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Help - intermittent momentary engine missing 93 500sl

Submitted by backert1 on 27 May 2008 - 2:03pm.
I could really use some help with this one. I own a 93 500sl with ~84,000 miles. Last summer and this spring I have noticed a very slight missing that occurs is noticiable at any constant engine RPM or road speed. The missing is very short lived, just feels like a little tug backwards. Most of the time it doesn't even show up on the tach - other times I believe I can catch a fleeting every so slight dip in RPMs. It can occur when the car is hot or when cold. It can occur when the weather is hot or cold. It can occur if the gas tank is full or near empty. It can occur if the car is idling or moving at 90mph. It can occur some days and not at all on other days. It can occur every few seconds or every few minutes. The only things I can tell is that when it occurs, the engine speed is steady (never notice anything during accelration) , and when it occurs it appears the missing seems to be consistent in time from miss to miss although that miss interval may be different different days. Any ideas? If it had any dependence on RPM, temp, car speed, gear, tank level, etc it would be easier to diagnose. Seems to irregular to be fuel filter, injector, fuel tank pressure, fule distributor, etc....... I worry about impending failure of some component but don't have an idea of where to start. Thanks, Todd
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Diagnosing 93 500SL engine misfire

28 May 2008 - 10:31am
Kent Bergsma

Todd, from your description the problem sounds to me like it is electrical in nature (not fuel). As you already know it can be very difficult to troubleshoot these type of intermittent problems. I can only guess at this point and most mechanics would be doing the same. I would go at it this way.

1. Thoroughly inspect your engine wiring harness for deterioration or cracks. Are the wires still pliable or hard and brittle? Does insulation break when you try to bend the wires. A failing wiring harness will cause the symptoms you describe. If harness appears to be ok then proceed to:

2. Unplug and clean the contacts on every wiring plug you can get to along your firewall and in your engine compartment. Apply dielectric grease to all pins and reassemble.

3. Change all your fuses in your fuse box. Eliminate all aluminum fuses

4. If it still cuts out after all this then I recommend you replace the voltage overload relay.

5. If miss persists I would then replace your ignition wires/ cap / and rotor

If none of this solves the problem then you have a real tiger by the tail... Kent

Thanks Kent. I was hoping

2 June 2008 - 11:26am
backert1
Thanks Kent. I was hoping there was a typical fix for this - a suspect component that was subject to just such a failure. I am familiar with grounding and poor connection issues as I have tracked many of them down over the past years. This one is just so rythmic that it doesn't have the feel of a bad connection which ususaly presents more randomly. I will have some time htis weekend to chase after this and see what I can find. Plugs are cheap, wires / caps / rotors are $$$.