intermittent fast idle getting worse - PeachParts Mercedes-Benz Forum

Marc · #1 05-04-2002, 02:09 PM

I have an 83' 500sl euro. We don't drive it much. Very little during the winter months. The weather is starting to get better and my wife took the car out. When she came back she mentioned that it seemed to be idling very fast and wouldn't slow down. I took the car out and it was fine. This was several weeks ago. We now have been experiencing this "fast idle" between 1600-1800 rpm intermittently. The car may be sitting idling fine and all of a sudden it will just kick up to 1600-1800rpm this is true also when driving, you can tell sometimes its idling normally and sometimes fast. Other than the idle the car runs great. This car used to chug just a little on startup and you had to wait 20-30 seconds and then it would run just great. I assumed this was the warm-up regulator and since we didn't drive it much this was no inconvenience. Now though with the fast idle I'm afraid we're going to damage the transmission by shifting with this fast idle. I've read some of the posts here. I've taken the idle control valve out and tried to clean it. If anything the problem has gotten worse. I 've read of the idle control unit. Does this model have one, and if so, where? Could this be warm-up regulator, idle control valve. Any thoughts would be greatly appreciated.
I'm slowing refurbishing the car, replaced most of the interior a year or so ago, so, I don't mind spending some money on the car but would prefer as much info as I can get before I start thowing money at some of these more expensive parts.

Thanks

Marc

blackmercedes · #2 05-04-2002, 05:08 PM

Hhhmm. Does this car have a Mass Airflow Sensor? Our 1988 Mazda 626 did the exact same thing, and the MAS was the culprit. We've had several member's cars here doing the same thing, and in most (all I can remember) the MAS was at fault.

Not sure about FI systems of that age.

Marc · #3 05-04-2002, 07:53 PM

It doesn't idle rough at all but, I did check the fuse on the ovp.

I read some of those posts about the solder cracks. I've worked on circuits boards in the past so, I can solder. I don't know where the idle control unit is on this car. There isn't much room between the engine and firewall, which is where I think it would be.

I don't know if this car even has one.

thanks

dpetryk · #4 05-05-2002, 01:31 AM

Intermittant hi idle could be caused by an improperly adjusted throttle body switch which commands the idle control system to engage when your foot is off the gas. Check the contact closure resistance of this switch and be sure it closes when the throttle is fully closed.

Marc · #5 05-05-2002, 10:41 AM

I took out the idle control unit and checked for cold solder cracks, the board looked fine but, I resoldered the contacts anyway.

Unfortunately this didn't help at all.

Yesterday I took out the idle control valve (someone else referred to this as a throttle slide). It had a lot of carbon in the two outlets. I tried to clean it. I didn't open it up but poured in some gasoline and shook around until it came out clean.

I seem to have had the fast idle problem since I've done this (its not intermittent anymore).

Any thoughts on if this part causes this problem or, did I destroy some sensor by the way I tried to clean it?

BTW, part number is 000 141 14 25

I appreciate your thoughts.

Marc · #6 05-05-2002, 06:31 PM

I took out the idle control valve again and started cleaning some more. Noiticed this time how the valve slides back and forth, it did appear to be stuck. I cleaned some more and realized that its spring loaded, by the time I was done the valve would spring back by itself if I pulled forward on the slide.

I thought for sure this was going to solve the problem. It didn't.

I did find something else. If I pull the connector off of the idle control valve while the car is running the idle speeds up even more, when I plug it back in the idle drops to something close to if not normal. When I turn the car off and restart I'm back to the fast idle.

Does this tell me anything? Defective idle control valve?

Are we having fun yet?

dpetryk · #7 05-06-2002, 09:29 AM

Yea its the valve. First off, you are using the wrong fluid to clean it. Gasoline is not a good solvent for that application. Its like using gasoline to clean a carburator. The deposits are formed from the gas and as a result gas is a lousy cleaner. You need something much stronger. I use brake cleaner. I have found that all brands are not created equal. It will disolve carbon. And you really have to work at it.

There are several threads discussing this very topic. Several by me. I suggest you read them.

It is normal for the idle to increase when you remove the connector. Thats a good sign. But the valve is still dirty.

Marc · #8 05-06-2002, 10:21 AM

Dave,

I actually saw one of the posts about cleaning the idle valve. Thats what made me go back and clean it again to begin with. I should have mentioned, I used a can of Gunk Carb Medic this time not gasoline. By the time I was done the valve would spring back freely if I inserted a small screwdriver and pulled forward on the slide.

Is there a definitive way to test the valve without having a second one available.

You mentioned in an earlier post about an improperly adjusted throttle body switch. Do you still think this is a possibility given the symptoms? Do they typically lose their adjustment or does something else affect it. Where is it? How would I check it. I do have a multimeter to check resistance.

Thanks for your help.

Marc

dpetryk · #9 05-06-2002, 11:38 AM

In my experience, it could be the throttle body switch. Im my 91 420sel, when the switch was not making contact when it should, the idle was about 1200-1300 rpm. Sometimes it worked and sometimes not. I checked the throttle body switch contact resistance and noticed that it bounced around quite a bit. The resistance should be less than 2 ohms when closed. It was bouncing around2 to 50 ohms and was "noisy" and not repeatable. It was a simple position adjustment of the switch itself. It was on the edge of switching when the throttle was closed - most of the time. After adjustment everything is OK.

Check the resistance of the switch and be sure it is closing when it should.

However even if the electronics is OK the valve seems to have an authority problem. Let me explain what I mean. It started with my 87. The idle was always hi. About 1200 RPM. I could not get it to come down. When disconnecting the valve connector - the idle went up. When applying 12 volts to the valve - the engine died. I connected a variable voltage source to the valve and was able to regulate the idle speed quite nicely. I was then convinced the problem was in the control module since the valve seemed to work. NOT. The valve had the ability to control the idle, I demonstrated that. So why didnt it work?

The control module was fine. It seemed that it was not able to supply enough current to the valve to bring the idle speed down far enough. Thats what I mean when I say it didnt have enough "authority" to control the idle. It was trying but it did not have enough strength. I bought a new valve and installed it and low and behold everything was fine. So now I was determined to understand what the problem was.

I took the "bad valve" apart - I mean completely apart. I could find nothing. I checked the stroke vs current & voltage signature from the new valve and checked it against the bad one. No observable difference. The bad valve had no observable difference to the new one except it was not shiny. So I accepted the fact that there was still so something I did not understand and I let it go at that until my 91 had the same problem.

This time I reasoned that there was something physical in the valve portion causing the problem. Thats when I got real agressive on cleaning the thing. I worked on that thing for hours until I could get no more carbon out of it. And it worked.

There is another phenomom used to drive these valves. The control module modulates the current applied to the valve with something called "dither". Dither is needed to overcome something called "stickshion" . Stickshion is best described as something like sticky friction. Its where the valve does not want to move shoothly and porportionally with the applied current. Because things get sticky it wants to jump rather than slide smoothly. So the electronics applies a low level rapidly varying current to it to constantly wiggle it and keep it free so it wont stick in one place. You can feel the dither when the engine is running. It feels like the valve is buzzing. This buzzing effect makes the valve move smoothly even though it is full of gunk and other sticky stuff.

My conclusion is that over time these valves get crudded up and wear mechanically and the result is that it takes more current to drive the valve shut. After a while the control electronics cannot supply enough current to bring the idle down and hence the idle creeps up. Cleaning it can cure it if it is not worn out. Dither makes the valve move thousands of times per minute and the resulting wear changes the stroke / current characteristic of the valve to something beyond what the electronics can compensate for. So try cleaning it really well and if that dont work, grab your wallet.

Marc · #10 05-06-2002, 12:56 PM

That was a great explaination. Completely logical.

I did no disassemble the valve to clean it. I was trying to clean it through the valve openings.

I may do as you described and apply voltages to see if the valve has the ability to control the idle.

The valve looks like it is held together with a crimped band around the middle of it. I assumed it was not serviceable. I may just take it apart and try and clean it, since my next step it to purchase the valve anyway.

I appreciate your help.

dpetryk · #11 05-06-2002, 01:27 PM

Dont take it apart. It will be destroyed in the process. They are crimped together and you must clean it without disassembling it. I took mine apart cuz I figured it was shot and had nothing to loose.

Marc · #12 05-12-2002, 02:36 PM

Well, I replaced the idle control valve. It looked like this fixed it. But, the next day I had the fast idle again. Its back to intermittent.

Yesterday, I got frustrated with it. I had been hearing a hiss from behind the outlets in the center console. I started disassembing to try to find a broke hose or something. In the process I took out the climate control unit. When I started the car perfect idle. The rest of the day, perfect idle and no hiss. So I figured something the control unit activates has a vacum leak. However, my wife took the car out and again the fast idle is back.

Disconnecting the connector from the idle control valve no longer causes the idle to speed up, nor does reconnecting it cause the idle to slow down.

At this point I'm at a loss.

Anybody have any ideas on what this sounds like?

I would appreciate the assistance.