1999 w210 Fuel pump intermittant

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jb - 03 Jul 2007 23:50 GMT

Occasionally, and more frequently as of late, the engine fails to
re-start after sitting for a few minutes. I suspected the fuel pump
circuit and validated this by attaching a switched buzzer to the relay
that powers the fuel pump(under the rear seat). Using the buzzer, I can
verify that the car is not started because power is not being supplied
to the fuel pump relay.
In normal function, the fuel pump runs when the key is turned on. Then,
I think, once pressure is built up after a few seconds, the fuel pump
shuts off. So there must be a pressure sensitive switch to cut power
once a specific pressure is achieved. If the key is then turned to
start, power is supplied to the relay and the fuel pump will run. Then
obviously while the engine is running the fuel pump is always running.
What reasons would the fuel pump relay not receive power while the
engine is cranking?

Thanks for your help
JB
99 E55
88 300E

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-->> T.G. Lambach <<-- - 04 Jul 2007 03:05 GMT

Fuel pump relays become intermittent and need to be replaced. YES the
fuel pump is ON for a second or two BEFORE the motor is cranked, after
the motor is running it is of course ON. The relay cuts the fuel supply
to a stalled motor so that a fuel fire doesn't occur if the car is
wrecked and the key left ON. In your car's case the relay has power but
the fuel pump doesn't - because the relay is shot.

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jb - 04 Jul 2007 20:37 GMT

> Fuel pump relays become intermittent and need to be replaced. YES the
> fuel pump is ON for a second or two BEFORE the motor is cranked, after
> the motor is running it is of course ON. The relay cuts the fuel supply
> to a stalled motor so that a fuel fire doesn't occur if the car is
> wrecked and the key left ON. In your car's case the relay has power but
> the fuel pump doesn't - because the relay is shot.

Thanks T.G. but I may not have made the point clear. The relay is not
getting power and thus it doesn't activate. I have the buzzer wired in
to the terminals that supply the relay with power. When the car is
cranking and not starting, power is not being sensed at the relay. I
wait a couple minutes, then start cranking and the buzzer activates, and
the relay activates, power is then connected thru to the fuel pump and
the car will start. The car starts every morning religiously, without
exception. The only time it doesn't start is after I drive for say 20
minutes, shut the car off, come back in 10 minutes and try to start it.
Then I crank it for a couple seconds, wait 30 seconds, crank again,
wait 30 seconds, crank again, and then eventually it starts. It has
never left me stranded. Am I making any sense? Thanks for your help.

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-->> T.G. Lambach <<-- - 04 Jul 2007 21:42 GMT

OK, so, once running, the motor never quits. It's only the hot start
that's intermittent. Roland suggests a ten second delay interval relay
is involved. I don't know. If that's so then I suggest that relay has a
heat related failure and that the fuel pump relay itself is innocent.

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Roland Franzius - 04 Jul 2007 12:41 GMT

jb schrieb:

> Occasionally, and more frequently as of late, the engine fails to
> re-start after sitting for a few minutes. I suspected the fuel pump
[quoted text clipped - 10 lines]
> What reasons would the fuel pump relay not receive power while the
> engine is cranking?

There is not a pressure sensor but a second 10s timeout relais switching
off the current if the ignition doesnt respond. So you should look for
the ignition - fuel pump relais signal.

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Roland Franzius

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Roland Franzius - 06 Jul 2007 10:34 GMT

Roland Franzius schrieb:

> jb schrieb:
>> Occasionally, and more frequently as of late, the engine fails to
[quoted text clipped - 16 lines]
> off the current if the ignition doesnt respond. So you should look for
> the ignition - fuel pump relais signal.

The easiest way to check is to unplug the fuel pump relay and to bridge
contacts 30 to 87 in the socket using banana plugs as we say and a wire.
If he engine is starting you should change the relay. If not a short cut
in or on the way to the warm start heating resistance could be a reason.
Even a cold or heat decomposed solder connection on the lower side of
the relais socket is a possibility.

If the engine doesnt start with a new relais the next steps are ignition
signal from cruise controller, coasting phase fuel switch off, climate
and automatic gear controls, that influence the idling.

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Roland Franzius

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Tiger - 05 Jul 2007 15:50 GMT

I think JB has a hot start problem... which indicates vapor lock... thus
leaking fuel injector.

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jb - 06 Jul 2007 00:58 GMT

> I think JB has a hot start problem... which indicates vapor lock... thus
> leaking fuel injector.

Tiger would that condition cause power to be absent at the fuel pump relay?

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