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Old 01-21-2006, 06:35 PM
1989 560SEL, 1998 CL600
Join Date: Jan 2006
Location: Bay Area CA
Posts: 47
Idle speed fluctuates - 560SEL

The idle speed on my 89 560SEL fluctuates. The idle is also not as smooth as I would like (more noticeable in park). The change in RPM is very subtle but you can hear the difference. The frequency of fluctuation is about 2-3 seconds with equal time at each of the two RPMs. You can't see the fluctuation if you watch the RPM guage on the dash, You can see it if you attach a frequency counter to pin 1.

I've hooked up a DVM (with duty-cyle) between pins 2&3 at the X11 connector. Interestingly the measurement fluctuates in sync with the engine RPM. Around 50Hz when the RPMs are high, 44Hz when the RPMs are low. Adjustment of the CO mixture screw will change the raise or lower both but not stop the fluctuation.

Also interesting is while trying to determine if the rough idle might be due to one cylinder I pulled each spark plug wire. Each time the rpm drops and stops fluctuating! (the engine idles rougher as well, I was unable to conclude that any particular cylinder was causing the rough idle).

Then something else happened that might also be a clue. Sometime while pulling the plug wires the check engine light came on. I wasn't watching it continuously so I assume that it came on because of a spark to the engine block. The check engine light's brightness was also fluctuating! It was hard to tell if the variation in brightness of the light was in sync with the rpm change, but they seemed to be at about the same frequency (2-3 seconds).

It seems to me that the engine control computer is causing the flucuation. Either it can't bring the engine into a steady state or something is wrong with it. Perhaps with OVP relay is sending out a flucuating voltage. Where's the best place to mearsure this?

What do you think should be my next step? I recently re-did the cylinder heads and during reassembly I've replaced most everything: plugs, wires, cap, rotor, fuel injectors, idle control value, vacuum hoses. Things not changed where coil, fuel distributor, eha value, fuel pressure regulator. Also keep in mind that this is a CA emissions car. I mention that the idle is not as smooth as I would like, Its not bad, my 2000 Ranger is no better. But I would like it to be nearly perfect. Am I being too fussy?


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Old 01-21-2006, 07:39 PM
Registered User
Join Date: Jan 2006
Posts: 7
Idle Speed Fluctuates

I had the same trouble with my car and changing the fuel injectors took care of the problem once and for all . Before that I changed every component that seemed suspicious mostly the obvious ( spark-plugs, wires, cap,rotor , fuel filter, fuel pumps) . I waited with the injectors because they were the priciest item but got lucky when I saw ad on eBay for W126 Injectors for the big boys for $40 a piece article # 8031671714 . Now they're even cheaper with free seals and free shipping . Mercedes CD says that "Idle Vibrations" should be corrected by changing engine-mounts and/or injectors . I had mounts changed and went for the injectors . I think $320 is a good price to pay to get very nice fuel ecomomy - low rpm idle - and responsiveness which I never
knew this car was capable of . Most members will agree that it's a must to change fuel-filter before putting in injectors . Procedure to change injectors is easier then changing spark-plugs . Hope this helps !?
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Old 01-21-2006, 08:09 PM
1989 560SEL, 1998 CL600
Join Date: Jan 2006
Location: Bay Area CA
Posts: 47
Injectors are new. (see my previous post)

Here's another interesting observation: The idle speed is fluctuating between 730-770rpm. If I disconect the vacuum line to the ignition control unit it drops to 690rpm and does not fluctuate. What does this indicate.

The higher than spec idle speed has made me think I have a vacuum leak. But I've checked and tested all of the lines with a vacuum gauge/pump. I've sprayed the injector seals with carb cleaner and didn't notice any idle change. I still need to test the vacuum at the brake booster. If necessary, I'll take the car to a shop that can do a smoke test to confirm no vacuum leaks.

Any suggestions on futher testing I can do?

Last edited by falconsprint; 01-21-2006 at 08:46 PM.
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Old 01-22-2006, 12:02 AM
1989 560SEL, 1998 CL600
Join Date: Jan 2006
Location: Bay Area CA
Posts: 47
Another clue, my temperature gauge never goes above 80C. At idle it likes to sit at 70C. I must have either a bad sensor, stuck thermostat, or the installed thermostat is set for 70C. Although the idle control uses a different sensor than the dash gauge.
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Old 01-22-2006, 03:58 AM
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zcc zcc is offline
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Join Date: Nov 2005
Location: Kuwait
Posts: 610

Make sure the the gasoline is clear and clean and has a good octane. Use two STP octane booster bottles (the orange bottle) for a full tank, if your problem solved then it's a gasoline problem, if not you need to check the spark plugs and the spark cables as well. Use only Bosch spark plugs and Beru spark cables (NOT BOSCH SPARK CABLES). All the best.
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380SE 1984 <------- Totaled by flipping accident
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Old 01-27-2006, 02:09 PM
1989 560SEL, 1998 CL600
Join Date: Jan 2006
Location: Bay Area CA
Posts: 47
Here's an update.

Changing the OVP Relay lowered the idle RPM from 770 to 670. That's good, but it didn't fix the rough idle.

The spark plug in cylinder #2 was getting fouled. I tried swapping the injectors, plugs and ignition wire with cylinder #1. Each time it was #2 that got fouled. Last night I swapped the ports in the fuel distributor. Fortunately you can do this with #1 and #2 without having to bend the hard fuel lines. Not only did the fouling stop in #2, but it has not fouled yet in #1. The idle is much much better. There is still a slight stumble at around 1200 rpm when accelerating from a stop.

I'm now assuming that I get a bad fuel distributor. It must be pumping too much pressure into that port. I assume that the bad distributor is also responsible for the stumble. I'm not sure why it is not affecting #1 as much as it did cylinder #2.

Am I on the right track? Is it possible for a distributor to go bad in one of the eight ports? I'm thinking that some dirt has lodged itself so that a valve is not closing all of the way.

P.S. Yes, I've have used Techron in my gas tank several times. It hasn't helped in this case.
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Old 01-27-2006, 02:54 PM
halman2228's Avatar
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Join Date: Jul 2004
Location: Houston TX
Posts: 304
With you all the way...


I wish I could help but I have the same annoying fluctuation (on my '90) - you can hear it but its not enough change to register on the tach. I've not done anything about it yet and I am WAY behind you in skill level/knowledge so I'm listening (reading) real closely...

Good luck. If I accidentally trip over a possible answer I'll make sure it gets to you...

"Dangerous with a wrench...and not smart enough to know it"

Stable (in order of acqusition):
'84 500SL, 280SEL (Euros)
'77 280SE. '90 560SEL
'79 450SEL 6.9,
'95 C280 (totalled by daughter 8/07)
'81 280SL (rescued) '88 300CE
'86 560SEC '95 C36
'01 CL55

DD's: '04 S500 4Matic (SWMBO)
'04 F150 Crew
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Old 01-27-2006, 06:45 PM
Registered User
Join Date: Nov 2003
Location: MD
Posts: 102
Hey Dave.
When the FD goes, generally there is fuel left inside the throttle body area after the car is off after running awile. This indicates a breech in the FD piston. Hope this helps!
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Old 01-28-2006, 01:09 PM
Registered User
Join Date: Jul 2000
Posts: 638
why #2 plug fouled?

You may have a valve guid or valve stem seal problem in #2 cylinder.
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Old 02-04-2006, 06:06 PM
1989 560SEL, 1998 CL600
Join Date: Jan 2006
Location: Bay Area CA
Posts: 47
A used fuel injection distributor from e-bay (from forum member derryb) has solved my problems. Smooth idle and no more stumble accelarating from a stop.

Thanks everyone for their advice.
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Old 02-04-2006, 07:11 PM
Registered User
Join Date: Sep 2005
Location: JAX, FL
Posts: 140
good to hear

thinking now that maybe i should have kept it as a spare for my 560SL. Glad I could help out.

65 190d sold
85 380SE sold
87 560 SL 180K
84 380SE 96K (mint)
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