How does the Idle Control Valve Work? - PeachParts Mercedes-Benz Forum

pentoman · #1 02-25-2009, 08:13 AM

Quote:

Originally Posted by ps2cho

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...

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!

ps2cho · #2 02-25-2009, 01:51 PM

Quote:

Originally Posted by pawoSD

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.?

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

Quote:

Originally Posted by babymog

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

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.

Quote:

Originally Posted by slk230red

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

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?

Quote:

Originally Posted by pentoman

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!

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.

Quote:

Originally Posted by pawoSD

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....

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...

lkchris · #3 02-25-2009, 05:40 PM

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.

ps2cho · #4 02-25-2009, 06:33 PM

Quote:

Originally Posted by lkchris

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.

pawoSD · #5 02-25-2009, 07:01 PM

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

Chowbow · #6 02-27-2009, 02:42 PM

Quote:

Originally Posted by ps2cho

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?

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.

pawoSD · #7 02-27-2009, 03:02 PM

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?

ps2cho · #8 02-27-2009, 07:38 PM

Quote:

Originally Posted by pawoSD

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:

Quote:

With a digital multimeter hooked up to read DC volts, start the car and let it warm up to a steady warm idle. With the engine at idle adjust the potentiometer body by pivoting it slightly until 0.70v (plus or minus 0.10v) is obtained

Just make sure its set at the correct voltage?

Quote:

Originally Posted by duxthe1

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.

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.

---

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.

wbain5280 · #9 02-28-2009, 12:09 AM

Quote:

Originally Posted by Chowbow

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

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.

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