[W126 Coupe] Stalling issue

[rltimmerma at cox.net]

More info on my stalling issue:

I had time to work on my  '90 560 SEC today.  After studying the tech. manual I decided to rule out fuel delivery by testing the fuel pump flow.  Results were 1 1/2 liters in 43 sec. (spec is 1 liter in 40 sec.)  that plus fuel delivery pressure of > 100 psi. (spec is 94 psi) leads me to believe that the fuel pumps and filter are doing their job just fine.

 Unfortunately I don't have a meter with duty cycle or on/off ratio measurement capability to check the diagnostic output on the X11 connector.  I did find a chart that lists current flow through the EHA under various operating conditions.
ignition on = 74.4mA (spec is 75)
engine running with cold coolant <80 C = 0 mA (spec is 0)
engine running at normal temp >80 C  I was running at +12.5 mA (spec is 0 +\-3)
Knowing that the EHA is used in conjunction with the O2 sensor to maintain A/F ratio I decided to try adjusting the Lambda setting.  Sure enough a minor adjustment brought the EHA current back to 0 +\- 1.5 mA.

increasing the throttle slowly causes the EHA current to increase then slowly settles back to small +\- trim responses.  closing the throttle back  to idle causes EHA current to go negative then again settles to +\- trim.  I think the EHA is doing what it is told by the CIS controller.

What doesn't happen is acceleration enrichment when blipping the throttle.  The chart says current should go to >+20 mA.  mine just sits there while the engine bogs.

Unfortunately there is no troubleshooting hint as to what may be causing the lack of EHA response to an acceleration demand.

Does anyone have any suggestions?  Otherwise I'm going to have to individually  test each one of the devices connected to the control unit.

As always thanks in advance for any replys.

Ray Timmerman


---- rltimmerma at cox.net wrote: 

=============
More background on the stalling issue.

Car has 136,000 miles.  I've owned it for 6 years and put an average of 3000 miles per year.  Mostly Sunday after noon drives with an occasional run up Palomar mountain and back.

The problem first showed up in June after a long period of the car sitting idle.  My first thought was condensation of moisture in the gas tank. (sloshing of water in the bottom of the tank by rapid acceleration got into the fuel lines  making the car stumble).  I  put a container of STP gas treatment in a half tank of fuel and drove it around town for a week to no avail.

Spark plugs and wires were replaced last summer,  Rotor and  cap appear to be in good shape.

Other than stumbling when I put my foot down the car runs fine. (although it's not nearly as much fun to drive as it used to be)

I've traced all the vacuum lines I can see without taking the intake apart.
The air flow /fuel metering plate moves freely. (slight resistance when pushing down and returns smoothly)

If it had a carburetor I'd be rebuilding the accelerator pump.

Thanks for the suggestions, I'll be studying the service manual CD to try to get an understanding of how the injection system handles off idle acceleration enrichment.

Ray Timmerman

---- Axel Wulff <axelwulff at hotmail.com> wrote: 

=============

Well, it could be a number of things, such as the EZL (ignition control module), the ECU, the fuel distributor or the fuel metering plate, among other things.

 

The fact that it only hesitate/stumble when hot may indicate that the EHA is malfunctioning.


How many miles on the engine? What is the age/condition of spark plugs, caps, cables, distributor, coils?


Axel Wulff
610-731-5453 Cellular
610-572-4611 Home



 

> Date: Sun, 27 Sep 2009 15:18:34 -0700

> From: rltimmerma at cox.net

> To: mbcoupes at mbcoupes.com

> Subject: [W126 Coupe] Stalling issue

> 

> I need some help with a stalling problem. My 1990 560 SEC has taken to dying on rapid acceleration.

> 

> If I try to accelerate sharply from idle the engine quits. It always starts right back with no problem and idles perfectly. If I accelerate gently it also has no problem including WOT acceleration from moderate speeds. I found while checking this morning that the problem doesn't happen until the engine warms up. (accelerates as it should when engine temp is cold.)

> 

> I checked the fuel pump pressure by disconnecting the supply line to the fuel distributor, installed a 0-100 psi gage and jumped fuel pump relay terminals 7 and 8. fuel pressure is over 100psi (it pegged the gage) I don't have the metric fittings to check the lower chamber pressure or test while the engine is running.

> 

> Am I correct in thinking this is a fuel supply problem? Is there some electronic enrichment on rapid acceleration?

> 

> Any help is much appreciated.

> 

> Ray Timmerman

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