W126 420SE 4.2 V8 (M116.965) High Idle Troubleshooting

[finnoctane]

Hi, new member here but been a Mercedes 190e 2.0 manual owner for a while now...it was my parent's car back in the 90s and I've now adopted it!

I recently purchased a W126 1988 420SE with the M116.965 4.2 V8 (Gen II - I believe so this may not apply to earlier V8s) to complement my 190e but have been plagued with a high idle problem since purchase last summer. A problem that appears all too common with the W126 V8 models. I've finally after nearly six months managed to solve it with the help from past posts on this forum and others with hopes that I've gone about it in a cost effective manner which may be of help to others. It's way too easy to throw new parts at a problem and not actually know whether the electronics of the idle system are functioning correctly.

From the workshop manual 7.3-112 Testing electronic idle speed control the following must be true at operating temperature:

1. Idle speed 600-750rpm in P or N
2. Idle speed 450-550rpm in D
3. Current draw of ICV 700-1,000mA at idle. May drop below 700mA above 2,000m altitude.
4. Resistance across ICV terminals of ICV 3.5-5.5 Ohms

My high idle presented nearly all the time since purchase, sometimes it would idle lower when cold but once the engine reached operating temperature the idle would settle at around 1,000rpm. During diagnosis I had done the following:

1. Cleaned the idle control valve (ICV) with a carb cleaner spray liberally. Appeared to help somewhat for a few days but returned to high idle after. Repeated several times with no permanent change to the high idle.
   
2. Tested proper function of ICV by applying 12V from the battery briefly across the ICV terminals. The engine stalled due to lack of idle air appearing to confirm proper function of the ICV - suggesting no significant vacuum leaks (don't have a smoke tester so that will have to do). Pulling the 2-pin plug to the ICV resulted in the engine speed increasing to 1,600rpm.
   
3. Checked the voltage going from the OVP to the idle control unit (ICU) connector block under the passenger floor mat. Looked inside the ICU by taking off the plastic cover - mine is appears to be a microchip version not the old style soldered PCB. All in spec.
   
4. Checked continuity going to the ICV from the ICU connector block. Checked resistance across ICV terminals at around 4 Ohms. All in spec.
   
5. Replaced the OVP for good measure as mine appeared to be original from the factory. No change to high idle.
   
6. Replaced the injectors and injector seals as they were well over 100k miles. No change to high idle but the engine seems to run a little smoother.
7. Ignition control unit (EZL) failed during this time. Sent to Avilec on the Isle of Wight for testing. Confirmed that the failure was unrepairable. Sourced a second hand unit from Germany and replaced the ignition coil for good measure as it appeared to be an original from the factory. Testing appeared to show the coil was in good spec but it's a cheap component and Avilec did mention the EZL failure appeared to be from over-voltage damage from either ignition or power side of the EZL. New second-hand EZL did not cure the high idle.
   
8. Replace the voltage regulator on the alternator due to the brushes being warn to below 1cm and again appeared to be original from the factory. This solved the low charging voltage of 13.6V at idle, now it's at 14.2V at idle.
9. With all the above done, the current draw of the ICV from the ICU should be in spec as per the workshop manual. I fashioned up the ICV test cable (102 589 04 63 00) by putting the multimeter in series across from the 2-pin connector to power the ICV. At idle the current draw was exactly 1,000mA. The workshop manual states anything from 700mA to 1,000mA is within spec. As 1,000mA is the maximum current the ICU can push to the ICV to reduce the volume of air flow, we must conclude the electronics of the vehicle are working correctly and the ICU therefore not mechanically functioning correctly. At 1,000mA the ICU aperture should be at its smallest aperture and conversely at 700mA at a wider aperture. The workshop manual also states at altitudes higher than 2,000m it is possible to see current draw of lower than 700mA to allow more air in due to the lower air density.

Although while in for service the ICV and ICU were swapped out with as far as is known working parts...we must bare in mind most of these parts are now 30 years old. The high idle persisted.

Based on the above I followed the advice of others and pulled the brass insert inside the ICV out to slacken off the spring pressure just enough that the plastic plunger just touches its stop. I've pulled the brass insert out probably all of 1-2mm, it still sits recessed to the surface of the ICV outlet by around 1mm. Pull the insert too far out and the car will stall easily when coming off the gas. While this may not be a perfect fix, it deals with what is perhaps a weakened solenoid due to time and wear not being able to overcome the return spring force.

ICV brass screw insert technique: http://www.peachparts.com/shopforum...r-control-slide-valve-confidence-check-2.html

More information about the ICV on a German site: http://w126-archiv.mercedesforen.de...odelle_reparieren_(LLS)_-_M116/117&oldid=2311

Now for the first time since owning the car, the car idles at around 600-750rpm and 500-600rpm in gear (best guess from looking at the instrument cluster). With the engine stone cold at around 10 Celsius, the idle initially goes to a 1,100rpm high idle and settles to 600-750rpm after around 20 seconds.

Hopefully I've done this in a reasonably correct manner, if there's anything else worth checking please let me know. Thanks

 

[star]

Didn’t bother reading post, too long. Do you still have a high idle? If so What’s your idle now in neutral?

 

[DREAMER NO2]

In my mind this hides some other problem . Just a quick fix,, or is there a reason for the reset of this ICV ..

 

[ianrandom]

The icv just gets tired. They're very expensive new, so the reset of tension works well as long as you don't do it too much.