92 300E idle surge - help!!! - PeachParts Mercedes-Benz Forum

DAVID LEE · #1 06-27-2009, 03:23 PM

I have a 92 300E 3.0L with a very frustrating idle surge. The surge only occurs when the car is in gear. If left in park, the idle surge NEVER occurs, and I mean NEVER. I have read several threads on this topic, and have read through the CD several times and I'm still baffled.

Things I have done -
1. I have disconnected the EHA connector thinking the surge would go away. Not so. As before, the car idles perfectly in park, but once put in gear the surge appears again.
2. I have run the car with the O2 sensor disconnected with exactly the same results.

When the surge occurs, the rpm will jump up to around 900 rmp for about 1 second, and then return to normal idle. When you're at a stop light and this does this, it is very disconcerting. Also, if I'm driving on the interstate and take my foot off the gas pedel, the surge also occurs.

Please help!!!! But as I said earlier, the surge NEVER occurs when the car is in PARK.

DAVID LEE · #2 06-29-2009, 08:14 AM

I spent quite a few hours this weekend thinking through this problem and doing some measurements with my DVM. The DVM has a frequency meter, but the measurements I've taken at the X11 connector are confusing me. At pins 2 & 3, the frequency measurement has a value of either 81.3, or 0 depending on how I adjust the idle mixture - very strange. I would think it should change fairly linearly as I adjust from lean to rich. This leads me to believe that possible the O2 sensor just isn't responding quickly enough. When I took VDC measurements at the O2 sensor, I am not getting a reading that is oscillating between .1 and .9 volts, but I'm not sure that my meter is capable of responding quickly enough. The "typical" reading on the VDC scale is about .75 volts, and when I disconnect a vacuum hose on the intake manifold and drive the misture lean, the reading drops down to around .15 volts, so I know the O2 sensor is responding, but perhaps it is responding too slowly.

What really has me baffled though is that when I disconnect the EHA connection and take the computer completely out of the loop as far as fuel/air mixture is concerned (please correct me if I'm wrong here), I still am getting the "surge". This morning I connected everything back up and drove to work (about 9 miles). There are several stops along the way, and I noticed that the closer I got to work, the fewer "surges" I encountered at stops.

Oh yes, I also did the "carburator & choke cleaner spray test" and could not find any vacuum leaks.

One thing I read in the support manuals was that there is an input to the closed loop system from a switch that tells the computer that the transmission has been placed in gear. Since my problem only occurs when the car is in gear could it be???? I do not know where this switch is, but more than likely it is located just below the shift lever cover. But once again, if the EHA is connector is disconnected then how can the fuel mixture be modified by the computer? There is some operating theory here I am missing.

My "logic" tells me that when I am putting the car in gear, that I am altering the fuel / air mixture to the point that "something" is trying to compensate. Putting the car in gear would also alter the vacuum as well, so....

Once more, I'm hoping someone out there has some ideas as to what is going on, or at least some tests that I can make.

ctaylor738 · #3 07-01-2009, 10:08 AM

Is this a 124030 car with a 103.983 engine?

DAVID LEE · #4 07-01-2009, 05:42 PM

Yes - 124030 with the standard 103.983 engine.

I've been doing quite a bit of reading and feel like I need to check the Air Flow Sensor Potentiometer. The sympton seems to point to this device because - when I put the car in gear, the idle speed is reduced to around 600rpm (so smooth though, it's hard to tell the car is running) and I'm thinking that the potentiometer is "worn" at the spot that the car normally idles and that the computer thinks it needs to "speed it up" and does. Once it detects it has 'sped up", the idle drops down and we go through the same process again.

I've found a guy that sells an aftermarket Potentiometer, but I'm wondering if the quality is there (your input on aftermarket devices would appreciated as well).

Anyway, that's where I am in the diagnosis process - been at work all day, and I probably won't do anymore troubleshooting until Friday (need to keep my wife happy), but I certainly would "process" any insight you have.

I've seen your name on this site for many years and greatly respect your input, so any diagnostic input from you would be greatly appreciated.

thanks

ctaylor738 · #5 07-01-2009, 08:12 PM

Thanks for the kind words.

I don't think that the mixture or the EHA has much to do with the idle problem. Why are you chasing this area?

As far as I can tell, there is no idle control unit, your car's idle is controlled by one of the control units. An unlike most KE Jet engines, it doesn't seem to have an idle air valve. So you need to find the mechanism by which the idle is controlled. Then, identify the inputs to the control unit that influence idle speed and verify that they are correct. Some likely culprits are engine temp, gearshift position, AC on/off. What signal is used to determine engine RPM?

If its like the 400E I have been messing with lately, there is an idle speed and cruise control module that varies the position of the throttle plate at idle. There is actually an electric motor that moves the plate.

I suggest that you get either the Startek or Alldatadiy subscription for your car - it will help you a lot with troubleshooting.

DAVID LEE · #6 07-01-2009, 10:06 PM

Chuck:

My car does have the idle air valve. Why am I messing with EHA? My thought was to eliminate influences and plain old "learning". I've had this car for 7 years and this is the 1st time I have done anything to the engine except replace plugs and the water temp sensor (which also caused a high idle, but consistently no matter whether the car was in park or in gear).

So...I've never had to troubleshoot anything engine related. I've had quite a bit of experience working through all kinds of issues on my old 77 350SL - a completely mechanical system. This is my 1st troubleshooting experience on the closed loop version of the same system. People learn in different ways and my way seems to be to do some reading and then go see what happens when you do "????". Anyway, the inputs that would seem to vary idle speed via the idle air valve are a/c on, gear selector, idle position microswitch, and the air flow sensor position as indicated by the potentiometer. There may be other influences, but these are the inputs I've discovered so far.

By completely disconnecting the EHA connector I managed to remove one whole set of inputs and subsequent outputs. Now to work my way through what's left of the inputs and outputs. What I've written concerning the O2 sensor and the EHA unit are for the most part curious observations and after thinking about what I've observed relative to how they influece the idle speed seem to be inconsequential to the problem I'm trying to solve.

Thanks for the response and I'll check out the sources you mentioned and come Friday get back under the hood. I'll get there - just a little slower than most.

DAVID LEE · #7 07-03-2009, 05:22 PM

It did turn out to be the air flow sensor. There was a spot worn through the carbon strip where the wiper made contact with the potentiometer. I'll order one and replace next week. I was able to position the pot so that the worn through spot was not in the path of the wiper, so that did get rid of the surge, but as I suspected, it idles a bit too fast.

I may try one of the $50 after market products and see how it works. Has anyone had any experience with them? If so, I'd like to know how it worked.