1990 500SL, Warms Up and then Goes into "Limp Home Mode"

[JamesO]

I have a 1990 500SL with 60K miles. I recently replaced the EHA when it was doing a similar thing. Heres the deal.. The car runs fine until it reaches 100degrees, or warms up...it then will stall at low speeds and when you restart and try to accelerate, it will only allow 2500RPM max,(limp home mode) however I can manipulate it to get the MHP up, (like 55mph)...when I take the foot off accelerator the RPM's immediately fall like its going to stall, seems like something electronic. Pushing the accelarator to the floor still only gets you 2500 RPM. The curious thing is It only does it after it warms up. Is this a Throttle Actuator, KE unit, Clogged Cat Converter, Airslide, potentiometer track gone bad. ASR problem? I am open to suggetions as we are stumped and my Mechanic can't pull codes with the model/year.

Thanks,
James

[JamesO]

Oh yeah sorry I forgot it is a 129 car...

[ScottinSoCal]

Originally Posted by JamesO

Oh yeah sorry I forgot it is a 129 car...

This is a guess, since I don't own a 129.

The O2 sensor doesn't work until it's warmed up, so chances are the engine computer is programmed to ignore input from the sensor until it reaches operating temperature. If the sensor is bad, it'll send the car into limp home mode.

[Roncallo]

Originally Posted by ScottinSoCal

This is a guess, since I don't own a 129.

The O2 sensor doesn't work until it's warmed up, so chances are the engine computer is programmed to ignore input from the sensor until it reaches operating temperature. If the sensor is bad, it'll send the car into limp home mode.

To elaborate on that just a little more. The car runs in open loop until warm. Most of the sensors including O2 are essentially ignored until the car is warmed up. O2 is a likely candidate but an OBDII scanner will tell you what the real problem is. You have two maybe four O2 sensors on that car. Do you feel lucky. I recommend you check the codes.

John Roncallo

[bobterry99]

The codes on this car can not be read with an OBDII scanner. But they can be read with an LED from Radio Shack. The part number is 276-209.

Limp Home Mode does not limit engine RPMs. It limits throttle position. I think the first thing to determine is whether or not the limited performance is due to Limp Home Mode.

[JamesO]

I got the following codes on the CIS-E
code 7 ?
code 18-current to idle air control valve
code 29-difference in ECT between CIS-E control Module and EZL ignitioncontrol Module

MAS codes
13-Excessive AC belt
14 Vehicle speed signal/circuit
15- short cirvuit detected in fuel pump circuit

DI codes-with car not running
17-crankshaft poistion open or short cirvuit

Can anyone help with solutions to these codes...this grey market 1990 500sl with early ASR is killing me.

Thanks,
James

[bobterry99]

Code 17 on the DI is normal with engine off...Did you erase codes and re-read to see which ones are fresh? Most codes are retained in diagnostic memory even when the fault has been corrected. So the presence of a code does not necessarily indicate the underlying condition exists.