PeachParts Mercedes-Benz Forum - View Single Post

stevebfl · #29 05-11-2002, 10:20 PM

How about a little more abrasion for the learning exercise. I agree with Dan. Nothing more frustrating than to give a bunch of good advice only to have the shot gun used anyway.

I am not trying to be hard on this problem or tech but this should be a learning moment.

What makes this frustrating is that good technicians almost hate a shotgun fix worse than no fix. It can often leave one not knowing what really was happening.

When dealing with these kind of problems one must find out what is missing or added to the proper equation.. Let me give an example of a current car in my shop at the moment unfixed,

The customer says the cars starts and as it is driven off the thing stumbles and can possibly stall. The first test drive brings nothing so a preliminary scope test for available voltage (coil/rotor problems) and/or other ignition irregularity. No prpblems present. Next closed loop operation was checked at x11 left fender. Pegged at 100%, now here is something. I adjust the mixture to 40-45% and drive again. During one of the early road tests the car had just briefly hesitated and now the car does well and I feel there is a good chance the problem is solved. An hour later the service writer wants to call the customer and I drive it one more time. Out the drive and at the street I almost get run over as the car bogs baddly studders as I head for the parking lane. I try again to enter the street and it dies but instantly restarts. Call the customer and tell them that we now have really experienced the problem but won't get done that afternoon.

I put on a fuel pressure gauge suspecting a pressure drop (possible fuel pump or relay problem) and I drive it a few times. Its back to running perfectly. On the third attempt it starts doing the problem and I am in the parking lot. The fuel pressure is constant at 6.5 bar. Its not fuel pressure drop. The car is continueing to screw up and it clears periodically like a switch is pressed. Further observation shows that trying to rev to motor causes great clouds of black smoke. Well now we are getting somewhere. This IS a mixture problem ....not caused by over fuel pressure.

The point to all my tests are to find the condition that exists so that I then can hypothesize a failure that can cause such; which will then be tested. Since system fuel pressure isn't the problem the gauge is removed. The next thought is that the car could be electronically enriched; either that or the mechanism of electronic control is acting without command.

The fuel gauge goes on the lower chamber pressure and the ammeter goes on the EHA and knowing it has been beaten the problem quits hiding. The next start I see my problem in full glory. After normal start-up enrichment the EHA current drops to 3-4ma and all of a sudden it goes to 67ma and the lower chamber pressure drops a whole bar. Now all of system control is usually done within a tenth of a bar or two. This IS my problem.

Thats where I left it on a busy Friday afternoon. I have a used KE controller that we will probably try but I didn't do that on Friday because I want to look at the start input to the controller to see if there is a reason for it to go to starting enrichment control levels.

The point to all this wasted bandwidth is that I almost never go fishing with parts. I seek what is missing or added and start with the basics. In the car in question, something WAS missing due to the poor connection. Untill the conditioned is labeled all efforts should go to monitoring fuel and ignition. One was missing or both. It would have been found within one or two events. The need for more than one event is that one must start somewhere. In my example we started with the concept of reduced fuel and found just the opposite. We could just as easily been monitoring primary ignition (which is what I assume was lacking due to the poor connection - although it could also have been a lack of electricity to the fuel pump relay). I often drive the car monitoring both fuel pressure and primary ignition. If its a steady problem, I'll do it on my dyno so I can watch the scope and not the road.