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Discussion Starter #1
Help! I have a 1984 380SL. I have a problem that nobody can figure out right now. When I sit traffic lights, in drive with foot on brake I get this irratic jolting sensation. It is as though one of the sparks is missing, but it only happens at idle and as soon as you pull off it goes away and all is smooth. Also the jolting decreases significantly when the air conditioning is on.<br> <br> What’s going on? What could be the culprits? Any help vastly appreciated.<br> <br> Thanks<br>
 

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Discussion Starter #3
If you believe a plug is misfiring, while holding a plug cable with a rag (or long plier), remove wires one at a time. When you do that, engine should shudder more since it is not getting the juice. When you get to a plug where removing the wire does not change the irratic idle, you have a culprit. See if the plug looks OK. If it is, insert a long screwdriver at the end of the plug and bring the metal part of the screwdriver close (quarter inch) to the car body (metal part only). You should see a spark jumping from the cable via screwdriver to the car. If not either you have a bad cable or need to check the cap/rotor/point.<br> <br> Make sure your timing and idle speed are correct, throttle body or carburator injector is clean, and air filter is good. Let's start from there and see what happens.<br>
 

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Thanks for the info. I will try all you have suggested. The thing is, I have already replaced all the sparks, wires, cap and rotor (ensured that the wires are in the correct order), but still have the problem. Actually, once I replaced all this the idle seemed worse than before.<br> <br> Your comments about timing interest me. I got to think there is some kind of idle controller that’s not working or a sticky jet. I don't want the expense of checking the jets yet until I have eliminated all other possibilities. Do you know of any idle controller? Or do think it could be some vacuum leak? Even so, how to you check for that?<br>
 

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Discussion Starter #6
Idel controller as in adjusting idle? Like screw in to decrease and screw out to increase idle? I've done that on other cars, but not on SL of that vintage.<br> <br> Vacuum leak: obvious is to check those darn little hoses for cracks, kinks and tears. If you system is similar to Honda (tens of hoses), I bid you lots of luck. One trick that some of the mechanics used is to spray carburator cleaner/WD-40 at vacuum connections (at throttle body, air-distribution box, etc.) a couple of squirts -- you ain't trying to flood it. You should sense a jump in idle/less rattle since a leak is fixed temporarily. It goes without saying that you have to know where you are spraying fluids so that you don't mess up other electrical sensors.<br> <br> If you have buddy with an emission scope (tester like Snap-on unit that shows Nox, CO and all the good stuff), you can hook it up to see if you are running lean or rich at idle. Maybe another obvious is that if you have dark sooty plugs, you could be running rich (too much gas or not enough air -- vacuum leak). Much luck.
 

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Discussion Starter #7
Is the jolting coming from the motor? What temperature does it happen at? How many RPM's is your tacho showing at idle- 800+? Is your transmission oil level correct or low? There are alot of things it could be. Sounds like you may need a tune.
 
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