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  #1  
Old 06-14-2015, 03:15 PM
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89 300E, W124, M103 High idle... SOLVED

Ahh yes.. the infamous nasty high idle, in my case 1500rpm !

What amazed me was the number of threads here with no resolution.
Soo have to love the responses ....''i dont know anything bout your problem but ....''

MB was running horribly due to a split in the rubber boot under the fuel distributor.

While the old one was off replaced both the hoses that go to the Idle Air valve. Even with the FD off, the hose going to the block is no fun. It was brick hard so replacing was mandatory. Finally popped it with an 18" pry bar. Great time to renew the two vacuum connections that are under the FD too.

Started right up after replacement and then the fun began. Enlisted the aid of Frank with his extensive knowledge to help me solve and fix the high idle.

List of checks.
OVP relay. checked good.
Fuel pump relay. checked good.
Idle air valve. checked good but swapped. No change. Idle would drop to about 1k when the connector was removed.
Throttle position switch checked, rechecked. Both checks were good.
EHA current checked good. EHA swapped. If you dont have a vise to help get the adjustment screw cover off, the top of the radiator frame works great ! Adjusted 1/4 CW. No improvement.
All vacuum points checks with throttle body cleaner.
Fuel accumulator swapped, no change.
O2 sensor checked for ohm value and voltage. Both good.

All this to not be able to find any thing that improved the idle speed.

Coolant Temperature Sensor at the rear right of the M103 proved to be the culprit. Its a temperatue sensitive resistor. With a cold engine several sites indicated it should read 38 ohms to ground and across both leads., mine was reading just under 4000 ohms. Finally a BAD reading.

Added a 1K resistor to ground from the green/red wire which brought the max idle down to 1k but the idle was never stable... 1k to ~500 back and forth.

The proof was by adding a 39 ohm resistor to the green/red wire to ground. Cold idle was now ~850 and stable.

Final proof will be popping a new one is which I understand is easier with the valve cover off.

My Coolant Temperature Sensor is the two pin variety. I have no idea which lead would be the right one for the 4 pin.

While I was at this EVERY electrical connection that was opened was cleaned and dielectric grease added on reconnection.
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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."
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  #2  
Old 06-14-2015, 03:36 PM
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Trevor Hadlington
 
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Location: Worcestershire in England
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IAV valve when conector is removed it should bring the reves up to 1500 1900 if all is well with it. If not its shot .
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  #3  
Old 06-14-2015, 03:48 PM
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That is not true on the M103
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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."
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  #4  
Old 06-15-2015, 12:12 AM
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There are a lot of unsolved high idle threads because the cause always seems to be different in every case!

I had various idle problems on my 300E, I am pretty sure it was due the very degraded wire harness on top of the engine, but I traded it before digging into it any further (rest of the car was not that great either).

My 420SEL had a high idle and it was due to a ton of vacuum leaks. Replaced ALL rubber parts on the entire engine(huge job), and problem solved. Idles 800 in park and 600 in drive/reverse.
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  #5  
Old 06-15-2015, 05:47 AM
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Trevor Hadlington
 
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Quote:
Originally Posted by TnBob View Post
That is not true on the M103
ok you give me a reason i have done this just weeks back now ,give us your answer
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  #6  
Old 06-15-2015, 06:03 AM
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Quote:
Originally Posted by optimusprime View Post
IAV valve when conector is removed it should bring the reves up to 1500 1900 if all is well with it. If not its shot .
Quote:
Originally Posted by TnBob View Post
That is not true on the M103
Quote:
Originally Posted by optimusprime View Post
ok you give me a reason i have done this just weeks back now ,give us your answer
I too agree with you optimusprime - well the high idles does happen on my M102 when I unplug my air idle control valve...

...but there's no need to start with the kung fu - Bob's a good chap. I can imagine he's a bit frustrated with all of the conflicting forum evidence that has been presented. I've got also got uneven idle trouble with my M102 - these CIS systems seem to bring out the worst in people - I don't know what the heck is happening with it just yet either. I saw a comment on another forum saying that CIS was probably responsible for three suicides per year!

I can add that my M102 has the four pin temp sender - but I'm not sure if that makes a difference but it is interesting to read that Bob's got a smooth idle by playing about with the resistance...
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #7  
Old 06-15-2015, 11:10 AM
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I am the Frank to whom TnBob referred. In the process of trying to sort out the cause of the high idle, a process which involved 50+ emails, one important point has emerged: There is not one common configuration of idle air control used over the production run of the M103 engine. Consequently, all the anecdotal evidence is very likely correct, and some of that evidence will conflict with other evidence. Some of the thermistors used for coolant temperature sensing have values of many thousands of ohms, and some have values of less than 50 ohms. Some of the air valves have a no-power, rest position that is wide open, and others close when power is removed.
So, when requesting aid in solving a high idle symptom, or issuing a challenge to deliver an answer, it behooves us to also provide a full specification of the system in question; there simply is not one correct answer. TnBob did a fine job in picking his way thru a system that was not configured as either he or I expected.
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  #8  
Old 06-15-2015, 08:57 PM
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Facts without proof are pointless.

Pic 1 shows the IAV with 12+ VDC applied. It is open mostly.

Pic 2 shows the IAV with 0VDC applied. It is closed mostly.

When the MB is started, 12+ VDC is applied to the IAV
by a signal to the appropriate circuit by the ECU.
Attached Thumbnails
89 300E, W124, M103 High idle... SOLVED-cimg0627-resize-v-.jpg   89 300E, W124, M103 High idle... SOLVED-cimg0628-resize-v.jpg  
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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."
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  #9  
Old 06-16-2015, 04:10 AM
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When I was fishing about for information I came across this



From here =>

Idle Air Control Valve - Airflow versus Control Voltage - MBWorld.org Forums

The guy who did the measurements reckons

Quote:

...that with no control voltage (e.g., the IAC is electrically disconnected), the engine will receive some idle air - enough in some cases to cause a high idle
This is what I have experienced (and apparently others too) - but I don't quite go along with the reasoning just yet. May be I don't understand the system - but with a choke on a carb you get a high idle by reducing or restricting the air flow don't you? So I have been assuming that the air idle control valve is open during normal running / when the engine is not at idle but the hole closes and restricts the extra air to help control the idle...

...I think I need to go back to the FSM and see if there's any more information to be gleaned...
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #10  
Old 06-16-2015, 08:16 AM
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Quote:
Originally Posted by Stretch View Post
...I think I need to go back to the FSM and see if there's any more information to be gleaned...
OK - that was a merry dance.

No information about this on www.startekinfo.com that I can find.

So I went and had a look in the German language version of the W123 FSM I've got to see if there was a functional description chapter for CIS in either the M110 or the M102 engine sections.

I found them both - but my German is not up to understanding it all. I did find a chapter number though =>

Chapter 07.3-001 that's the one explaining in nice terms how CIS works!

Back to startek - couldn't find that chapter in either the W201, W124 R/C107 or W126 FSMs

I eventually found it "elsewhere" in a W126 FSM

Here's a snippet describing the air idle control valve - from this information it does indeed seem to be working much like I expect a choke to work





It strikes me to be a fancy electromagnetic choke - there's a basic AC signal (what would you call it DC modulated?) that fluctuates at about 200Hz to energize the magnet - the basic DC amplitude pulls the opening to different positions.

This is in some way related to the temperature sender - I need to read some more but have to bugger off elsewhere in a minute so I haven't got the time to get to the bottom of it just yet!

I think the problem I have is possibly related to a leaky diode(s) on my alternator which might be arsing up the excitation voltage for the air idle control valve...

...perhaps that's also happening for you Bob?
Attached Thumbnails
89 300E, W124, M103 High idle... SOLVED-snippet-w126-fsm-chapter-07.3_001-about-air-idle-control-valve1.jpg   89 300E, W124, M103 High idle... SOLVED-snippet-w126-fsm-chapter-07.3_001-about-air-idle-control-valve2.jpg  
__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #11  
Old 06-16-2015, 12:27 PM
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" May be I don't understand the system - but with a choke on a carb you get a high idle by reducing or restricting the air flow don't you? "

The answer on that is, no. On a carbed engine the idle speed is increased using an eccentric cam on the linkage. This is both to warm up the engine quicker and to draw more air to compensate for the enriched mixture. Kind of a "robbing Peter to pay Paul" method. You can't use a controlled vacuum leak for engine speed because then you would have a low vacuum condition which causes problems due to the disruption of the venturi effect which is what carbed systems use to introduce fuel into the air stream.

On a fuel injected system you can and do use controlled vacuum leaks because the fuel is injected under pressure as an aerosol and does not require the vacuum to draw the mixture into the intake runners.
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  #12  
Old 06-16-2015, 12:47 PM
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Quote:
Originally Posted by Mike D View Post
" May be I don't understand the system - but with a choke on a carb you get a high idle by reducing or restricting the air flow don't you? "

The answer on that is, no. On a carbed engine the idle speed is increased using an eccentric cam on the linkage. This is both to warm up the engine quicker and to draw more air to compensate for the enriched mixture. Kind of a "robbing Peter to pay Paul" method. You can't use a controlled vacuum leak for engine speed because then you would have a low vacuum condition which causes problems due to the disruption of the venturi effect which is what carbed systems use to introduce fuel into the air stream.

On a fuel injected system you can and do use controlled vacuum leaks because the fuel is injected under pressure as an aerosol and does not require the vacuum to draw the mixture into the intake runners.
You've lost me Mike. I'm thinking along the lines of an upstream butterfly valve type choke as described by this

https://en.wikipedia.org/wiki/Choke_valve#Automotive
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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #13  
Old 06-16-2015, 01:12 PM
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The choke valve doesn't increase the RPM's. It only enriches the fuel mixture.

I was referring to the increased idle speed you mentioned.
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  #14  
Old 06-16-2015, 01:47 PM
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To Stretch goes a tip of the hat, and a bottle of cheap bubbly, for the information he has supplied in posts #9&10.

In #9 the photos are of a rotary idle air valve. The armature and the valve pieces rotate to open and close the window in the air passages.

In #10 the drawings are of an axial idle air valve. The armature and the valve piston slide lengthwise on their axis to open and close the air aperture.

The rotary valve (as used on the OP's M103) has a no-power rest position that is slightly open, but apparently not defined, leading to observations of differing no-power idle speeds.
The axial valve does have a spring loaded, defined, no-power rest position that is wide open.
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  #15  
Old 06-16-2015, 03:17 PM
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Great pics Frank !
Nice input Stretch !!

Regardless of other systems and their different Bosch parts I am outlining my
findings on my motor.

On my particular M103 6 cyl in line, with the IAC connected, the idle is ~1500rpm.
With the IAC disconnected on two differenct IAC's, the idle drops to ~1000rpm.

With the Coolant Temperature Sensor connected, the idle is ~1500rpm.
With a cold start motor and a 39 ohm resistor to ground from the green/red
lead of my two point connection, the idle is right at ~800rpm.

My cold resistance of the Coolant Temperature Sensor is about 4,000ohms.

I have no idea what the M103 4 lead Coolant Temperature Sensor ohm reading should be.

FWIW, my cold start injector is quite different than the one I found in a 190E M103 motor. The IAC is the same.
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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."
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