300sl hot start problem - Mercedes-Benz Forum
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post #1 of 15 (permalink) Old 02-15-2012, 12:24 PM Thread Starter
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Vehicle: 1991 300sl auto Red/parchment. 1982 300sd. Teal/beige
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300sl hot start problem

I have checked out a couple of other threads regarding my problem - but none of them came to a definitive conclusion. 1991 300sl, 175xxx miles, fairly well taken care of over the years (I'm not the original owner). Cold starts are fine (takes a couple of clicks) smooth idle, smooth acceleration. Hot starts within a few minutes of shutdown are also fine - smooth idle, etc. It's a hot start after the car has sat for about 5 mins or more that become difficult. Hard to start (usually feathering the gas helps) and then there is a low speed "miss" which evens out at higher rpm's - when the car goes back to idle, the roughness returns - this can go on for several "stop and goes" and then it eventually evens out again. I sometimes detect gas fumes -but not always. Recently changed dist. cap and rotor (as well as the distributor suppressor plate - which some threads indicate can be a problem). Ran some injector cleaner. Checked fuses for corrosion, cleaned the heat sensor contacts. None of this has helped.
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post #2 of 15 (permalink) Old 06-23-2019, 08:57 AM
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Hi,

Did you solve the problem and found the cause?
I'm happy to hear what it was.

Regards
Christian
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post #3 of 15 (permalink) Old 06-23-2019, 03:55 PM
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Hi,

Did you solve the problem and found the cause?
I'm happy to hear what it was.

Regards
Christian
Quite possibly poor fuel holding pressure due to either faulty accumulator, regulator or leaking injectors.
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post #4 of 15 (permalink) Old 06-23-2019, 04:00 PM
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Could also be due to a vacuum leak in the EGR vacuum line. When hot the switch over valve operates, then the unmetered air leak causes the engine to run rich due to erroneous compensation by Lambda closed loop.

So, to eliminate possibilities, starting with previous post, check fuel pressures at idle and engine off.

Then check duty cycle.
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post #5 of 15 (permalink) Old 06-23-2019, 10:12 PM
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Thank you for the reply.
The fuel pressure regulator is a brand new one since a few weeks.
So I was suspecting the accumulator. Your feedback "fuels" my idea.

I'll start with the accumulator and injectors.
Previous test showed me perfect fuel pressures while running.
I' m going to do the tests again and do the tests with the engine off for at least 30 minutes. (after running)
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post #6 of 15 (permalink) Old 06-23-2019, 10:51 PM
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Do you know where I can find this valve or line?
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post #7 of 15 (permalink) Old 06-24-2019, 01:16 AM
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The accumulator is next to the fuel filter, underneath the car to the rear, just fore of the trunk wheel well, under a black plastic cover.
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post #8 of 15 (permalink) Old 06-24-2019, 01:17 AM
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To test if accumulator is ruptured/leaking, there should be no fuel coming out of the small rubber hose when pump is running.
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post #9 of 15 (permalink) Old 06-24-2019, 10:28 AM
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I did some tests and there is no fuel coming out of the small rubber hose.
System fuel pressure is 6.4 bar as it should be but I noticed an immediately drop to 3.2 bar when the engine is stopped. Is that normal?
Not sure how correct my gauge is but the manual that I found mentions that even if the pressure slowly drops below 3.5 bar the accumulator is broken.

http://manual.startekinfo.com/maniak/JSP/e2/2_1/m32.jsp
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post #10 of 15 (permalink) Old 06-24-2019, 03:43 PM
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Quote:
Originally Posted by Christian Omloop View Post
I did some tests and there is no fuel coming out of the small rubber hose.
System fuel pressure is 6.4 bar as it should be but I noticed an immediately drop to 3.2 bar when the engine is stopped. Is that normal?
Not sure how correct my gauge is but the manual that I found mentions that even if the pressure slowly drops below 3.5 bar the accumulator is broken.

http://manual.startekinfo.com/maniak/JSP/e2/2_1/m32.jsp
New regulator.
Accumulator tested ok.
Leaking injectors? Or leaking cold start valve. To test CSV check spray pattern for good atomisation/spray pattern. It should not drip fuel from the nozzle.
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