Yet another high idle question - PeachParts Mercedes-Benz Forum

Harro10 · #1 04-23-2010, 11:10 AM

I have a 1989 560SL with 63,000 miles on it which I have owned for about one year. The car runs smoothly but idles at about 750 RPM when first started and in gear. When the car is warmed up and I put it in park the idle increases to about 1200 RPM. When I put it back in gear it returns to 750. This is constant, not erratic. The engine runs smoothly at both RPMs with no stalling or missing issues.
I am not very mechanical but have searched the boards and found many possible causes. I visually checked the vacuum lines I could see and reconnected one that was disconnected - no effect. I disconnected the idle control valve while the engine was running and the idle surged to about 1600 RPMs. I removed it and sprayed carb cleaner in it to no effect. The idle control unit does "hum" with the ignition turned on and I do not know if that has any significance. . Before I start throwing parts (idle control valve, idle control unit) can anyone tell me which part is the most likely to be causing my particular issue?
Even if I can not fix this the car still runs great and is a pleasure to drive.
Thanks

rsmartin · #2 04-23-2010, 03:56 PM

my 86 560SEL w/ 82K started doing the exact same thing about a week ago

idles normally when cold, but idles fast when hot

no running problems and drives fine.

i cleaned the icv too. ordered new overload relay just in case. waiting for it to arrive. some on here have said they have taken the relay apart and resoldered but mine is aluminum case crimped to the plastic 5 pin piece so don't know how to get that open without destroying the case. read on here that some people found that to be a fix. my ABS light comes on and the seat belt buzzer keeps buzzing after it's supposed to time out, but at a much quieter volume if i don't fasten my seat belt. didn't used to do that.

Harro10 · #3 04-23-2010, 04:07 PM

Good luck with the fix. Let me know how it works. I have had no problem with the ABS light or I would have looked at the over voltage relay first off but I know it is still a possibility for my car. I went back out to the garage and took the idle control valve off again to shake it. It did rattle slightly and that is supposedly a good sign according to an old thread. The hunt continues.

rsmartin · #4 04-23-2010, 05:10 PM

yes, my icv rattles too. i took it off and gave it a good spray with throttle body cleaner. it's nice and clean and rattles slightly too. in addition, if i unplub the icv while the car is running the idle increases as well. just like yours.

ctaylor738 · #5 04-23-2010, 05:43 PM

Have you checked the voltage between the pins at the ICV with the engine at idle? It should be around 4 volts.

If you are getting that voltage, it suggests that the ICV is bad or that you have a idle air or vacuum leak. In other words, the control unit is sending the signals, but can't control the idle.

If the voltage is low, then the control unit is probably bad, or you isn't getting enough juice from the OVP relay.

Harro10 · #6 04-23-2010, 08:02 PM

I will see if I can get my son to return my meter and check that. If I do not get four volts from pin to pin then it might be the Idle Speed Control Unit or the Over Voltage Protection Relay. If I do get close to four volts then keep looking for a vacuum leak or the Idle Control Valve.
Thanks. I see in other threads that some owners have spent months trying to track this problem down. I am lucky that my car is still driving well in spite of the high idle.

ctaylor738 · #7 04-23-2010, 08:40 PM

Remember, you slide the connector back just enough to get the meter's probes on the pins.

Harro10 · #8 04-23-2010, 09:34 PM

Thanks again.

rsmartin · #9 04-24-2010, 10:12 AM

i get 3.74 volts on ICV when car running.

the other day when i was cleaning the valve and had the air cleaner off, i replaced a rubber vacuum connector that was broken. it's part of the EGR circuit. it connects down in under the throttle body at the front. had to use long needle nose pliers to get to it. i see a number of other connectors under there but how in the world do you get to them?? looks like you'd have to dismantle the whole throttle body and fuel distribution system!!!

can't see what condition they are in but there are a bunch of rubber and plastic lines underneath the throttle body area

again, my car only has 82K but it's an 86 so time certainly trumps use in my case

ctaylor738 · #10 04-24-2010, 11:50 AM

If you are getting that voltage, then the control unit is doing its part to control the idle, but either the air valve isn't working or you have an air leak somewhere.

rsmartin · #11 04-24-2010, 12:30 PM

is there a way to test the valve? is it an open and shut valve or does it vary in it's open and shut status. i.e. partial open

ctaylor738 · #12 04-24-2010, 01:04 PM

The ICV is a frequency valve, constantly opening and closing in response to signals from the control unit. The higher the voltage, the more often the valve is closed resulting in less air and a lower idle.

I just pulled out the CD and looked at the 560 specs. My bad. Unlike the 380, the measurement on current is actually 700-1000 milli-amps to the valve. You would measure this by disconnecting the connector on the valve, connecting one side with a jumper wire, and putting your meter in-line on the other side and seeing what you get with the engine idling. I still think you are OK with 3.5-4 volts, though.

There is a spec of 3.5 - 5.5 ohms resistance between the pins on the valve.

rsmartin · #13 04-24-2010, 01:19 PM

why does engine temperature affect it so much. the hotter my engine the higher the idle.

i tested vacuum on the line to the intake manifold. the line that comes out of the back of the top of the engine and connects to the Y at the firewall in front of the brake booster.

reading of 50 cm/Kg at @ idle of 1000 RPM

rsmartin · #14 04-24-2010, 01:29 PM

zero ohms showing when placing multitester leads on pins of ICV. needle goes all the way over to zero

ctaylor738 · #15 04-24-2010, 02:55 PM

Well, there are two tests on the CD for the ICV.

1. 3.5-5.5 ohms resistance
2. Clicks when battery voltage is briefly applied

If you can't find any other reason for the high idle, I think a new valve would be called for. Sorry.

My theory on the cold/hot idle, and I stress both "my" and "theory," is that the engine is getting too much air which means the mixture is leaner than it should be when cold, so it idles low. As it warms, it is able to use the extra air more efficiently, so the idle speed increases.