vegasdan

Quick Question. I have an 88 928 s4, 31k miles. I had a new fuel pump installed, as well as all fuel lines checked. Car runs perfect without any power loss. If I am in traffic or using the car all day, the fuel pump gets loud. Car still runs normal with no power loss and has almost a full tank of gas. If I turn the car off and let it sit for an hour, the pump sounds normal again. I replaced the pump twice, thinking I had a bad pump. One time I did smell vapor and the pump was loud again. Had all lines checked and nothing. Before I bring the car back in, can any of you give me some ideas of what might be causing this issue? There are no fuel leaks or loss of power or anything. The pump just gets noticeably louder after running in traffic or after using the car for some time. Thanks in advance for any input or advise.

FredR

Has the in tank fuel pump been checked for integrity of the pump to outlet boss hose connection? That assumes your model year has an in tank pump - never sure which ones have and which have not got one.

vegasdan

@Fred, its my understanding the 1988 does not have the internal fuel pump, however, that is a good question I will let my shop know. Just to be sure. I know there is a "filter" of some sort that I was told by my shop that they don't like to mess with, as it could result in having to replace the entire fuel tank. Not sure why that would be the case, as I would replace that filter regardless. I'm on fuel pump two now. We thought perhaps the first one was bad. Not sure why it sounds perfect most of the time until it is hot or running for some time.

hacker-pschorr

The filter they are talking about screws into a metal insert. Over time they can gall together and when you try to remove the filter, it takes the metal insert with it and there is no known way to re-install that insert.
The same thing can happen when trying to remove the in-tank pump. So yes, when this happens the tank is essentially ruined.

However, there is a solution to this problem your shop is probably not aware of sold by 928's R Us:

https://rennlist.com/forums/928-foru...-up-issue.html

I talked to Roger yesterday at 928's R Us, he has them in stock.

Kevin in Atlanta

I would like to add that when it comes time to remove the intank pump or filter that using a hose clamp to hold the tank tight against the metal insert can help avoid turning the insert out. Also, good for when you tighten them.

FredR

Eric has covered the insert issue and every time one removes the in tank kit there is always the big worry that the process of removing the internal kit will spin the insert and historically the tank is fubarred. There is a solution to this issue - I have no personal experience of it but it seems promising.

Regarding your issue both the in tank pump and the models without such are kitted with a "strainer". The strainer removes larger particle sizes that if ingested into the pump can cause it to jam up. A filter is what is on the fuel pump discharge that removes very small particulate matter that would otherwise clag up the injectors.

As to what is causing you problem remains to be seen if your model year does not have the in tank pump. Pumps that makes noises generally do not improve with use so it is wise to monitor the situation as you try to work out what might be the root cause of your concern.

worf928

Does the pump make as much noise if the lower cover is removed?

NickTucker

The pump is mounted on rubber bushings.. I wonder if, as they are old and brittle, that under some situations they transmit vibrations to the car.. I found on mine that I could hear it whine sometimes.. Don't remember if it was when the ambient temp was hot or cold, or car temp for that matter.
I did do some clean up work there which did include replacing the pump and filter and now all is quiet.

vegasdan

Good idea Nick. I will ask and see if that is checked. Perhaps while under "hot conditions" that could cause the pump to sound a bit louder. It is working fine and is brand new. Even when the sound is a bit elevated, the pump is still working great and the car is running perfect. Perhaps what you are suggesting is the issue. Thanks for the input everyone.

I'm wondering if the shop did change that filter or if I should tell them to change the filter. I'm just nervous if that causes further issues when the car is running perfect now. The pump must be getting the fuel it needs. If that filter was clogged, it would not run right or have issues. Just my thoughts.

dr bob

I just drove the car this morning, first drive since last fall when it was put up for winter storage. I can hear the fuel pump running in the cabin at 40 MPH, a bit of a tribute to how quiet the car is these days.

I'll get it up on the lift soon, and inspect/replace any isolation mounts/bushings that may be tired. Maybe add some sound-deadening around it, the filter and the S pipe while it's all exposed. Actual conducted noise is pretty intriguing, since the tank has foam isolation at the top, rubber isolation at the straps, and the pump actually hangs on the straps as the only metal path for conduction.

The car runs perfectly after almost six months of hibernation this time around. Thanks for asking...

worf928

It's not uncommon to find the worm clamps for the filter or pump oriented so that the 'worm' part contacts the cover once installed.

jpitman2

Couple of observations. If the filter has recently been changed, I would look into it being done correctly, and there is no noise transmission path visible due to anything coming into contact with the cover, as remarked above. Changes in its sound level would indicate a change in the load it is seeing, or the power available - try checking its current drain when quiet and noisy. Last time I looked at mine it pulled 7A, although its a different model from the OP's. I use a meter like this https://www.aliexpress.com/i/32884020715.html

Second, I spun the threaded insert in my tank years ago, and was able to seal the leak with PC11 for a long time until I got a replacement tank. I gave this tank to another , who managed to remove the insert, remove the frozen strainer threads, clean up the outside, pull it into place from inside the tank, coat with epoxy, and seal it in. Leak free for years now. Somebody is also making a special part to replace the original threaded adapter.
I have NEVER been able to hear my pump out in the open, only in an enclosed space like the garage, as in when it gets the priming run in CIS cars.
jp 83 Euro S AT 57k

realhudda

It's possible that your fuel pump is failing or has already failed. To test the fuel pump, you can try the following:

1. Listen for the fuel pump: When you turn the ignition key to the "on" position (but not all the way to start the engine), you should be able to hear the fuel pump activate for a few seconds. If you don't hear the fuel pump come on, it could be a problem with the fuel pump, the relay, or the wiring.
   
2. Check the fuel pressure: You can check the fuel pressure using a fuel pressure gauge. The pressure should be within the manufacturer's specification, which you can find in your vehicle's repair manual. If the fuel pressure is too low, it could be a problem with the fuel pump or the fuel pressure regulator.
   
3. Check for power at the fuel pump: You can use a test light or a voltage meter to check for power at the fuel pump. First, locate the fuel pump fuse in the fuse box. Then, with the ignition key in the "on" position, check for power at the fuse. If there is no power at the fuse, the problem could be a blown fuse or a problem with the wiring. If there is power at the fuse, the problem could be the fuel pump or the relay.
   
4. Inspect the fuel pump relay: The fuel pump relay is a small electrical component that controls the fuel pump. To check the fuel pump relay, you will need to locate it in the relay box under the hood. Consult your vehicle's repair manual for the specific location of the fuel pump relay. You can test the fuel pump relay by swapping it with a known good relay.
   

jpitman2

Note that the pump only runs at turn on in early cars - IIRC , CIS types. This is done by a pump relay specific to those models. A 928's pump relay is under the the passenger footwell - pull back the carpet, undo 2 screws and remove the wooden cover. 'Inspecting' the pump relay externally is of no value at all. Even if you open the case there is unlikely to be any visible evidence of failure. To check for pump noise you could jumper 30->87 in the pump relay socket. You should NOT normally be able to hear the pump at 40mph, considering all the stuff between it and you. Louder than engine, exhaust and road noise?
jp 83 Euro S AT 57k

realhudda

I see, thank you for the information. It's important to note that tampering with the pump relay or any other electrical component in a car can be dangerous and potentially cause damage to the vehicle. It's always best to consult a professional mechanic if you have any concerns about the operation of your vehicle. They have the tools and expertise to safely diagnose and repair any issues.