoses under the air filter are reattached. Many can come off without you even noticing.

The throttle linkage, I would leave it alone. The Idle control Unit electronically adjusts idle issues rather than setting them with the throttle linkage. Older other model cars do adjust with this linkage, so you are tempted to think that is the answer, but not really with the 420 SEL unless you screw it up and then the symptom will be that the car stalls out.

Finally check that the cable running from the Ignition Control Unit to the middle of the distributor does not have broken ends exposing bare cable. This can happen if you turn the distributor cap back lock the wrong way and chip away on the cable plug. It's easy to miss this problem. But this issue usually just causes hard to start after a rain or completely hard to start, which is not your issue.

Hope that helps.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top