Mechanics

MERCEDES BENZ 420SEL ENGINE PROBLEM

1986 Mercedes Benz 420sel • 175,000 miles

G’day,

I bought a 1986 Mercedes 420SEL for a parts car, but would like to know what's wrong with the motor and whether it's worth fixing, because the car I bought it for is having other problems, but don’t get me started on that.

Here are the symptoms:

When it's cold the economy gauge needle sits at about a centimetre (half an inch) from the extreme left of the gauge scale, at idle. As it warms up the needle moves closer to the left side, but even when fully warmed up it never really reaches the left side, as it does on my other 420SEL, even at idle and in neutral.

There's no smoke when the engine is cold but when the temperature reaches about 80 Celsius heaps of blue smoke starts coming out the exhaust.

Surprisingly, in neutral, it idles reasonably smoothly, but when it's put into gear the engine shakes quite a bit.

I also noticed that when I rev the motor, while it's in neutral, the economy gauge needle actually goes to the extreme left, and when I take my foot off the pedal it bounces back toward the middle of the scale before settling where it started from.

I bought the car from a mechanic, who claimed that he had changed the timing chain, just in time, because it was very loose, but he left the old parts in the boot (trunk) and, on inspection, the chain guide/rail that attaches to the chain tensioner had been stripped bare to the metal at a large section of it.

So, now I'm wondering if the guy I bought it from was exaggerating when he said that he changed the chain in time, and whether - in fact - the chain was so loose after the guide/rail material broke off that it skipped a tooth or two on the cog/s and messed up the valves.

I’d like to know if the symptoms described above are consistent with my theory and, if not, what else might be wrong and how I should go about finding out what the problem is.

I look forward to your reply and thank you for it in advance.


Kind regards,
Nick
Avatar
Nick Vlahos
December 15, 2012.



The economy gauge is vacuum actuated and the difference in vacuum when cold, warming up and revving would cause it to fluctuate. Not moving to extreme left could mean low vacuum at manifold and a vacuum test should clarify this.

The shaking in gear could mean either the engine mounts are bad or the engine is not running as smoothly as it should or the idling speed is too low.

Blue smoke coming from exhaust means oil is getting to it, either via the cylinders or the exhaust valve seals. It could also be remnant of oil from previous problems if any repairs were carried out.

When timing chain breaks, it is likely to cause a lot of damages if it is caught in between. For this engine if the timing chain is not installed correctly, the engine would not be able to run smoothly, if you are able to get it started.

KHLow2008
Dec 15, 2012.