Another high idle mystery

1971-1989: 280SL, 280SLC, 300SL, 350SL, 350SLC, 380SL, 380SLC, 420SL, 450SL, 450SLC, 450SLC 5.0, 500SL, 500SLC, 560SL
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matthewz108
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Another high idle mystery

Post by matthewz108 » Tue 02 Oct, 2018 7:28 pm

Hi all,
I've found myself a 1982 r107 500sl (m117), and it has a high idle problem which I've discovered is quite common with these things. It starts and drives well, idles smoothly, however when warm it idles at about 1400rpm. It starts and idles at about 900rpm, then once warm, the idle rises. I've cleaned the idle control valve thoroughly and measured the voltage at the icv as about 5.2 volts when running warm which I believe is about right. When it's unplugged, the idle increases slightly to about 1500rpm.
I've checked for vacuum leaks by spraying wd40 at all vacuum hose connections and even along the hoses (read this method somewhere else so not sure if this is even correct, but i don't have any sophisticated tools or other knowledge?), with no change in engine running.
Linkages seem original and free.
Can I investigate or eliminate any other possible causes?
Any help or ideas would be appreciated...
I have some other aesthetic queries, but they aren't anywhere near as annoying as this issue so will leave them for another time...
Thanks in advance.
1971 W108 280SE 3.5
1982 R107 500SL

CraigB
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Re: Another high idle mystery

Post by CraigB » Tue 02 Oct, 2018 9:50 pm

I will watch this thread with interest. I have blindly ignored the k-jet system for years, mainly because it has been completely trouble free on my cars. I left a car a bit long, fuel went off and now learning lots. You've probably seen lots of you-tube videos when you search like i have. The one i recall with high idle they came back to air leaks - 'false air' I think he said. Mine was running fine but now runs like crap. Dropped the fuel, new filter and checked fuel pressure at about 90psi which is right for the dual pump 560 (not sure about your car). Then I removed the filter that is part of the inlet union and noticed the little screen in it was broken and i think this may have added to my trouble. Then I pulled the injectors and some of them felt almost loose and rubbers very hard but a spare engine i had they were nice and firm to pull and rubbers nice and soft. Pretty easy to pull an injector if you wanted to check that. Checking US sites, all those rubbers can be had fairly cheaply - if its a keeper might be worth a couple of hundred to replace those rubber manifolds and all the seals and make sure ruling that out and forget about it for another 20 years. Surprising mine had been running so well but when you said about the WD40, I wonder if that would work if all of them sucking just a little air. Have you checked your plug colour after a good run and warm and then idling for a while a the high idle? I don't know but wondered if the leaness would show up in a pale colour - If the suggestion is (from this youtube) that the leaner mixture from the air picks up that idle speed a little.
I set up my injectors with another set of lines i had and their spray patterns are shocking, all going off at different pressures and dripping, some running after shut off. So i know where i need to start working. I'm soaking the set with the nice rubbers tonight and will try those tomorrow - fingers crossed. Then i will see how it idles and if I need to look further.
The other thing i have heard about, but didn't pick up from the youtubes etc., is the rubbers between the manifold and plenum i think it is - under the top part there are 8 rubber rings and can get air leaks there I have heard.
Anyway - just thinking out loud and hopefully something in there useful and might trigger things to try on yours. I know there are other parts of the system that others might have knowledge of.
If i think of the collective km of k-jet driving, its a good trouble free system, just can't wait to get back to being in that condition again!
Craig Baulderstone
Lurch
280s's
280SE3.5
280SL
350SL
500SEC's
560SEL's

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TheMadRacoon
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Re: Another high idle mystery

Post by TheMadRacoon » Tue 02 Oct, 2018 11:39 pm

CraigB wrote:
Tue 02 Oct, 2018 9:50 pm
I will watch this thread with interest......

the suggestion is (from this youtube) that the leaner mixture from the air picks up that idle speed a little
Me too Craig.

Now just wondering - in general, and nothing to do with whether a car has kjet or djet - about the chemistry of this phenomena: My 350SLC idles a little high when cold, until it warms up to (I forget exactly) a temperature where the idle air valve starts closing. If I try driving off too soon then have to release the throttle, the idle rises (until the ecu cuts fuel supply; that bit is djet, but putting that aside) posts here & and other sites on high idle, as you note, advise to check for air leaks.

Now when cold the engine is running rich. Here comes the chemistry question: adding air, whether via the open idle air valve or due to vacuum leaks, technically makes the fuel/air mixture leaner, but not lean (in this scenario) so I 'get' that more power is released and the engine turns over faster. Does that happen too in a warmed up engine experiencing high idle? Leaning out the mixture due to vacuum leaks when the fuel/air mixture is expected to be about right (engine at operating temperature)...would that leaner mixture cause a faster idle? I never got my head around that one.

Hoping you or the collective wisdom can clarify that for me. Not meaning to hijack the thread Matthewz108, hoping it helps us all.

By the way I tried the WD40 test too (once engine was warm) but also found no difference. Perhaps when warm rubber hoses expand and seal better and hide the problem? Is it worth testing with WD40 while the engine is still cold?
Emad,
TheMadRacoon
1975 350SLC Astral Silver (725) / Blue (2012, 284,900 km - present.... a real Benz, getting better and better)
1988 190E Deep Blue (900) / Cream (2006, 190,000 km - 2007, ~ 215,000 km .... FSH and still spent big $$$)
1974 280E Reed Green (860) / Bone (1993, 316,700 km - 2004, ~490,000 km and still A1)

gazman
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Re: Another high idle mystery

Post by gazman » Wed 03 Oct, 2018 12:03 am

I don't think it's a mystery, I think it's sucking air. There are many potential air leaks hidden under that top inlet manifold. To rid myself of high idle hell, I replaced everything rubber in the valley along with new injectors and o rings. All the old rubber's were rock hard and for sure were sucking air especially the donuts that join the two halves of the manifold together. If the cars a keeper its worth doing along with the timing chain, guides, cogs, tensioner and oiler tubes, especially on a 500 as I believe they were only a single row chain. As I have found you do need to throw some money at these older Merc's after purchase but ya gotta love em. Great cars. Enjoy.
1989 560 SEL King of the road

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Re: Another high idle mystery

Post by CraigB » Wed 03 Oct, 2018 1:14 am

pretty sure the only single row chain was early 380 but still, a very important point. Double or not if you can't be sure work is done you just do it again. Its a 'game over' situation. Even if its not a keeper, absolute minimum is that LHS inner guide, the only one on the inside run of the chain. That's the one that snaps and jams between chain and cog, throwing chain/breaking..... valves into pistons. There is a theory (that makes sense to me) that a worn tensioner adds to that problem because it allows the chain to 'slap' that guide and if brittle break it, so for the cost of a tensioner might be worth thinking about. Its a $20 part and it is possible to replace with just removing the rocker cover and loosening off the PS pump. You can measure chain stretch and check shape of teeth on cogs - winding a new chain through is not a huge job. You didn't mention what mileage it has done but i agree if its a keeper and higher mileage and you do all those things it will really freshen it up and you can just enjoy it and not touch it for a very long time.
Craig Baulderstone
Lurch
280s's
280SE3.5
280SL
350SL
500SEC's
560SEL's

matthewz108
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Re: Another high idle mystery

Post by matthewz108 » Wed 03 Oct, 2018 6:11 am

Wow, thanks to the consistent quality contributors on this forum. No one's hijacking any posts - all good.
The cars got pretty high mileage at about 220k. But when I bought it after I had it checked out, I drove it 10 hours home with no problem and plugs looked fine when I gave it the once over at home... it's funny because it idles smoothly, kicks back and runs well through the rev range otherwise.

Sounds like everyone is leaning towards vacuum leak or maybe some other not-so-obvious gremlin (i even read somewhere it may be engine mounts?) which was my thought as the system is not that complicated...

Looks like I'll invest in some parts and start from one end and start to replace replace vacuum pipes/ seals. It would probably cost me a couple of hundred for someone to diagnose it as it is, so maybe it's worth a shot. Or maybe I'll google how to make a smoke machine to check vacuum thoroughly....
1971 W108 280SE 3.5
1982 R107 500SL

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Michel
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Re: Another high idle mystery

Post by Michel » Wed 03 Oct, 2018 6:54 am

My 500SL also idles high when it's warm (and when it feels like).
Alas I have not investigated the possible cause, but this thread is a reminder that I should do that sometimes!
Michel (in Sydney)

W116: 1978 450SEL 6.9 - 690SEL # 3312
R107: 1985 500SL AMG - LHD
C208: 2000 CLK430 Coupe
W163: 2003 ML320

gazman
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Re: Another high idle mystery

Post by gazman » Wed 03 Oct, 2018 9:17 am

You can't access many of the rubbers to check without removal of top half. 220k an 36 years old = cooked donuts. It's hot in there. If you decide to go ahead take pictures draw diagrams ect of vac line routing. You don't really want to get lost, And beware of frozen bolts, a couple of sharp blows using an Allen socket and hammer help's shock them into freedom.
1989 560 SEL King of the road

CraigB
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Re: Another high idle mystery

Post by CraigB » Wed 03 Oct, 2018 9:32 am

Good to discuss these things! 220k is not a heap of miles for one of these but i would make that point again about doing something about that chain guide before using much more. You would be surprised how often there are posts that start with "I just bought this car and......"
After a long run I don't think the vacuum would show up because any leakage would be a small proportion of the 'mixture'. It would be after idling for a while. Most of those lines should be fairly visible. Do you have an economy guage? If that's not working that is a hint that there is a line off or something.
From the posts i have read so far the main seals I would be looking at are injector seals and those rubber manifolds and big hoses are pretty cheap. These get the most heat and my thinking is that if you think of the forces of the vacuum those injector seals are right there where it is sucking as the piston goes down. The fact that some of mine were basically loose in there (on a 220k km car) is not a good sign. Even local suppliers are not that expensive for those injector seals, but following recommendations on here i checked out fcp euro and all this stuff was very cheap and the shipping estimate was good to.
I want to give an intelligent response to Madracoon, and i know the d-jet and K-jet behave differently when trying to adjust idle - but if i shortcut the basics as i understand it, there is a sweet spot for maximum power that will be leaner, but you run the risk of damaging things like melting valves etc. and so you run it a bit richer. I know my AMG on the dyno produces good power but the mixture graph shows it could produce more if leaned out a bit in the higher revs, but there are limits on how you can adjust that on the standard system and it is on the safe side for the engine. If you go back to tuning your simple old single barrel carby you wound the mixture screw until it reached maximum revs and then wound it back something like half a turn.... from memory. I still need to get more familiar though as to how the enrichening system works on the k-jet.

Good advice from gazman - need to find that part number or name for the 'donuts'. And photos are good to with the diagrams - im still getting the hang that i have this camera always there on my phone.
Craig Baulderstone
Lurch
280s's
280SE3.5
280SL
350SL
500SEC's
560SEL's

gazman
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Re: Another high idle mystery

Post by gazman » Wed 03 Oct, 2018 2:46 pm

It's only a 10 min job to remove L/h rocker cover and have a look at the colour of the guides if it's coffee coloured it's overdue for replacement. I would put this on the top of the list, like Craig says if it breaks the engine is scrap. if you only replace that one guide it's cheap insurance for a while.
1989 560 SEL King of the road

matthewz108
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Re: Another high idle mystery

Post by matthewz108 » Fri 05 Oct, 2018 6:27 pm

So the 3 thing's to do are:
1. Check timing chain
2. Check timing chain; and then
3. Check the timing chain.
I think I'll leave the timing chain to an expert. I'd rather pay a bit and have it done right... with some insurance...
In the meantime, I'll start looking through the vacuum issue, and I'll try to diagnose it before i go changing components...
The issue with pulling it down is not having access to parts as required, then the job taking weeks (or months...) to complete.
1971 W108 280SE 3.5
1982 R107 500SL

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