W126 500 M117 - High idle help

[500Landyacht]

Hi All,

I'm trying to diagnose a high idle on my 1985 500SEL and am after some help. I know this topic has been done to death but the threads never seem to lead anywhere. Also, I'm not expecting anyone to tell me what my problem is, just how I can troubleshoot the IACV further.

Since buying the car a year ago, it has idled at about 1200 + rpm. Starts at where it should be when the engine is turned over and then slowly rises. So far I've done the following;

- Found several vacuum leaks and replaced all the offending hoses.
- Replaced the OVP relay (shot in the dark really but it was the original and looked like it had been tampered with before

- Cleaned the IACV and observed the following;
1. Nothing happens to the idle when it's unplugged
2. If I unplug the valve and put power to it, it opens and the engine slowly cuts out. So at least I know it's not just stuck completely.

So my question is, before just blindly throwing money at a new IACV, how can I determine it's actually faulty? Please note, besides doing very basic multimeter checks, I'm a total novice when it comes to electrics.


Thanks in advance!

 

[optimusprime]

To find information on this problem , you can use the search box at the top of the page, this is just one subject that i found in there .Put this in the forum search box > Thanks For Information On High... Idle < - - hope this helps you .

 

[500Landyacht]

Thanks and I had a look but I'm not after a random thread that just says a million reasons for high idle. I'm looking for specific troubleshooting advice on the IACV

 

[Bellow]

My experience with IACV isn't with a Mercedes, is with (probably) a different fuelling system, and, I'm uncertain whether opening or closing an IACV results in maintaining idle speed against increased load - nonetheless, as we are looking for activity at the IAVC it shouldn't matter.

Is there any sign that the IACV is responding to change of load? For example, I know that mine is because if I simultaneously throw the steering full lock and engage drive (or reverse) the idle speed doesn't change. If the IACV wasn't functioning the rpm would drop during that. How is it with yours?

Does your engine have a throttle stop screw? Mine has (although it is supposed to be tamperproof) and when I adjust it, the IACV stabilises the idle speed - another sign that it is working.

If you aren't seeing anything of the above, possibly your IACV is not seeing a signal from the ECU (as the IACV moves when fed power independently - a sign I'd say confirms that it is OK) possibly due to damaged broken wiring between it and the ECU. Any sign of physical damage? Theoretically, it should be possibly with a multi-meter to measure an ECU output on the wires with changing load - but you'd need to know which wires. (Looking at the wiring diagram for mine, there are 4 wires in 2 pairings, each pairing driving the IACV in one direction or the other. Green/black and green/white is one pairing, blue/black and blue/white the other. Kinda obvious where to take that). You have though, identified pairings in yours when you powered it.

IACV usually has a reset sequence (for new install or when they've been removed and disturbed). If you haven't performed it, it might be worthwhile.

One other thing (at this stage) I'd have a look at is that the ECU is actually seeing that the engine is warming. A faulty temp sensor/broken wire/etc could cause that. My engine has 2 temp sensors - one for the ECU, the other for the temp gauge.

 

[optimusprime]

500 Good luck i will let you carry on .