Idle Speed Air/Control/Slide Valve Confidence Check

[BabyBlueBenz]

As a recent acquirer of a 1986 560SL, I too have recently begun to experience the aptly described "high idle hell". I've read the related Service Manual pages and the many threads on this forum and others. I did some preliminary diagnoses this afternoon...specifically, a few tests of the Idle Speed Air Valve [ISAV] (a.k.a. Idle Control Valve, Idle Air Valve, Idle Slide Valve). I understand that there are many prerequisite items to check in resolving this dilemma, and I've done some already with more to follow in the days ahead. The attached photo shows a close-up of the ISAV (MB component ID of "Y6") currently in my car. And Yes, the 50-degree Thermovalve (labeled 7 in the photo) has been temporarily "loop-capped" as part of my EGR-removal task in progress. And Yes, I capped the vacuum fitting at the rear of the engine that lead to this Thermovalve. I will be double-checking those hidden-in-the-rear and buried-in-the-V vacuum fittings again.

As suggested at Fast Idle Issues

Originally Posted by ctaylor738

One thing to check. When you get the high idle, slide the connector off the pins on the valve a little ways, and use a meter - carefully - to measure the voltage going to the valve. If it shows somewhere around 4 volts, the relay is trying, so replace the valve. If no voltage, replace the relay.

The plug and pins mentioned are labeled "2" in the photo. With the engine warm and churning away at 1500 rpm, I measured the voltage across the pins at 3.3 volts. Does this pass? I heard this component described as a "frequency valve"...is that another way of saying it is variable control? I never heard the term used before.

Per http://www.benzworld.org/forums/r-c107-sl-slc-class/1305243-560sl-fast-idle-speed-once-engine.html:

Check the ICV's coil resistance to see if it hasn't gone open circuit first. Often they gum up and won't close fully giving you a high idle. If you lucky you can clean out the inside with alot of carby cleaner (try soaking it in it) and it may work again. An ultrasonic bath may also be a good idea.

Shutting down the engine, I removed the ISAV from its rubber-lined loop clamp along with the short rubber hoses (labeled 3 & 4) and the interconnecting rigid vacuum line (labeled 5). I cleaned these four parts with the brake cleaner "Brakleen". The interior of the ISAV and hoses were dark with soot, but cleaned up quickly. I followed the cleaning of the valve with a rinsing spray of WD-40. Once dried off, I measured the electrical resistance between the two pins and obtained 4.5 Ohms. The Service Manual specified a target range between 3.5 and 5.5, so I considered the test a pass.

Per the second page of Service Manual document 07.3-112 "Testing electronic idle speed control", I applied 12 volts to the two pins and heard a slight click. When reversed, the current yielded another click. Tom Hundt's excellent tutorial on the high idle problem at http://www.fly.net/~thundt/mercedes/high_idle_hell.htm describes the result of this test as a "loud clack" with a spring-induced click upon removal of the voltage. I did not perceive this to happen with mine. Also, the ISAV rattles when I shake it. That's my primary question here: Does the lack of spring-return and the rattle suggest a bad ISAV to you? It does to me, but I'm a newbie to these electro-mechanical systems.

For extra credit, opine on this factoid:
The last page of Service Manual 07.3-112 "Testing electronic idle speed control" contains a note regarding the "Distributor line for idle air". It details the revision of two parts related to the idle speed control system (the hose is labeled 6 in my photo). My engine number happens to fall before the implementation was made. Does anyone think these revised parts are worth pursuing?

Thanks for any and all input.

[Strife]

1. A "click" is OK.
2. Rattling is good. NOT rattling is BAD.

[BabyBlueBenz]

Originally Posted by Strife

1. A "click" is OK.
2. Rattling is good. NOT rattling is BAD.

Thank you for the helpful answer.

I will proceed to look elsewhere for the cause of my high idling.

[donbryce]

I went through much the same diagnosis on my '86 560SL last year. I actually swapped the IACV from my '85 380SE, and it made no difference (idle on the 380SE is correct BTW). I bought a replacement control module with the right part number off EBay, noting that the one in my car was not the correct part number, obviously installed by someone else at some point. This did make a difference, and at last time the car was on the road (Oct '08), my idle was perfect on start-up, running high for a minute or less, then settling down nicely to about 750. But, it still seemed to be a tad high after the engine had warmed up to operating temperature, not 'settling down' like it did after a cold start. In the spring, I'll try changing the temperature sender to see if it is sending a false signal to this control module. Perhaps this might be something to look at in your case..I'm not sure which sender to check either..

[ctaylor738]

I would say if you only measured 3.3 volts at the valve, then you need a new control unit. The most reasonable source of dependables units is the ProgRama rebuilts sold by a variety of online vendors.

The idle valve is a frequency valve that regulates the amount of air to the engine, and hence the idle speed, by closing more or less of the time. The control unit sends ground signals to the valve to close it. For a reason that escapes me this can be measured as voltage. The higher the voltage, the higher percentage of time the control unit is sending the ground signal. So at 1500 RPM, a good control unit would be close to 5 volts trying to slow the idle down.

You might also make sure that your throttle valve is closing completely. Unhook the throttle and cruise control linkages, and see if that improves the idle. It's not unheard of for the linkage to bind and give the engine just enough "false air" to raise the idle. I suggest this because 1500 RPM is quite high, even if the idle system is completely non-functional.

[Strife]

A caution about reading a pulse with voltmeter: GENERALLY, analog meters will go up and down linearly with pulse width. So, with a 5V signal at 50% duty cycle (50% 5 volts, 50% 0 volts, and note that the actual low signal is likely to be above zero volts), you should read AROUND 2.5 volts, but this is dependent on the qualities of the meter movement.

HOWEVER, digital meters may not react this way, depending on how their internal sampling and averaging algorithms work. Generally, a more expensive meter is going to behave better than a cheap one in this area. Setting the meter to "AC" generally won't work because non-true RMS voltmeters are designed to treat every AC voltage as a sine wave, and adjust downwards accordingly. Real "true RMS" voltmeters, still not cheap, actually try to calculate the area under the curve of the sine wave of utility AC, or the area under the rectangle (a pulse).

[ctaylor738]

I have tried twice, both times unsuccessfully to get the switch out with the throttle valve on the manifold. It's not that big of a deal to remove the air guide, except that you really need some force to get the collar off the throttle valve neck. Once you've done that, the throttle valve comes off easily. Be sure to check the air guide for cracks, and you may want to replace the band and lock that holds the collar to the throttle, as well as the vacuum line connectors.

Since you've got the injection mostly undone, this would be a good time to do the injector seals as well. Cheap.

[BabyBlueBenz]

I received a new Idle Speed Air Valve and Throttle Valve Switch (TVS) today. I will probably just install the ISAV first and see how she responds to that and bypassing the currently faulty TVS' idle signal. Then I'll (delicately) tear apart the top side to gain access to the throttle assembly and replace the TVS.

Based on the helpful advice above, I'll order the injector insulating sleeves & o-rings. I've already got fresh rubber vacuum elbows for those connections at the throttle body.

2/14 UPDATE: I replaced the ISAV and started her up. No significant change to idle...unless I bypass the faulty TVS by jumpering its connection pins #1 & 2. Then the idle drops nicely...but still indicated on the tachometer as over 1000 rpm. Not great, but an improvement.

I went ahead and pulled off the Mixture Control Unit and Air Guide as an assembly to gain access to the throttle. As mentioned by another, it wasn't too hard (except for separating the Air Guide throat from the throttle).

The next step will be to see if I can reach the two TVS screws with the throttle still on the engine, or whether the throttle has to be removed first.

I'm semi-documenting this operation, in case others would like it outlined. See The 560SL Throttle Valve Switch Replacement Narrative.

[BabyBlueBenz]

Originally Posted by ctaylor738

...and you may want to replace the band and lock that holds the collar to the throttle, as well as the vacuum line connectors.

I got around to replacing my faulty Throttle Valve Switch (TVS). It required pulling out the throttle assembly, which is held in by four small Allen-headed bolts (and a fused gasket). Once out, it took longer to clean the tar-like goo coating the underside of the butterfly than it did to swap the TVS. If anyone thinks it useful, I've posted a new thread with the details at The 560SL Throttle Valve Switch Replacement Narrative. On hindsight, it wasn't too hard. My idle speed has improved (read dropped), but is still not down to the target of 650 rpm (more like 950 for me now).

ctaylor738, the big hose clamp holding the Air Guide throat onto the throttle body broke the moment I tried to tighten it. I can't identify this part or find it on a cursory search at Fastlane, ******** or PerformanceParts...any suggestions on a source? I'm starting to look for a generic hose clamp equipped with a side-ways screw setup.

[ctaylor738]

You can get the parts from a dealer or Phil should be able to find them for you. There are actually two pieces, the band and the lock, best to replace both, since it's very easy for false air to enter the system at that joint. I would look up the PNs but the EPC is down at the moment. I will try again later.

You did check the air guide very carefully for cracks?

[450slcguy]

Originally Posted by BabyBlueBenz

the big hose clamp holding the Air Guide throat onto the throttle body broke the moment I tried to tighten it.

Mine did the same thing, but it wasn't broke, the pieces just separated. It took a bit of figuring out and several attempts to put the pieces together again correctly, but eventually they came together and worked just fine. Patients was the key. I had also though about replacing it with a standard metal band hose clamp, but the access to get a screwdiver in a horizontal position to tighten the screw was virtually impossible.

[BabyBlueBenz]

Originally Posted by donbryce

I'll try changing the temperature sender to see if it is sending a false signal to this control module. Perhaps this might be something to look at in your case..I'm not sure which sender to check either..

I'm trying to identify the appropriate temp sender involved in the idle control...Tom Hundt's High Idle Hell refers to a "16 deg C temp switch", but the MB service manual diagrams don't mention it specifically...only as the "coolant temperature output from pin 9 of CIS-E control unit". I know it's not the S71 Cold Engine Lockout at "top right rear of engine", or the S25/5 105/115 deg C Temp Switch at "top front of engine" which are both involved in the the climate control system. I'm guessing that it might be the sender near the TVS junction at S29/2x1. UPDATE: Never mind, I've found a clue in an old post: "You might check by making sure the engine is warmed up and then unplugging the oil temperature switch located down on the right side at the front of the engine, just above the oil filter can. If the idle goes down unplugging the switch, the swith is defective. The terminal on the switch should have continuity to ground cold and be open when warmed (above 16C) up." I'll be checking that out ASAP. UPDATE: Visit High Idle on my '85 380sl for my self-discovery on this sub-topic.

Originally Posted by 450slcguy

Mine did the same thing, but it wasn't broke, the pieces just separated. It took a bit of figuring out and several attempts to put the pieces together again correctly, but eventually they came together and worked just fine. Patients was the key. I had also though about replacing it with a standard metal band hose clamp, but the access to get a screwdiver in a horizontal position to tighten the screw was virtually impossible.

Crap. That's probably what happened to mine, but the small C-shaped piece that popped off is in a garbage bag now. I'll try to retrieve it, and then see if I can get the clamp parts back together the next time I pull off the Mixture Control Unit and Air Guide. I've been considering a standard hose clamp, but like you said, it would seem impossible to tighten that thing from the very limited access.

Originally Posted by ctaylor738

You can get the parts from a dealer or Phil should be able to find them for you. There are actually two pieces, the band and the lock, best to replace both, since it's very easy for false air to enter the system at that joint. I would look up the PNs but the EPC is down at the moment. I will try again later.

If you can, I would appreciate those part numbers. Even if I get the old one reassembled, I'm not excited about reusing it.

Originally Posted by ctaylor738

You did check the air guide very carefully for cracks?

I gave it a good look, but nothing jumped out at me. I'll be checking it again soon...any particular areas to look for cracking? The edges? The folds? The throat where it's clamped?

I'm also considering doing the full intake manifold removal to clean up the gunk accumulated on the bottom plenum and replace those Manifold Seal Rings (aka rubber donuts). I recently bought one new one to compare its softness to those on the car...and they're quite hard. [Sigh] My To Do List isn't getting any shorter.

[SMinn560sl]

For what it's worth, I've been fighting a slightly high idle on my car for quite some time now. Mine not only idles a little high, but surges around 1500 RPM and stumbles slightly at idle. General prognosis has been an air leak somewhere. I've taken the intake off so many times now that I think I can almost do it blindfolded (well, not quite).

After working on it myself for months without success, I took it to a Bosch fuel injection expert. He sealed all of the holes in the intake manifold and used a smoke generator to pump smoke under pressure to detect any leaks. He reported back that the manifold was leaking underneath, in the area of the rubber seals between the manifold halves, so I took the car home, removed the manifold, and replaced the seals. Incidentally, I previously replaced the idle control valve, the control module, injectors, and all rubber parts in the idle system and manifold in general (including the boot between the fuel distributor and throttle and all injector seals).

When all was back together, it ran the same. I subsequently took the car back to my Bosch expert, he smoked it again, and again proclaimed a leak under the manifold. This time I authorized him to repair the leak. He again replaced the rubber seals between the intake halves and again replaced the injector seals. The car is now a lot better, but still idles a little high and surges around 1500 RPM.

I'm convinced that I still have an air leak somewhere and plan to go through the idle control system thoroughly when I get another weekend at home. Of course, I guess it could also be the idle control valve not completely closing or allowing a varying amount of air to enter the system as it gets around 1500. I just don't know.

By the way, I'm not certain, but I do not believe my idle control valve has a screw adjustment as the pictures above indicate, but I'll check the next time I'm under the hood.

Edit: Curiosity was killing me. I just went out and pulled the hose off the back of the valve and verified that I do not have any adjustment on the back of my idle control valve. My car is an 86 560SL.

[rgp717]

Not to be the dope here, and I have a 85 380SL, which looks similar, but how (1) do you disassemble the Air Valve, and (2) make an "adjustment"? I too am in high idle hell.

[dpetryk]

IAC valve needs to removed from the car. 1 electrical plug and 2 hose connections.

Look at the picture in post #18. Find that spot on your valve. Its one of the hose connections. Remove a plug that covers the threads inside the valve. Insert the screw as shown in post #18. Pull the screw and brass piece out a SMALL - repeat - SMALL amount. The brass piece is press fit into the aluminum and some force is necessary to move it. I used a small prying device (screwdriver) under the nut to move it out. I use a vise to move it in.

Re install and test. Your idle should be lower. If not low enough repeat the process. If your idle is too low - go the other way.