How does the Idle Control Valve Work?
 

Can someone explain to me how this works exactly for the M103 motor?

Fighting an idle misfire.

Is there a way to test it? I have unplugged it via the electrical cable and the misfire continued. Any other way to make sure it is functioning correctly?

Didn't you have the misfire before you replaced your head gasket? Is the misfire only at idle?
How much current is your EHA and ICV drawing at idle?

I would think that a misfire is either electrical (spark?) or a fuel issue. The idle control valve just varies the amount of air getting to the engine at idle to keep it at an optimal speed....

When my 300E was misfiring it was from fouled (non resistor) spark plugs. I put in a nice set of NGK resistor plugs and it has run awesome ever since.

Where would I attach a DMM to find the currents?

The idle control valve works by letting air leak into the intake manifold in a controled manner or modulating it. The Idle Control processor controls this along with the ECU. You can easily tell if the ICV is working properly by unplugging it. If the idle speed increases, it's working. It might be a good idea to clean it.

The EHA on the fuel distributor modulates the fuel pressure between the upper and lower chamber if the FD, under control of the ECU.

The O2 sensor tells the ECU just how much to modulate the EHA.

The fuel pump relay has a processor inside to turn on the pump and activate the cold start valve.

Plug your meter (in mA mode) into one side of the connector, it has to be in series. I designed a male/female plug for this purpose. Usually a problem in the ICV circuit is high idle.

But, on to your misfire problem. As pawoSD stated, your problem is more likely to be ignition related. Can you describe your misfire condition....is it a miss at idle that clears up when you accelerate?

Of course the obvious things are: rotor, distributor cap, spark plugs/wires, ignition control module, etc.

New OVP
New Coolant Temperature Sensor
New Voltage Regulator
New Alternator
Injector's+Seals (previous owner + smoke tested)
New Fuses
Non-Resistor OEM Bosch H9DCO's
New Bosch Ignition Wires (checked for arching as well)
New Distributor Rotor + Cap + Housing
New Fuel Filter
New Breather Hoses
Used Fuel Distributor
New Ignition Coil
Used Fuel Pressure Regulator
New O2 sensor
Used EHA Valve
New Motor Mounts
-- Head work done yesterday:
New Head gasket, intake and exhaust gasket, front timing cover gasket.
Head skimmed and pressure tested
New Water pump.
New Valve Guides + Seals
-- Smoke tested for leaks
-- Compression is 185psi across the cylinders
Swapped Jetronic Brain /w used one
Swapped Fuel Pump Relay /w used one
Swapped Cold Start Valve /w used one
Swapped EZL /w used one

^As you can see I'm not sure what is really left. I'm going to test fuel pressure next I guess.

In bold above...but yes exactly what you said. It clears up on acceleration. I was in my dads old Buick Wagon the other day and I felt a misfire on acceleration for the first time -- Mine does not have this! Idle only.

The ICV of an M102 (almost certinaly identical) has a 'zero' position (i.e. if you disconnect it) of slightly open. This means that if the wiring system fails then the idle control valve will be left slightly open ensuring your car idles ok if a little high. This information may help you diagnose.
I think that a little current to the ICV closes it up, but then applying more and more pushes it past that point and on to opening it right up.

You can squeeze the pipes going into it - the idle will be caused to dip but the engine should respond by opening the ICV and the idle will raise up again.

I had an idle-only misfire once on a 1986, it was due to the pipe going from cam cover to air cleaner coming loose.

Okay so it realistically shouldn't cause a misfire, only an increase in RPM's if it was faulty, correct?
I have no vacuum leaks as I had the car smoke tested about 8 month ago, and last night sprayed a ton of carb cleaner everywhere and it had no effect on idle speed. I got a little gremlin somewhere...

Have you ever tried swapping out the ignition control unit on the fender? Or the computers back by the battery? Perhaps one of those devices has a glitch thats causing an electrical irregularity....and thus a misfire.?

Have you pulled your spark-plugs and compared them to look for one that is different than the others?

You have certainly done a lot to your car. Your head gasket job was done very well with attention to detail.

Have you checked for any DTC codes?

The next thing I would do is:

1. Test the fuel pressure
2. Measure the EHA Current
3. Test the Throttle Position Sensor
4. Test the Air Flow Potentiometer

He said it was still misfiring after his head gasket replacement, I'm assuming he had brand new plugs in it at that point....

I vote for an electrical irregularity. If I was you I'd have probably had a MB tech diagnose it at the dealer at this point though....

I believe you should not get a misfire, correct. You could check by pullnig the hoses off and seeing what happens.

FWIW, I have found these CIS cars will run with the ECU removed without misfiring. Throttle response will be a bit weak and cold running will suck, but it shouldn't have a single cylinder misfire or anything. May help you rule out certain problems that info?
Good luck; bear in mind it could be something you thought you've checked!

Yes the EZL is the ignition control unit on the fender.

I haven't since the head removal...but the misfire has been with me since I got the car...I will pull the plugs tomorrow to see if there is any indication if I am running rich or lean.

That is for sure ;)
How would I test the air flow potentiometer? In the M103, I know there is an electrical connector that goes to the pot, but what does that control? I thought there was no traditional potentiometer on this engine...
Also does the M103 have a TPS?

By the ECU, you mean Jetronic brain? With that removed, what components of the ignition system are turned off exactly?

That is always my fear...but if that part was replaced and it made zero difference I couldn't have got a part that was an exact replica of the misfire I experience...any change at all -- I would notice.

That is the plan if I don't find anything wrong with the aforementioned. Would I get charged astronomical prices for a diagnosis only? I have only been to the dealer a few times for parts and they were crazy prices...That's why I am afraid of asking them to look at the car...

A little misconception regarding function of idle control valve.

Whenever a cold engine is started it requires a rich mixture and a little more than normal throttle opening to keep running.

On carb cars, there was the choke blade and the fast idle cam.

On FI cars, there's the extra injector or a computer program to create rich running and there's the idle control valve to bypass a little more air, i.e. creating same effect as a fast idle cam but not actually opening the normal throttle blade. These are both controlled/monitored by an engine temperature sensor input to computer.

Not much action from idle control valve once engine's warmed up, except that it's supposed to be closed. It indeed "controls idle" during engine warmup, but after that it doesn't do anything.

In the mornings when my engine is cold...for about 5-10 seconds I do not have a misfire then it slowly gets worse for the next 10 seconds before it feels the same as when it is warm. Could there be a possibility that the valve is not closing when it should be? Would that be considered a vacuum leak? (I have had the car smoke tested and I have no visible vacuum leaks at all.

If the valve was not closing then you'd just have a high (but not misfiring) idle....around 1500ish....

Okay that's ruled that out then. Thanks!

Did you ever check for trouble codes?

No trouble codes....only trouble code I have ever had in the last 4 years and 30k miles has been an altitude correction capsule which I replaced with one from the junkyard...unfortunately not my misfire, but codes are clean now.

I heard that the EZL has trouble codes stored? Is that true?

Later EZLs have a DTC readout but not in 88. You are aware that in 89 Mb changed the camshaft profile for a smother idling engine. There very well may be nothing wrong with your car as the early cams are known to idle a little rougher than the new profile cams.

Personaly I think the early cam has the potential to make better power than the later cam. If I remember correctly the early cam has a smidge more duration that they took away from the later cams. I'm going to do the head in my 90TE soon and I'm going to pull the cam and rockers out of my 92TE parts car b/c it has an 89engine in it. I've done a lot of work to my 90 but it still just doesn't pull down low like the one with the 89 engine in it did.... not even close, really. I've chased and fixed a lot of demons in the 90TE but I think that the weak low end is the later style cam. I've got several mods happening when I do the head on my 90 so a stronger engine won't prove my point but if it is still weak down low then it will at least disprove it and I can keep looking for my low end power.

You can check the potentiometer through the instructions detailed on this site: http://sg-motorsports.com/AFS%20Pot%20write-up.htm

Mine was bad, and I had to replace the whole air flow sensor though because I couldn't find the potentiometer separately. It solved my idling issues but my misfiring issues were resolved through other means. I had to do new plugs / wires / rotor / cap, EZL ignition module (pulling the vacuum line on it, you should be able to hear the air flow), and the last piece was my #5 injector was a little loose and that cylinder was choking up. Found that by pulling all plugs and then realized that the injector was loose.

Don't think the M103 has a TPS, but there is a microswitch for idle. You can see it with the air cleaner off, towards the firewall. $25 cheap replacement. This is more for controlling idle speed rather than any misfiring issues though I think...

Good luck.

Is it a "Real misfire" or a rough idle? There is a difference......a real misfire would rob the engine of power and you'd have a fouled plug(s) and get terrible MPG's.....

Does it misfire all the way through the RPM range?

Rough idle then. Does not misfire through RPM range. I get terrible MPG (15mpg average 50% F/W 50% City), Plugs look fine. I am robbed of about ~40HP according to iWrock when I let him drive my car.

Great link on the potentiometer. Thanks. It is on for replacement though...so I am only checking:
Just make sure its set at the correct voltage?

It's an actual rough idle...not normal. I can hear the engine surging rhythmically along with very poor power and MPG as above. It's not normal when the whole car shakes. My visor shakes all over the place some times.


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Also my mixture is tough to get right. I can never get the idle and 2000rpm to be within 10% of each other. I currently have the idle set to ~45% but after I rev to 2000rpm it sits at ~60%. Any thoughts on that? From what I understand, it should be within 10% of each other.

the idle control valve will not cause a misfire.....

[QUOTE=ps2cho;2120255
Fighting an idle misfire.
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You stated that you checked the spark plug wires for arcing. Did you check them in pitch dark, if possible?

Another misfire thread:
http://www.peachparts.com/shopforum/showthread.php?t=162549&highlight=fuel+accumulator

The M103 does have the air flap position sensor. The throttle has fully closed and fully open switches.

Yes I checked it at night with a spray bottle. I wasn't expecting to see any arching with brand new wires.

I will check the fuel accumulator tomorrow as well.

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Any thoughts on the mixture problem? It clearly states in the MB CD's that idle to 2500RPM must not differ by more than 10%....Mine differs by 15%. What could be causing this?

I have a question: Why does the car stall after I disconnect then reconnect the ICV? (electrically that is)

Good to know. Are these in the air flow sensor itself?

If you do it while the car is running the computer shuts down the Idle Control System when you unplug it, and doesn't re-initiate it until the car is shut off/started again.