Author Topic: Fluctuating idle on my 350  (Read 3398 times)

oscar

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Fluctuating idle on my 350
« on: 14 July 2006, 04:09 AM »
Hi fellows,

If it's not one thing it's another. >:(

My MPS is still fine but earlier tonight whilst the engine was still warm, on startup the engine revved up to 2000 then back to 500 doing this just over once every second.  It did this on the rare occaision last year, mostly on cold mornings but not every morning and it didn't necessarily have to be cold.

Drive it less than 1km then park it the idle is normal.

Any thoughts?
1973 350SE, my first & fave

Denis

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Re: Fluctuating idle on my 350
« Reply #1 on: 14 July 2006, 08:48 AM »
MPS again ...

Denis

Paris, France

Denis

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Re: Fluctuating idle on my 350
« Reply #2 on: 14 July 2006, 08:52 AM »
...maybe it will go away ...
mine did the same thing and IT went away  ;D

MPS means : Many Problems Source  :P

Denis
Paris, France

Nutz

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Re: Fluctuating idle on my 350
« Reply #3 on: 14 July 2006, 10:43 AM »

Did you tighten the vacuum hose down well on the pressure sensor or is the hose itself collapsing?You can also check temp sensors 1 and 2 circuits + sensors to eliminate them as culprits.

But before all that,what shape is your ignition system in?

oscar

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Re: Fluctuating idle on my 350
« Reply #4 on: 15 July 2006, 03:49 AM »
Thanks guys,

I was hoping for a, "oh, that'd be just (x) part.  Costs a couple of bucks.  Replace that and you'll be right"   :D :D

Ok not quite.  I'm adamant the MPS is ok (still holds vac) but tomorrow I'll test the terminals too just to be sure.  I found your troubleshooting section on benzworld Nutz.  Very handy, and you mention a couple of things above that point to some factors I read about whilst doing a general net search.  ie igniton and vacuum.

The igniton system has had its problems.  On that spark plug thread I said how two Bosch plugs stopped working even after a clean.  However I read a good article last night about a Toyota vehicle where after some exhaustive testing and component replacing, their igniton issue was solved by replacing leads.  It was all to do with the increased resistance of dirty plugs and the spark burning the lead by finding a less resistant path inside the boot. 

Given the history of my black plugs, cracked valve seals, leaky MPS and 4 very old leads and 4 used leads, I'm guessing I should stop discounting the importance of new leads.

I'll check as much of the vacuum system as I can as soon as I can.  I've replaced check valves to the brake booster and central locking etc within the last 2 years.  A few actuators have been replaced with the front passenger door and trunk to go.  But since we're talking manifold I don't know.  I held a stethescope around gaskets trying to hear hissing which I couldn't.  I've read injector seals could also be a problem.  The MPS line could do with a clamp IMO and a closer inspection. It's old too. 

Someone suggested using Propane thru a hose onto gaskets and listen for the change in revs if there's a leak. :o :o  Don't think so.  I'll stick to the stethescope for now.
1973 350SE, my first & fave

Denis

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Re: Fluctuating idle on my 350
« Reply #5 on: 15 July 2006, 10:18 AM »
Hey oscar

You car has so many problems that diagnosis is difficult. As I said, my car did like yours and "fixed itself"  :P

Check your vacuum line to the MPS. Cut off half a centimenter from both ends, if there are no cracks at the ends and no burn or damage along the hose, refit it , do NOT replace it. This is how you end up inthe poorhouse - replacing everythings in sight  ;D

Denis

Paris, France

oscar

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Re: Fluctuating idle on my 350
« Reply #6 on: 15 July 2006, 10:38 AM »
Thanks Denis will do.  I don't mind replacing things if there's a guarantee it only gets replaced once.   :-\  But there's no such thing. :)

Otherwise the car is running really well.  It's so smooth and the engine sounds softer but better, especially over 4000rpm ;D.  This morning the car didn't flutuate, just revved upto below 2000 and it slowly comes down when the engine warms and/or the vacuum builds.  There's a strong immediate vac on the MPS hose but I'm still suspicious.  I'm going to shorten that hose later today, it's definitely stretched to a larger diameter at the MPS and probably at the manifold too.  Cheers.

BTW, taking my crappy diaphragm to a "metal man" today.  We're going to see what he can come up with reproducing these things.  Of course don't hold you're breath, even though we are the first in the world to attempt this. ::)
1973 350SE, my first & fave

Nutz

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Re: Fluctuating idle on my 350
« Reply #7 on: 15 July 2006, 01:42 PM »


 4 very old leads and 4 used leads, I'm guessing I should stop discounting the importance of new leads.

Curious what the resistance is on those old leads  :D I'd recommend before anything that you get brand new leads before purchasing anything else.I bet it alone will solve a lot of your problems  ;)

 
Quote
Someone suggested using Propane thru a hose onto gaskets and listen for the change in revs if there's a leak. :o :o  Don't think so.  I'll stick to the stethescope for now.

It's quite safe,just don't light it to test  ::)
« Last Edit: 15 July 2006, 01:43 PM by Nutz »

Denis

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Re: Fluctuating idle on my 350
« Reply #8 on: 16 July 2006, 04:08 AM »
Nutz, Styria

I have always been amazed how after cleaning them, a set of ignition wires can look great, have nice clean connectors and fit snugly, perhaps also improving the engine idle. And then, replace this pristine harness with a new one : MUCH better !!!

This is one thing NOT to buy used despite the fact that cleaning dirty wires can help a good harness.

Denis

Paris, France

Very hot, sunny day, blue skies and reasonably quiet - the Parisians have left the city to go on holidays near the sea, except me  :(

s class

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Re: Fluctuating idle on my 350
« Reply #9 on: 17 July 2006, 12:14 AM »
I suggest for ignition leads, you should only use genuine OEM.  Aftermarket stuff will multiply your problems and confusion.  I found a Bosch set on Autohausaz.com for a good price.  Did wonders for my 450SL.  (it now idles and accelerates with 8 cylinders).

Ryan

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

oscar

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Re: Fluctuating idle on my 350
« Reply #10 on: 17 July 2006, 07:47 AM »
Hi Guys, guess what.  I've got my answer!! :)

I posted the same question on the New South Wales MB club site and was told it's caused by an ECU versus Auxillary Air Valve situation.  During warm up the AAV opens but if the revs reach approx 1600-1800rpm, the over-run fuel cut-off is activated.  The engine dies until the FI kicks in again.  Sounds simple.  Heres the link to the post.  It's not long and has got a couple of fixes.  I haven't looked at the AAV yet so I can't describe what to do, but have a read.

Nutz, I checked the resistances of the MPS, 86 and 342 ohms.  The leads have a big history which I wont go into but they all read 1kohm apart from one which read 5.  Mind you that suppressor has 5kohms stamped on it. ::) I know, again, long story, but I promise I'll replace em.  I agree with what you've all said re leads.

BTW, the car's still running really well, I can't believe the comparison.  But I've also realised I've just wasted 2 litres of suspension fluid by not replacing the return line in time. :'(
Regards.
1973 350SE, my first & fave