Thump noise, feels like car wants to die

Asked by Jan 02, 2015 at 10:51 PM about the 1991 Mercedes-Benz 420-Class 4 Dr 420SEL Sedan

Question type: Maintenance & Repair

My 1991 bentz 420 sel started doing a thumping noise when I am stop at a red light. The oil pressure drops and goes back up. But when I put it in netural the car is fine. Following day car was fine then today after I put gas in the car it started in again. It feels  like if my car was a manuel tranmission and if i  pullled the clutch out to far And its a automatic. Check engine light isnt on or anything. Checked all fluids and everything is normal. No problems when driving it either just when its stopped

10 Answers


something unusual there with idle control. Like surging, Try cancelling all climate control settings that run your a/c compressor, while it is acting up. Or, run with and then run without. Since you put it in neutral, and it improves, (that would give an rpm rise) This is what I would show my tech. Simple things you can do ? look for detached or burned hoses, or anything odd with the caps or dipsticks under the hood. it should be sealed up and everything installed properly and tight. If you can make the thumping come and go, it will be easier to demonstrate, but difficult to guess without being there. unusual a/c problems during winter can be giving a symptom but cannot say definite.

Thanks for your answer. Now its happening while in park. I am thinking maybe its a spark plug or somethings clogged. I haven't checked the filter yet I am going to check it tomorrow morning and then take it in I am going to have it looked at tomorrow. I only had the car for 2 months and it only has 70,000 original miles on it. Garage kept and wasn't driven that often. Bought it from an old lady.


I suppose it is worth looking under the hood running in the dark, for an arcing wire. Spritzing it with water in a spray bottle can make one show up that happens under load. on a 91 dont be shy when it comes to replacing bosch ignition parts with fresher stuff. Finding it cheap can take forever but some kits are on ebay. using other brands always causes trouble.

Called a few repair shops and they all think the vacium holes is clogged.


what does that mean? vacuum hose is clogged? To what component ? You have a low idle, in gear, and thumping. Then does it in park too...........what's it mean, "feels like i let the clutch out too far" a clutch without pressure on the pedal is fully engaged, so to me, that means you are describing RPM too low and engine misfiring or idle speed to low for staying I must ask, is there an idle control valve ? an idle air bypass ? the older ones had hoses them that I worked on, They shut when cold, and opened as temp increased, they had an electrical connection. They set a code after the system became capable of doing so. That leads to: have you checked for codes ?

It was the fuel injector


Then you could say you had an engine miss that was fuel related, so then on a 91 you would have nozzles right? and a fuel distributor? Or does that have electronic solenoid type injectors for some EFI system. or is it CIS-E (aka, K-jetronic). The nozzles can plug i suppose or wash out? liquid erosion, I have read about in bosch engineering bulletins ? Did you clean it? or replace one or the set? Just being nosy for experience.


I show nozzles. in a search. like the amount is measured and delivered, then sent to the nozzle for atomization into the cylinder. must have been blocked? or washed out resulting in puddling ? Questionable.

needs tune up.does this happen when commpressor on


Tough one to converse with, no info, it was like a misfire, then he says it's all caused by an injector. then stooped replying. All I can say he must have found a tech who understood.

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