Getting too much fuel! - PeachParts Mercedes-Benz Forum

#1 08-24-2002, 03:37 AM

I have a 1990 560 SEL that's getting too much fuel. I have replaced the fuel distributor, EHV, O2 sensor, pumps, etc. The CIS checks out OK, and after cleaning the engine with a MotorVac it runs OK for a while, then starts to choke on fuel again. Seems to do this right after I fill the tank, mileage goes from 19 mpg to <7 mpg within a few miles before car won't run anymore.

stevebfl · #2 08-24-2002, 08:10 AM

Figuring that all those parts work, I would check fuel pressure first and then EHA current.

The car can't run right and run rich. Monitoring EHA current determines the electronic component of mixture. If the fuel pressure is right the mechanical component should be close with a new fuel distributer.

#3 08-24-2002, 11:14 PM

Thanks Steve. I'll check the fuel pressure. The EHA was replaced and I have had all the dignosis done by a master MBZ mechanic, so I am pretty sure that he checked the EHA current. I know that at some point this car had some rear-end damage, so I am working on the theory that there is some obstruction or crimp in the fuel lines near the tank. Have you ever heard of anyone having excessive fuel pressure because of a corroded or crimped line near the tank? I recently replaced the fuel tank cap because it was leaking, but now I wonder if there is excessive pressure in the tank.

stevebfl · #4 08-25-2002, 08:46 AM

I have seen crushed return lines cause high pressure.

The 560 has a separate fuel pressure regulator that does such be returning fuel to the tank. I really doubt the credability of a tech that replaced a fuel distributor without testing fuel pressure or EHA current. Same goes for replacing EHA without either an external leak or internal pressure problem. I suppose it would be appropriate if it were open or short circuited, but I have never tested for such. EHA is a controlled leak that alters the differential pressure. The person diagnosing this system should understand this relationship and how EHA current alters the pressure.

I had a 560 about six months ago that went into cold start enrichmment +60ma every once in a while. The actual problem was severe corrosion in a 12 pin connector that was messing with the start signal to the controller. The only way we found the problem was to watch the EHA current and then speculate on how it could happen. First choice a goofy controller. Lucky that we had one to get that silly thought out of our head.

My next thought was that the start signal was being seen somehow by the controller. We found a large connector (from the wiring diagram and went there to test and found the corrosion. I was lucky to have guessed the right spot to test. I WAS not lucky to know what to test. It was based upon analysing the electrical and hydraulic fuctions.

BTW the differential pressure changes a full bar due to this current. For those who don't know, this is a huge difference in a system that does all lambda control within less than .2 bar (a tenth each direction)