Help analyze my EHA current and other (O2) sensor readings... - PeachParts Mercedes-Benz Forum

caliwagon · #1 05-11-2011, 01:28 PM

The car is a 1986 Mercedes Benz 2.3-16, all stock CIS with a 130,000 miles. It has been very well maintained--all ignition components are new (with proper resistor plugs), and the valve clearances are all within spec. I have checked extensively for vacuum leaks. I replaced a few old vacuum connectors, and I think the system is now free from leaks.

I'm chasing down some moderately long cranking times (not too bad, just longer than it should be) and an intermittent somewhat high idle when warm (1200-1300 rpm). I'm also trying to improve my mpg, which is currently hovering around 17-19. Lastly (and this is probably related to the poor mileage and maybe also the high idle), the decel system is not working properly at operating temp. It takes far too long for the engine to return to normal idle, and the car itself doesn't slow down as much as it should when coasting in gear. However, the decel system does seem to work fine with the air conditioning on (the idle is also always correct with the AC on). Other than this, the car runs and drives very well--no hesitation or lack of power.

I know for certain that my CIS coolant temp sensor is somewhat off. Based on the charts I've seen, the resistances in the sensor are reading higher than they should be (e.g. reading about 400-440 ohms at 100C instead of 140-220 ohms). I will be replacing it soon. Voltage to the sensor is 5 volts, which I believe to be correct.

With the engine fully warm, if I tap the O2 lead under the floormat while driving the voltage does seem to bounce around as normal, ranging from .9xx to .0xx volts. When accelerating it steadies out around .75-.85 volts, which I assume is the acceleration enrichment. The voltage drops below .5 during decel. FYI the O2 sensor light on my cluster is not illuminated (the bulb is good--it lights up when with the others when you turn the key).

As for the decel circuit, I have tested the throttle microswitch. The throttle cable doesn't seem to be sticking--it contacts the switch properly. The switch itself tests out correctly: zero ohms when contacted, otherwise infinity (or maybe I have that backwards, but either way the resistance switches with the switch closes).

Now on to the EHA. With engine off (but at operating temp) and ignition on, I get a steady +50mA going into the EHA. With the engine at idle, it's around -4 or -3 mA (fluctuating a little bit). Disconnecting the O2 sensor or the coolant temp sensor does not change the current to the EHA in either case. Resistance of the EHA is 20.7 ohms, which I believe to be in spec. Related to the decel problem, the current to the EHA does not change like it should when I rev the engine and let it return to idle. I believe the current should go to something like -45 mA when the throttle closes and the rpms drop (to reduce differential pressure and cut off fuel to the injectors), but mine doesn't.

I'm just looking for input. Do I need a new O2 sensor? When else can I check that might be related to the decel problem? How about the idle? Any thoughts are appreciated.

mbdoc · #2 05-12-2011, 08:17 AM

Since I have had about 6 of those cars, and even raced one for 10 years..

Sounds like a problem with the air flow position sensor.

That part can cause ALL of the above issues.

areces.cf · #3 05-12-2011, 08:32 AM

The deceleration fuel cut off do not work with the car sttoped, try the test driving the car to 60 MPH, and release the throttle.

caliwagon · #4 05-12-2011, 11:34 PM

I took some quick videos today to show some data...

1. Here are the duty cycle readings, from pin 3 in the x11 connector. KOEO I get about 52%--is that right? I thought it was supposed to be 70%. When I first start the engine (already at operating temp) I get a steady 49-50% until I first rev it. Is that normal, or a fault code (e.g. for the potentiometer)? You can see what happens when I rev it. In settling back to idle it goes to 54% until it reaches idle. These readings are with everything plugged in as normal, including the O2 sensor.

http://youtu.be/6v4yAH4ozEM

2. Here are some O2 sensor voltage readings driving along in 4th gear, then eventually decelerating back to idle. I'm just tapping into the lead here--the O2 sensor is still connected. If you turn up the volume, you can hear when I accelerate once or twice--it goes toward rich. It's a little hard to follow because the camera is sideways and the meter automatically changes scale so it keeps switching from V to mV readings. Ignore the clicking sound--it's the autofocus on my phone's camera. Let me know if you see anything noteworthy...

http://youtu.be/GIFHbYQCK-Q

caliwagon · #5 05-13-2011, 02:34 PM

Quote:

Originally Posted by mbdoc

Since I have had about 6 of those cars, and even raced one for 10 years..

Sounds like a problem with the air flow position sensor.

That part can cause ALL of the above issues.

I measured the resistance between pins 1 and 2 (the top two) on the AFM potentiometer. At rest the resistance is about 1.8-2k ohms, then it increases to about 3k ohms as you barely move the plate down. Then there's a dead spot (resistance goes to infinity) after which the resistance moves from about 5k ohms all the way up to 14k ohms. At 14k ohms the plate is still not all the way down--as I push it down the final portion the resistance decreases from about 14k ohms to around 9k ohms.

All these values seem far too high, and then there's the dead spot. But I've heard people say that you can't really test the pot on the car very well. What do you think? It seems like a PIA to replace the pot, so I only want to do it if it's very clearly the cause.

mbdoc · #6 05-16-2011, 09:01 AM

Testing that POT is difficult without having a recording imaging scope.

That POT (airflow position sensor) IS most likely the issue!