Jump to content
Impact Bumpers
carrpet

Noise coincident with start of fuel pumps - 930 66

Recommended Posts

Posted (edited)

When I start the fuel pumps from cold with the air flow switch disconnected, coincident with the start of the fuel pumps there is a noise which lasts for about 2 seconds from the engine bay. After that just the noise of the fuel pumps running. I was wondering if anyone else has experienced this. It only happens first time I start the car during the day with 0 psi fuel pressure

It sounds like the noise of a dry moving part. My initial thought was a fuel pump but wondered about the sound of the fuel accumulator diaphragm being pushed back against its spring as the pressure builds? The fuel accumulator is < 500 miles old.

The temperature in the garage is 6C and the cold pressure is 12psi.

All of my fuel pressures are in specification - cold, warm and system plus the fuel system retains its pressure for much longer than the minimum specification.

Would welcome any input on what it might be.

Peter

Edited by carrpet
Missing info

Share this post


Link to post
Share on other sites

I don’t have anything in my engine bay that’s runs momentarily when the pumps start.

Its hard to tell without hearing the noise but I think it’s most likely related to your fuel system. 

When I run my pumps from cold without the engine cranking they have a different pitch for the first few seconds. The pump seems to run faster and it sounds like it’s pumping air. This is what you’d expect as the system pressure will be v low so needs to build - it runs at nearly 7 bar so there is work to do. There are 2 pumps in series, one in the n/s wheel arch and one under the tank where it is on an SC. Both run together so the rear pump has little flow or pressure for a second or two. 

My relays (fuse box) cycle on and off (like a buzzing noise) when I’m running the pumps without the motor running (so not charging). It shouldn’t do this but the loom and fuse box is a v poor design. There is a fix but I doesn’t seem likely from your description. 

David

Share this post


Link to post
Share on other sites
10 minutes ago, flatsix777 said:

I don’t have anything in my engine bay that’s runs momentarily when the pumps start.

Its hard to tell without hearing the noise but I think it’s most likely related to your fuel system. 

When I run my pumps from cold without the engine cranking they have a different pitch for the first few seconds. The pump seems to run faster and it sounds like it’s pumping air. This is what you’d expect as the system pressure will be v low so needs to build - it runs at nearly 7 bar so there is work to do. There are 2 pumps in series, one in the n/s wheel arch and one under the tank where it is on an SC. Both run together so the rear pump has little flow or pressure for a second or two. 

My relays (fuse box) cycle on and off (like a buzzing noise) when I’m running the pumps without the motor running (so not charging). It shouldn’t do this but the loom and fuse box is a v poor design. There is a fix but I doesn’t seem likely from your description. 

David

Thanks David

It may just be pump cavitation which is a sound I am familiar with.

I will get close to the pumps while I get someone to operate the ignition switch. I would not hear the noise if the engine was cranking over.

My AEM Fuel pressure gauge reads 0 after about 6 hours.

Perhaps it's nothing to worry about.

Peter 

Share this post


Link to post
Share on other sites

Hi David

The noise is definitely not fuel pump cavitation having stood in my pit under the car while Ian Comerford fired up the fuel pumps. The noise definitely appears to be coming from the Fuel Accumulator - will confirm tomorrow !

On another note I have read that the yellow relay on the LHS of the engine compartment should fire up the fuel pumps (priming ?) for about 1 second when the ignition switch is in the run position and the air flow switch is connected "in-circuit" as normal ? Is this a phenomenon that you are familiar with and does your car do this ?

Peter

Share this post


Link to post
Share on other sites
23 hours ago, carrpet said:

HlThe noise definitely appears to be coming from the Fuel Accumulator 

On another note I have read that the yellow relay on the LHS of the engine compartment should fire up the fuel pumps (priming ?) for about 1 second when the ignition switch is in the run position and the air flow switch is connected "in-circuit" as normal

Hi Peter, 

The noise I hear with mine could easily be from the accumulator - I’ve never tried to pin point location other than it seems to come from the n/s rear of the engine bay. I wouldn’t worry if the car starts and runs OK. 

My 930 doesn’t have a yellow relay and definitely doesn’t prime the pumps with the ignition on. I claim no particular knowledge but I had thought the yellow relay was only on US spec cars post 86 (when the 930 was reintroduced to the US market). Mine has the normal round red relay in this location. 

David 

Share this post


Link to post
Share on other sites
1 minute ago, flatsix777 said:

Hi Peter, 

The noise I hear with mine could easily be from the accumulator - I’ve never tried to pin point location other than it seems to come from the n/s rear of the engine bay. I wouldn’t worry if the car starts and runs OK. 

My 930 doesn’t have a yellow relay and definitely doesn’t prime the pumps with the ignition on. I claim no particular knowledge but I had thought the yellow relay was only on US spec cars post 86 (when the 930 was reintroduced to the US market). Mine has the normal round red relay in this location. 

David 

Hi David,

Mine definitely has the Yellow Relay.

You can however replace the yellow relay with a red relay. The yellow relay is alleged to have an extra property which ensures that the fuel pumps don't cut out due to air flow switch "bounce".

An excerpt from Rennlist 

https://rennlist.com/forums/911-turbo-930-forum/315180-what-s-so-special-about-the-magic-yellow-relay-2.html

"The answer to "why Porsche put the magic Yellow relay in the fuel pump circuit instead of a regular red relay". For starters the red relay and the yellow relay pin connections are the same so a red relay will work. As you know, in the yellow relay there is a circuit board with diodes and transistors on it. I enlisted the help of a great friend who is an EE to tell me what the circuit board does. Between my understanding of the fuel pump circuit and his understanding of the circuit board I think I have the answer. As many of you know when you have the key in the on position and the engine off, if you push the sensor plate down or disconnect the plug at the plate the pumps will run. This switch on the plate is a very unstable "switch" in the world of electronics being it is mechanical and is prone to transient movements given its enviroment. So, there is potential for the plate to make contact or bounce against the contact and very momentarily cut the fuel pump off if the plate is swung violently up. (high revs,high engine load, quick shift type of scenario). Within the circuitry of the yellow relay there is a de-bounce function on pin 85 which is the ground potential coming from the plate switch to the tune of about 282milisec. What that means is if the plate for some reason is swung up and makes contact with the switch and completes the ground path to pin 85 of the yellow relay while the engine is running there is a delay of aprox 282milisec. (which is about two blinks of the eye) before it completes the circuit and cuts the fuel pump off. This potential for cutting the pump off in a turbo motor even for a short amount of time or if done repeatedly could theoretically be very damaging to the engine as we all know. So, the red relay will work but you run the risk of cutting the fuel pump off momentarily if you use one. Perhaps its a little overkill on Porsches end but very well thought out I think... "

As far as the noise is concerned, it did not happen this morning and it started first time, but I am convinced it is the accumulator diaphragm compressing the coil spring which is making the noise. The spring provides stored energy for a period of time, maintaining fuel pressure after the fuel pumps are switched off to ensure that the car warm starts. I haven't seen anything which convinces me that the accumulator has a cold starting benefit.

Regards

Peter

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...