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  #1  
Old 10-18-2007, 11:56 AM
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Cold running problem 1988 560sl

Hi New member here and have searched earlier posts but not quite the problem I have.
Starts up great all the time, but seems to shake or skip when cold. More noticable in gear than neutral. Clears out and runs smooth when warm after about 10 min. No problems at all warm. Not really noticable at all after 1500rpm cold. Any ideas from past experience?
Thanks in advance
Dave

1988 560sl automatic 123K

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  #2  
Old 10-18-2007, 05:39 PM
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Quote:
Originally Posted by dilligaf View Post
Hi New member here and have searched earlier posts but not quite the problem I have.
Starts up great all the time, but seems to shake or skip when cold. More noticable in gear than neutral. Clears out and runs smooth when warm after about 10 min. No problems at all warm. Not really noticable at all after 1500rpm cold. Any ideas from past experience?
Cold pressure could be too low.
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Old 10-18-2007, 05:57 PM
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"Cold pressure"? What exactly is cold pressure? I've only heard of this in regards to tires.
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Last edited by tecqboy; 10-19-2007 at 10:46 PM.
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Old 10-19-2007, 02:20 PM
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Cold presure

OK what is cold presure and what presure could be low?
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  #5  
Old 10-19-2007, 03:20 PM
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Not sure I agree with the pressure...sounds like it could be coolant temp sensor not sending computer enough info to enrich fuel mixture when cold.
Have you run some 44K though the system to clean out fuel delivery system?
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  #6  
Old 10-20-2007, 03:17 PM
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I guess that they are talking about fuel pressure when the engine is cold. There are two flavors, system and control. System pressure is what is delivered to the fuel distributor. Control pressure is basically a measure of how much fuel is being diverted back to the tank, which in turn controls how much fuel gets to the injectors. The lower the control pressure, the greater the fuel going to the injectors. So at cold engine temps or when you are accelerating, you want a lower control pressure to achieve a richer mixture.

This is all managed by the control unit, which is a small analog computer. One of its inputs is engine coolant temp, which it gets from the above-mentioned sensor. If the sensor sends the wrong signal (resistance), the computer will not make the right adjustment to the mixture.

So the sensor is a possible cause of your problem. But it could also be any number of other things.

My own experience with this exact problem on my 560 turned out to be a bad plug wire and a tiny vacuum leak in the small rubber hose that connects the cabin vacuum line to the intake manifold. Finally, the plug wire got bad enough that it became an all-the-time miss.

My advice would be to eliminate the potential problem areas that you can deal with: replace dist cap, rotor, plugs and wires, check all vacuum fittings. Then if you still have the problem, you will be able to take it to someone with a known baseline. You do not want a shop replacing these items at list prices, which is what they will probably want to do.
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'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Last edited by ctaylor738; 10-21-2007 at 08:01 AM.
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