m103 Low idle
 

Hey guys,
Have an interesting problem with my 1988 300CE m103 idle.
it starts fine and idles around 1000 to 900 rpm in park when cold.
BUT...when fully warm the idle goes down to around 600 in park and 500 in drive
drives ok and idles nicely in drive but if i shift it back to park or neutral the idle stays around 500 as if nothings changed! (lots of shuddering and lumpy idle)
I have changed all the vacume lines including the boot for the air flow meter,
have tried a couple of well cleaned ICV valves (new vacume lines) the cap rotor and wires are around 2 years old. the coil is brand new. Temp sensor is brand new.
have checked the OVP relay and since cold start is fine i assume its ok.
Crank shaft sensor gives me a reading of 947ohms which is within range.
I'm at a complete loss as to what is causing the problem. Any ideas?
Thanks in advance.

I wouldn't say your idle is worrisome. My 103 idles in a range of 550 to about 700, depending on load, weather and other ambient conditions.

thanks for the reply. the thing is it used to idle around 800 but just changed to low idle without any other indication. and its pretty lumpy in park (smooth with a slight shudder occasionally in drive). Changed all the 'usual' things like idle valve temp sensor ect thinking it was some kind of sensor failure. The duty cycle fluctuates around 50% at idle and 45% at 2500 rpm. the engine mounts are about a year old.

When is the last time you changed your O2 Sensor? That and also check the hoses for cracks on each side of the IACV.

O2 was changed about 2 years ago and the two hoses along with the air flow sensor boot was changed last week. from what i gathered an air leak would give these engines high idle not low?

Is there power and ground to the ICV when this occurs? Is there any change in the idling, when this occurs and you disconnect the ICV?

yep have about 5.2v to icv in idle, disconnecting it increases the idle to around 1K. is there a auto transmission shift position sensor or switch on these cars? what tell teh ECU that the car is in PARK?

yep thats true.

got that on mine

I also have an 88 300CE, and it was doing the same thing. Replaced the OVP and it cured the problem. Two years later, it started doing it again. Replaced the OVP again, problem cured again.

really? will try to source on and see if it helps! weird because all other functions work well like cold start and A/C compensation.
Thanks guys. will see how i go with a new OVP.

I kind of have the same problem..mechanic told me that it could be the electrohydraulic
actuator that is giving me a rough idle while stopped in "drive". This does not happen all the time though,mostly when exiting the freeway and coming to a stop. My indie
checked current on the EHA and found it to be low ..I probably need to get a new one or scrounge
one from the yards, luckily this part is shared with many 300 series and 190 2.6 series I believe .. I hope to have a new or used replacement soon, and see how it works .
Lots to learn about these fuel systems ..I will look forward to your next posts to see
what results you get ... Thanks css190e 2.6 180k/daily driver/ "it's always something"

Checking the current on the EHA only tells you it is compensating the mixture, it has nothing to do with how it is functioning in relation to needing a new one. It is simply telling you the system is running too rich/lean and the EHA is adjusting for it. Sounds to me like it is doing it's job :)

Check the throttle switch and decel microswitch.

On mine it was the...
 

Little rubber elbow that goes between the valve cover and the cylinder head located in the middle of the valve cover by the air filter. Mine was old, brittle and cracked and it idled fine when replaced.

checked that line mate, but seems ok. (wouldnt it idle higher if there was a vacume leak?)
replaced the OVP with a known good one but problem remains the same. i'm stumped! any ideas?

That isn't a vacuum line, but I don't know what it really is.

But a theory, since that line goes to the valve cover and it leaking causes low idle, maybe your oil filler cap is bad. I've had that problem on another car. I never would have guessed but the right crankcase pressure is important, I think it's a smog thing.

Thanks mate will check it again doesn't look like it's leaking though but it's trial and error currently! Idles nicely without the ovp plugged in around 900 in park and 600ish in drive. Cold starts suck though! My cis manual mentions a gear lever switch position switch but cant find any reference to it not sure if they mean the neutral safety switch?

And to theorize further
 

It might be the valve cover gasket. They do get old and hard and crack and need semi regular replacement.

Just got the car back from mechanic after installing new lifters mate. new Valve cover gasket was put in with the job.
I do have a tiny exhaust manifold leak could that be causing any problems?
have ordered a new gasket set just to be safe though.

Check the air meter plate for proper deflection and make sure it isn't binding in the bore.

Try inducing a vacuum leak by unplugging a line that isn't switched or valved. See if the idle picks up and smooths out, indicating that you are correcting a rich condition.

Verify your cold start injector isn't leaking. It's somewhat common for them to act up when the cold weather really hits. In warmer climates, they don't get used for most of the year. Then the first cold snap hits and they stick open or partially open after firing for the first time in a year.

Other possibilities:


1) Bad OVP relay. Though this usually shows up along with inst. cluster lights. However, rough, low idle symptom is identical as the IAC no longer functions.
2) Dribbly injector. Old injectors can start leaking when they warm up, or anytime for that matter. This would create a rich condition in one cylinder, the 02 sensor would tell the computer to compensate for this resulting in one rich and 5 lean cylinders.
3) Failing fuel distributor. The diaphragm inside the distro gets stretched to the point where it starts to block the outlets in the (upper I think) chamber.
4) A vacuum leak in some place you haven't yet looked. There is a hose that accepts the cold-start injector, and also connects to air intake ports underneath the exhaust manifold. It's hard to see and a total pain to replace. When it starts to leak, intermittently at first, rough low idle is common.

To follow up on what Dux said:

Put a small bit of vacuum line behind the idle microswitch contact arm (so the computer does not think the motor is at idle) and see if you still have the problem.

thanks guys will check the microswitch...
The hose from the ICV that excepts the cold start injector is new and i do agree total pain to replace, i ended up removing the air flow meter to get to it!
replaced the boot under there while i was at it.
unplugging a vacume line doesnt really do anything with the idle, maybe a little bit rougher but hardly any difference to idle.

unplugging the TPS increases the idle to around 900 and smooths it all out. not sure if thats relevent. checking with a multimeter seems to give me the correct results though.

Our M103 (300SEL) idles around 650 when warm (in drive).

The cold start injector gets used everytime the engine starts when at ambient temperatures, i.e. car not run for several hours.

mine idles around 550 in drive and pretty much the same in neutral (575-600 max) when shifting in to park it revs to around 800 and then slowly comes down to about 550.

Just did a cold start injector test and it works fine :( the injector stops spraying after 5-10 seconds.

just feels like a sensor thats sending the wrong signal to the ECU. unplugging the OVP gives me a nice high ~900-1000 idle in park and about 700 in drive, which is a bit high but good as a mechanical only idle i think? (i presume the ICV valve will be in open position with no electrical input from the ECU hence the higher idle?) cold start sucks with the OVP unplugged though!

Hmmm. That's odd. Mine (on my M103) doesn't fire unless its fairly cold at ambient. My temp sensors are tested and accurate. Strange. Perhaps this is why I often have to give it a little gas on startup.

Sorry if I mislead anyone.

thinking its the crank shaft position sensor now? probably the only sensor i havent replaced along with the EHA. might have to bite bullet and replace it? i'm completely lost for things to check now.

crap not the crank shaft sensor! :( i give up!!!
anyone know if theres a sensor for the CIS from auto box to indicate which gear its in? my problem is the idle speed in drive and park are exactly the same (~550rpm) when shifting from drive to park or neutral the idle doest change up liike it used to....

I would like to assure him that there is no need for him to worry about his idle. It is only natural that his idle stays around 500. In my case the idle normally is in the range of 500 to 700. When I showed it to a mechanic he told me that it is perfectly normal and that I need not worry about it.

vacuum leak
 

I would check for vaccum leaks especially at the breather hoses-I had one a bit loose and the car would not start at all. I read that one must always check for air leaks and in my case that was perhaps the best advice i have received. A vacuum leak does not have to have a raised idle if there is too much air. Consider what happens when the car sucks in too much air and not able to compensate by adding fuel...It stalls.