Fuelling oddities (190) - PeachParts Mercedes-Benz Forum

pentoman · #1 08-17-2008, 09:35 AM

Can anyone tell me why my 190E (2.5 petrol) exhibits these symptoms :

Engine warm, at idle:

Blip the throttle - the engine bucks a little, has a stumble.

Disconnect the EHA/control pressure actuator, engine warm, at idle:

Blip the throttle - engine responds more cleanly, no stumble or bucking

This makes no sense. The EHA is supposed to provide enrichment?? Yet it's better without it???

Go out on the road though, you'll find the characteristics aren't duplicated. Instead, without EHA it is pretty laggy to respond to the gas pedal. With the EHA back on it is "okay" only. Bunch of rubbish this KE.

stevebfl · #2 08-17-2008, 09:43 AM

The EHA provides fuel correction, which just as easily means less fuel as it does more fuel. Plus the circuit is polarity specific, if the wires get reversed the system does exactly the opposite of what is desired.

pentoman · #3 08-18-2008, 07:38 AM

My connector has been broken by a previour owner/abuser/mechanic so I thought it could be on the wrong way round, but I checked the wiring diagram and it's correct

. Even tried it on the wrong way round, man that was really wrong, you could hardly touch the accelerator without it falling over flat on the floor! The EHA really can make a huge difference and bring the car to a complete stop.

But back to my problem - why does it do the job wrong - introducing this stumble? Getting a bad signal perhaps, though how do you know? It's been too much time I've spent testing the sensors and wiring and there's nothing wrong with any of it. It's a whole replacement FD, AFM, FPR, injector lines and injectors complete bought-as-one unit on there now but the symptom is the same - crap stumbley response that makes me long for my 2.0 190E with its sharp reflexes and the kick in the back it had without stumbling and pausing first.

New ECU perhaps? Just checked the model number, it's the correct ECU for a 2.5.

stevebfl · #4 08-18-2008, 09:17 AM

The way you check a system.... any system.... is to understand what it does and montor the function.

In your case, the EHA is the sole electrical component of fuel control. Control is accomplished by sending a small current through the device which causes a calculated leak of lower chamber pressure to be effected. This effect can be monitored and has given specifications. The current has a definite profile. EHA current during starting is around 50ma, cold running around 10-20 (depending on temp), add the two if one is starting cold.

Once warm and with fuel mixture proper the EHA current goes to zero! If it wasn't zero one would adjust mixture till lambda control hovered about zero. Lambda control with a good sensor will go lean until rich correction and visa versa. That sweep should eb 3-4ma. A sudden acceleration should cause a 2-10ma (positive) corection due to the airflow potentiometer.

If you ground the O2 sensor lead the EHA will go to 10-13ma positive enrichment. If you place one hand on a 12v connection and the other on the O2 sensor lead the opposite will take place and the EHA will go -10-13ma. Thus the whole of O2 sensor control is from -13ma to +13ma or maybe a little less (some cars go further than others but will be consistant at whatever your are exhibits).

If what you see doesn't look like that then you need to figure why. If it looks like that and has an issue you need to evaluate whether the current does the proper thing to the lower chamber pressure. The lambda control will make about a .2bar difference in lower chamber pressure from rich to lean. The differential pressure (the difference between upper and lower chambers is about .4bar so lambda will change it from .3 to .5 (or a .2bar range).

If that isn't the issue then one must run the individual distributor outlets including injectors through a differential flow meter at three different flow rates with less than ten percent difference cylinder to cylinder at each flow rate.

Of course it could be ignition related.........................

pentoman · #5 08-19-2008, 12:08 PM

The car is a non-lambda car because I'm European

.

Identical system but the closed loop is ruled out - it just uses the EHA for acceleration enrichment, cold starting enrichment, and deceleration shutoff. Therefore should be nice and simple..... unless something is wrong with the signals the system's generating, or with the mechanical fuel parts, or there's an air leak. Just trying to learn what the symptoms are telling me. It's not that bad, and probably should put it down to being an old car with mechanical parts that wear out. But I know there's something wrong with it, something is missing.....

Oh I even rebuilt the head and still no solution. There's nothing major left but the ECU, fuel pumps/lines and ignition...!

Ignition: plugs and dizzy are new, I've tried a spare ignition computer, and the idle and high idle advance were checked, tested OK. Also primary and secondary (?) signals were fine according to the guy who tested them.