Rough start warm, fuel accumulator? - Mercedes-Benz Forum
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post #1 of 33 (permalink) Old 05-14-2019, 11:01 PM Thread Starter
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1985 500SEC When it starts cold it's fine and reaches operating temp or nearly then has a hesitation. Sometimes when it's a warm start it takes awhile to start and then runs rough for a bit and smoothes out. I thought the accumulator must keep pressure because cold starting was fine but now I read somewhere that the accumulator can affect warm starts so might be up for a new one but will that affect performance when running at operating temperature if it has an issue? It seems to be when the cold start injector cuts out I have an issue or it's masking something.
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post #2 of 33 (permalink) Old 05-15-2019, 07:49 AM
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Do a system pressure test to see if the system holds pressure. There are cold start aids that mask pressure problems when cold, but when warm the engine can't quickly develop enough pressure to deliver fuel.
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post #3 of 33 (permalink) Old 05-15-2019, 04:59 PM
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1985 500SEC When it starts cold it's fine and reaches operating temp or nearly then has a hesitation. Sometimes when it's a warm start it takes awhile to start and then runs rough for a bit and smoothes out. I thought the accumulator must keep pressure because cold starting was fine but now I read somewhere that the accumulator can affect warm starts so might be up for a new one but will that affect performance when running at operating temperature if it has an issue? It seems to be when the cold start injector cuts out I have an issue or it's masking something.
Hi there,

John is correct, doing a system pressure check is what needs to happen here.

The main function of the accumulator is to maintain 'rest' pressure for that hot restart, say 5 to 20-ish minutes from the time the warm engine has been shut off.

As the engine runs, the accumulator provides a damping effect as well.

"Takes a while to start and then runs rough for a bit and smoothes out." indicates to these eyes, that one or more of the injectors are likely dripping raw fuel into the ports when shut off, thus causing low/no rest pressure.. and the plug fouling that clears with driving.

The proper operation of the cold start valve means it only operates when the starter motor is engaged, and never on that hot restart condition.

Good Luck,

MBL
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post #4 of 33 (permalink) Old 05-16-2019, 05:52 AM Thread Starter
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Thanks for your help. I'll have to get the fuel pressure tested then. Did I read some where that's around 5.3 to 5.5? The hesitation I get when warm would still be from injectors then? I've had a miss in it but this feels definitely fuel related and cuts in about 70 degrees. I thought it might be fuel filter but there's a definite pattern to it. The other thing is if I play with the air/fuel mixture it seems to come good for a day and then plays up again.
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post #5 of 33 (permalink) Old 05-16-2019, 06:28 AM
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I experience (randomly) the same symptoms as described here. After replacing the accumulator, filter, and both pumps (somewhat of a preventative thing, as all of the above had a fair amount of corrosion including all clamps, and all appeared original from 1991), I still have the issue as well. Since the car is a weekend pleasure car, havent given much attention to the issue, but would like to eventually address.
My question(im new to working on fuel injection systems, other than filter changes): How does one troubleshoot this:

"one or more of the injectors are likely dripping raw fuel into the ports when shut off"
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post #6 of 33 (permalink) Old 05-16-2019, 07:39 AM Thread Starter
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Yes mine is just a weekend car too. It's just annoying because I got it running really sweet and it was nice to drive. Changed the thermo and now I have issues again.
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post #7 of 33 (permalink) Old 05-16-2019, 09:17 AM
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I experience (randomly) the same symptoms as described here. After replacing the accumulator, filter, and both pumps (somewhat of a preventative thing, as all of the above had a fair amount of corrosion including all clamps, and all appeared original from 1991), I still have the issue as well. Since the car is a weekend pleasure car, havent given much attention to the issue, but would like to eventually address.
My question(im new to working on fuel injection systems, other than filter changes): How does one troubleshoot this:

"one or more of the injectors are likely dripping raw fuel into the ports when shut off"
I suppose a leak down test might indicate leaking injectors if you're sure everything else is tight. Best is to pull the injectors and test them. I use a homemade tester.

Another possibility is the seal at the bottom of the control piston in the fuel distributor. If it's leaking at rest, you're going to get extra fuel puddling in the rubber boot underneath. You can check this with a strong light and mirror.
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post #8 of 33 (permalink) Old 05-16-2019, 10:06 PM Thread Starter
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Does all those suggestions relate to the hesitation that starts at operating temp as well? Or is that a separate issue? It does feel like more of a fuel issue than a spark issue but I could be wrong.
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post #9 of 33 (permalink) Old 05-17-2019, 12:07 AM
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Mine used to do the similarly (although M103 motor; required often two starts when warm, took a few seconds to start running smoothly), new injectors cured the problem.

Good luck,

Seppo
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post #10 of 33 (permalink) Old 05-17-2019, 07:55 AM
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Does all those suggestions relate to the hesitation that starts at operating temp as well? Or is that a separate issue? It does feel like more of a fuel issue than a spark issue but I could be wrong.
If you're referring to mine, it depends on the severity of the injector or control piston leak. You need extra fuel at a cold start so those leaks could be masked. They can definitely be an issue for warm starts.
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