I have a 1983 380 SEL and from time to time it sometimes while going on the highway doesnt want to go over 100 km/h with your foot to the floor! When this started happening there was also a hissing sound that developed under the car. I looked at it and thought it was the muffler or catylytic converter but if that was the problem it would do it all the time right?? I need help!

The car also makes a very slight pinging under hard acceleration but mechanic said not to worry. Is this OK?

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99 C43
98 S420
99 C230K
01 C240

BenzOnline,

Pinging from engine is definetly NOT ok... if it is actually pre-ignition (some call it spark knock) this is very detrimental to any internal combustion engine. What it means in the most simple terms is that your piston skirts are beating themselves against the sides of the cylinder walls... as you can imagine, this is not good.

Pinging can be caused by improper ignition timing or low octane fuel, among other things. Also check for EGR valve operation and proper engine operating temperature.

As far as the hissing sound... it sure sounds to me as if the catalytic converter is becoming plugged, however I cannot explain the "intermittent syndrome".

My advice, re-check the catalytic converter/exhaust system for blockage... this will cause all your symptoms

Good Luck!

mbmatt

mbmatt's response is excellent, but I would like to add one thing as food for thought. A hissing sound could be a serious vacuum leak, although you would probably not hear a vacuum leak while driving. In the case that it is indeed a vacuum leak a lean condition accompanies a vacuum leak. A lean condition is also a source of preignition.

As mbmatt said preignition is definitely not okay. It is to be repaired immediately. It will put a hole through the top of a piston. I've seen that happen.

I would put a vacuum guage on the engine and ensure that no vac leak exists.

Good luck,

Thanks guys for your replys! As far as the knocking I put mid grade gas in and a bottle of octane booster and it stopped but, the mechanic did advance the timing for more horsepower which is causing this. If the piston does get a hole what would happen? Would the thing blow or something?

__________________
99 C43
98 S420
99 C230K
01 C240