Knocking sound sometimes at start
 

Hello fellow Mercedes-friends,

I have noticed that sometimes when starting my 1984 500 SEL I get a knocking sound which is similar to what you get when starting an old diesel engine.

Anyone got an idea if this is normal and if not what it could be and what I should do about it?

Many thanks,

Mikael Westerberg
Vanersborg Sweden

Mikael:
This sounds like the problem I was having with my 1985 500SL. The tapping/knocking sound was most prevlent at start up, but soon became less pronounce. It turned out to be a worn cam lobe and rocker. With the hydraulic valve lash adjusters, this excessive lash will result in noise until oil pressure to the adjuster takes effect. Of course, loose big ends or mains could cause similar symptoms, but low oil pressure would also probably be a problem as well.
Regards
BBAY
w198 300SL 1963
w107 500SL 1985

Hi Bbay,

Its good (I hope) that You write "I was having" about the noise.
I only hear it about one out of twenty start-ups. what did You do about it and how do You think I should start the root-cause analysis?

Happy motoring,

Mikael

Mikael:
Well it seems that 'sometimes' was the operative term that I overlooked in your case. Intermittent noise can be frustrating to track down. In my case pulling the valve covers and inspecting the cam lobes quickly revealed the source of the noise. Scoring of the lobe and associated rocker increased valve lash to a point that noise was the result. Changing of the rocker without changing the cam is admittedly a temporary fix, but worked in my case. It's possible that a 'lifter' is dirty or binding and takes a while to remove the lash. Hopefully a more experienced mechanic can jump in and point you in the right direction.
BBAY
500SL '85
300SL '63

I don't know if your benz has a smog pump, but my 300 E had that sound only when cold starting. Someone on the forum said it was the smog pump going bad. A little elf with a pitch fork on my shoulder told me to disconnect it and it would go away. So I did. Car runs the same and the dieseling sound is gone.

Thanks a lot guys,

I will go out to the garage now and get my hands dirty on things. Hmmm I beleive that little elf is present here as well.

Thanks again.

Mikael

My 500SEL often knocks once or twice briefly on start but I know I need to replace the engine mounts . . . . , somtime anyway !

Sounds to me like timing chain slap. Maybe even a broken timing chain guide. I would not take this problem lightly. Check it or have it checked now before you have catastrophic engine failure.
If a timing guide is broken it will probably be the left side inboard upper guide.
Brian

Thanks a lot behind the star,

You got me a bit nervous now with Your mail. How do I check the timing chain both in terms of chain tension as well as the condition of the chain guide?

I dont have a repair book so bear with me for my stupid questions.

I´m quite experienced with engines and transmissions but the Mercedes ones are quite new to me since I have had the car for only a few weeks.

Thanks in advance,

Mikael

Mikael,
As far as checking the chain, you pretty much just have to go by mileage. If the car has over 100k on it I would replace it and the upper guides and tensioner for preventative maintenance. I know I will get a lot of disagreement on that, but I have seen many of these engines jump time and bend the valves. As far as checking for a broken guide, pull the left (drivers side ) valve cover and look at the inboard guide rail very carefully. It will probably be broken where the bottom pin goes through it. It will be hard to see, but it can be seen.
Brian