Red FP Relay Clicking?

[Funracer]

I have two red FP relays, one old original and one much newer. They both click quite loudly when the engine is running. Sounds a bit like when you shake a spray paint can
but not quite that loud. Touching the relay you can feel the rattle.

I’ve had the car for 3 years now and never noticed this continuous rattling clack.
Is this normal?

Thanks

[tsuter]

No, its not normal. It is not the relay though...it is doing its job. The fuel pump is not holding pressure...could be more than one cause: fuel pump going bad, leaking check valve, cracked fuel line introducing air....

[Funracer]

System pressure is 72 psi in range and all other WUR pressures are as well.

However, car is slow to start. Takes 6-7 seconds on cold start to fire.

FP and check valve are new-ish less than 500 miles on them. Of course this means nothing.

Anyone else had this happen? How was it fixed?
Thanks

[spuggy]

OP, you don't state model or year. 2 red relays could be an indication of a 930 - but I've seen photos of 911's with a full row of red relays above the fuse box too, so...

On a CIS car, I would suggest to check the air flow sensor safety switch and/or air flow plate adjustment - sounds like the plate isn't correctly adjusted and fuel pump relay is incorrectly switching on/off when it should be on continuously (eg motor running). The safety switch is designed to stop the pumps from running when the motor isn't running/making vacuum.

A 930 has a downdraft air flow meter; when system fuel pressure falls with the car off - eg overnight (or sooner with a leaky accumulator) - apparently it's quite common to get a "prime" action when switching the ignition to "On" before starting. Fuel pumps run, system pressure builds, the fuel distributor piston moves, moving the air flow plate, the safety switch goes off again and the pumps stop. Once you crank it over or the motor starts to run, air plate moves off the stop from manifold vacuum, pump(s) run continuously.

On an SC, think the air flow plate is updraft so you don't get the "gravity prime" effect - but otherwise works pretty much the same.

On a Carrera, pretty sure the DME relay controls the fuel pump. Maybe a bad contact?

There's nothing I'm aware of sensing fuel system pressure - and controlling fuel delivery accordingly - in either a CIS or Motronic car. However, a 930 has an overboost sensor that can interrupt fuel delivery, as well as a speed relay and other junk.

AFAIK, any pumps should run 100% of the time with the motor running. System pressure regulation is done via the system pressure regulator (in the CIS fuel distributor) on a CIS car, or by the fuel pressure regulator (on the fuel rail) on a Motronic car.

[Funracer]

Sorry about that. I’m on here pretty regularly and sometimes I forget that not everyone knows my car. It’s an 83SC stock CIS. Only one red relay. I was just saying I had another one and swapped it and there was no change.

I have adjusted the AFS a few days ago. I raised it slightly hoping it would help my slow start. It did not. Maybe its too high.

[spuggy]

Originally Posted by Funracer

Sorry about that. I’m on here pretty regularly and sometimes I forget that not everyone knows my car. It’s an 83SC stock CIS. Only one red relay. I was just saying I had another one and swapped it and there was no change.

No worries, thanks for the clarification.


I have adjusted the AFS a few days ago. I raised it slightly hoping it would help my slow start. It did not. Maybe its too high.[/QUOTE]

[wazzz]

Don't forget that on a CIS the FP relay coil is NOT energized once the engine is running. I know it sounds weird as we often tend to think it is the other way around, but this relay coil is indeed energized only during cranking.
So once ignition key is released from crank to run position, FP relay is de-energized and switches back to another circuit that allows the FP to still be powered.
Check the diagram.

[7783911]

wazz

however, and easy to test, the relay has to be functional for the pump to run (at least on my 77)..disconnect the relay..engine quits

that is why people seek the german made red tops which are superior to chinese knock offs

[wazzz]

Originally Posted by 7783911

wazz

however, and easy to test, the relay has to be functional for the pump to run (at least on my 77)..disconnect the relay..engine quits

that is why people seek the german made red tops which are superior to chinese knock offs

Yes, the relay is indeed required in all circumstances because even when its coil is not energized, FP current still flows through the other section of its internal switch. It is a SPDT.

[timmy2]

Slightly Spread the male contacts on the relay with a razor blade.
May just be an intermittent contact problem.