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Old 06-15-2011, 02:11 PM
Registered User
Join Date: May 2011
Location: Chattanooga TN
Posts: 8
R129 Slow/Hard Starting????

Hey folks,
Car is a 91 R129, 300SL, 6-cyl engine. I've read a buncha threads about this small issue, but everyone else seems to have a different problem. Since I purchased this car, this has been a consistent problem.

When the engine is cold (sitting several hours or several days), I have to crank the engine over for 30 seconds to a minute before it runs. After it's running, whether its 50 or 90 degrees, it runs beautifully. No warning lights or check-engine lights are on. When I turn the switch on, I hear the fuel pump do its little prime-up for a couple of seconds and shut off. I replaced the fuel filter and checked to plugs, wires, cap, etc. All these appeared to be pretty new. Pressing the accelerator partway or fully down doesn't affect the issue. When the engine is warm, it cranks within a few seconds of engaging the starter.

The car always starts, but seems like it oughta start sooner than this. I've owned a lot of older carby cars (Mustangs, Scouts, etc) and a few diesels, but I'm not a knowledgeable FI mechanic yet. Any help would be great!!!

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Old 06-16-2011, 12:18 AM
Senior Member
Join Date: Nov 2002
Posts: 786
just because the parts look new, doesn't mean they are correct. bosch or ngk plugs only, no platinums, bosch included. bosch or bremi cap and rotor, no exceptions. karlyn or bremi wires, no bosch, no oe-which is bosch in the mb box. these items tend to be exspensive and there's lotsa cheap imitators out there.

perhaps the coil is weak. you need to see if the available kv is strong enough for a cold start. needs a scope to check that.

could be a fuel issue, maybe too lean or control pressure too high. there's lotsa variables to a hard start issue. good luck, chuck.

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