Idle Speed Air/Control/Slide Valve Confidence Check

[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.

[BabyBlueBenz]

SMinn560sl, you're discouraging me.

If you've done all that effort and still have the same problem, then I may just settle for my current 1000 rpm warm idle.

Honestly, I didn't pay attention when I bought my car a few years ago and note how it idled back then. It definitely got worse a few months ago, easily reaching 1500 by itself (no stumbling though). With the Throttle Valve Switch recently replaced, the engine sounds "normal" to me.

Armed with Strife and other's suggestions to date, I have a few more things to check (alternator output, OVP, Oxygen sensor, temp sensor) before I give up on this task for now and move on the next item of concern for me...timing chain & guides health.

BTW, both my old and new Idle Speed Air Valve's were VDO brand and had the same threaded insert as dpetryk's photo. Here's a picture of my dissected ISAV showing the inside of the business-end:

[SMinn560sl]

Thanks for the picture B3. I have to admit that I didn't read the description of the adjustment fully and was looking for a screw protruding from the back of the valve (yeah, I know...). I'm off this weekend, so will give the screw trick a try. I'd expect the thread to be a standard metric thread instead of an SAE thread, but I'll try the size recommended.

Incidentally, if anyone is interested in building a smoke generator, it's really pretty simple. My Bosch guy's tester was just a can with a twist off cap like the type that paint thinner comes in. Essentially he had mounted a diesel glow plug toward the bottom of the can as well as an air hose fitting (quick release) into the top. As I recall, he soldered a small metal tube into the cap after drilling a hole in it.

To prepare it, he filled it up past the glow plug with hydraulic fluid (brake fluid), connected an air hose from his compressor to the hose fitting and slid a length of fuel line over the spout that he soldered into the cap. He attached two leads from a 12V battery to the glow plug and the can began pumping smoke under pressure from the compressor. He slid the free end of the length of fuel line over one of the vacuum nipples on the manifold and the system pumped smoke into the manifold. If you've disconnected all of the other pipes and hoses that feed the manifold, any smoke that escapes is a leak.