86 300E high idle, done search, need some tech answers - PeachParts Mercedes-Benz Forum

JFAZ · #1 11-25-2005, 08:11 PM

Hey everyone,

I have spent the past three days reading all 90+ posts re. "300E idle" and have still not been able to diagnose the problem...

My 86 300E w/ 135K miles idles at 1450 rpm in park. When in D or R it drops slightly to about 1000 rpm. Also when a/c is switched on it drops to about 900rpm. I have lubricated and checked the throttle linkage, it is fine. I have removed the idle control valve and cleaned thoroughly. I have also disconnected the wires going to the ICV and throttle valve switch- no change in idle. The TVS is getting 12v from the wires and I do hear the "click" when the throttle is adjusted down. When i pull back the plug on the ICV and measure (with the plug still partially connected) it reads about 4v. It seems like no matter what I do the idle will not drop below 1450 in P or N. FYI, the oxygen sensor and OVP have both been replaced in the past two weeks.

Through reading all the threads re. high idle on the 124 it seems like there are lots of little things that vary on the different year engines in these cars that could control idle... At this point I am trying to decide if I should just buy a new ICV and try that or if the symptoms thus far should point me in a different direction..? Am I missing something here on ways to test the components that control idle?

Thank you very much!

joel · #2 11-25-2005, 08:55 PM

coolant/water temp. sensor, ICV, and OVR fixed my problem.

JFAZ · #3 11-25-2005, 10:00 PM

joel,

Where is the coolant/ water temp. sensor located? OVR = OVP relay? Were you having the same problems?

Thanks!

joel · #4 11-26-2005, 10:42 AM

in my car, water sensor is located atop the engine head closest to the firewall. it has two prongs with wires attached to it.

OVR, overvoltage relay. updated ones have two fuses on top.

JFAZ · #5 11-26-2005, 04:13 PM

joel,

Do you know of any way to test the water temp. sensor? What were your problems leading to the replacement of it?

C'mon, I know there are some resident idle experts out there!

duxthe1 · #6 11-26-2005, 05:33 PM

A bad coolant sensor will give cold engine idle speed, yours is higher than that. Not likely the problem. If you can disconnect the IAV without a change in idle speed it means that the IAV is in the default wide open position and is either bad, or not receiving the proper electronic control. Easiest thing to do is swap in a known good IAV and see if it responnds differently. You have to make sure the throttle valve switch closes at idle, or it may always want to stay in decel mode, causing a high idle (though usually not as high as yours). The OVP and related circuits can cause problems. The ovp supplies current to the valve and the ecm provides a ground that varies in duty cycle to adjust the idle up or down. Gotta make sure all that works. A bad vacuum leak that has been adjusted for will also cause a high idle as well as an IAV connected backwards. Don't laugh, I've seen it

anthonyb · #7 11-26-2005, 05:40 PM

Does the throttle valve switch give continuity at both idle and WOT positions? Also, does the microswitch on the accelerator linkage work properly? Both signals are necessary for the engine to determine that the car is at idle.

The coolant temperature is measured by resistence, and the manual gives two values:

20*C: 2.28-2.72 kOhm
80*C: 290-364 Ohm

In my case, a high idle cold was caused by a malfunctioning microswitch.

JFAZ · #8 11-26-2005, 08:05 PM

anthonyb:
OK, just checked- the TVS gives continuity at idle but NOT at WOT. What does this mean? How do you verify that the microswitch is working properly? FYI, my car runs at this high regardless if the engine is warm or cold...
Thanks!

anthonyb · #9 11-26-2005, 08:24 PM

On the TVS, did you check continuity using the WOT contact? Either the L or R contact (can't remember which) is idle, the other WOT. THe middle is ground. Continuity should occur only when the switch is at the respective position (idle for idle, WOT for WOT). If that doesn't work, you can try spraying some air intake cleaner on the butterfly valve, and exercising the linkage to open and close the valve a few times. Still, because you're troubleshooting an idle problem, lack of WOT enrichment probably isn't that big a deal for you now.

For the microswitch, find the switch on the accelerator linkage. Check to make sure that the linkage presses against the little swing arm when the accelerator is at idle, and that the linkage positively actuates the switch with a click. Mine didn't - the arm would ride on top of the linkage. If the arm works, then check for continuity when the switch is activated. All it does is send an on/off signal to indicate when the engine should be at idle.

JFAZ · #10 11-26-2005, 08:52 PM

anthonyb:
My TVS only appears to have two electrical posts the third middle one just seems to be a plastic guide post... Also, the switch on the TVS is working fine, it positively actuates with a "click" sound. There is continuity when the switch is activated at idle position.

duxthe1:
So when I disconnect the ICV and nothing happens, how do i trace it to make sure it is recieving the proper electronic control?

Thanks!

sbourg · #11 11-26-2005, 10:48 PM

Quote:

Originally Posted by JFAZ

I have removed the idle control valve and cleaned thoroughly. I have also disconnected the wires going to the ICV- no change in idle....When i pull back the plug on the ICV and measure (with the plug still partially connected) it reads about 4v.

Ok, this should tell you that the engine is able to get air to idle high without using the idle air valve. If the ECU is supplying voltage to the valve, it is unaware of the air flow you have. I.e., the airflow meter is NOT metering the air now flowing to your intake ports. Only one way this is possible - an air leak bypassing the throttle. If you have verified the throttle is fully closed, then you have an air/vacuum leak. Nothing else will do it.

Steve

anthonyb · #12 11-27-2005, 03:13 AM

Quote:

Originally Posted by JFAZ

anthonyb:
My TVS only appears to have two electrical posts the third middle one just seems to be a plastic guide post... Also, the switch on the TVS is working fine, it positively actuates with a "click" sound. There is continuity when the switch is activated at idle position.

I think sbourg is on track with the source of your problem. But for future reference, the switch you're looking at is the microswitch. The TVS is instead located on the opposite end of the throttle valve from the accelerator linkage. It has a black wire pigtail that runs to a three-pole connected just outboard of the idle control valve.

JFAZ · #13 11-27-2005, 04:13 PM

anthonyb, thanks for the info.

sbourg, IAV = ICV right? So you are suggesting that everything is ok with this part right? Where do you think is the best place to look for an air/vacuum leak considering that the throttle is fully closed? Sorry, but I am kind of new to all this idle troubleshooting- I appreciate all your help.

Victor300E · #14 11-27-2005, 04:29 PM

when i changed air meter potentiometer i put it in without adjustment first time it caused high idle around 1500-2000 rpms. i read on this forum how to adjust the potentiometer idle went down to 650rpm. search on forum about air meter potentiometer and how to test it. maybe yours is faulty. hope this helps

sbourg · #15 11-27-2005, 07:07 PM

Quote:

Originally Posted by JFAZ

anthonyb, thanks for the info.

sbourg, IAV = ICV right? So you are suggesting that everything is ok with this part right? Where do you think is the best place to look for an air/vacuum leak considering that the throttle is fully closed?

Yes, and no. The idle air valve is an air 'leak' around the throttle valve. Air that passes through it therefore comes through the airflow meter, then back into the intake manifold. Any rupture in the plumbing that routes the air in this path will allow non-metered (and unfiltered) air to enter the manifold. Further, if the leak were between the idle air valve and the airflow meter plenum, the valve should still be able to control it - and this appears to not be the case. Most likely, then, is an air leak in the plumbing leading to the intake manifold.

My experience with this vintage is that all the rubber air plumbing will have hardened, possibly cracked, but in any event seal poorly at all connections. It will also certainly disintegrate if you attempt to remove it. Best procedure is to order all new plumbing and replace it all at once. This includes the branch that goes from the manifold to the breather port on the valve cover - the part that is between the pcv orifice and manifold also causes problems.

Note that none of this absolves the idle air valve, but you can't judge it until the air leak is fixed, which will likely solve the problem.

Steve