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Old 03-24-2001, 09:54 PM
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Join Date: Mar 2001
Posts: 51
My 560SL has 40,300 actual miles and is in outstanding mechanical condition--except I am having problems with the fuel delivery system. Twice in the past month the car would not start after having been driven lightly for 30 minutes or so. In both instances, the fuel pump was apparently not getting any power, since I could not hear the normal hum of the pump after the key is turned on.

My independent tech replaced the fuel pump relay after the first incident. It occurred again several days later and he then replaced the fuel pump (only has 1 pump--not 2). Yesterday it happened again, but this time it died at a stop sign while it was idling.

When these events occur, the car actually starts for about a second when the starter is engaged, but immediately dies. What do you think is causing this? Could I have another "bad" relay? I know from looking at the wiring diagram that the CIS-E Control Unit and the Electronic Ignition Control Unit are involved in the fuel delivery function. How likely is it that either of these "budget busters" are the culprit? Thanks for any advice.

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Old 03-25-2001, 08:07 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
First of all has your technician ever seen the car when it wouldn't start?? If not then you will have to go on Guessing about the problem. I would always suggest replacing both pumps on a 14 year old car, even low mileage.
Has he inspected the ignition system, cap/rotor/coil/wires??The fuel relay gets RPM info from the ignition module(EZL).
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Old 03-29-2001, 08:05 PM
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Join Date: Mar 2001
Posts: 51
M.B.DOC: Thanks for your speedy reply. I've been away for a few days, so could not answer you. Yes, the car would not start for the tech on the first occassion--that's when the problem was defined as the fuel pump relay. The second time the car acted up, it eventually started the next day and when we took it back to the tech, he concluded the fuel pump needed to be replaced. After the third occurrence (when I posted my message), I decided to change service facilities and had the car towed to the Mercedes Dealer. It would not start for them either. I gave them the receipts for the previous work, so they could see what had been occurring. Today they called and said it was fixed--"bad" fuel pump relay!! I guess it's possible, but 2 bad fuel pump relays seems unlikely, and may indicate some other component is causing the relays to fail. Guess I'll have to wait and see if it really fixed. Thks again for responding.

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