namasgt

Hello guys

I changed my original fuel pump because of the loud noise it was making for months.
The replacement pump still made sound regardless of how much fuel I have in the tank. It seems like it would over heat and start making noises. So, Roger gave me another new one as replacement, however after only a week its having the same problem again. When the car seats for a few hours and the pump cools down the sound goes away, until another hour of drive that would cause it to make noise.
Note: fuel filter and internal fuel tank screen were replaced during the pump change.

Any idea if this is normal?

Thanks
Ali

NC928S4

How would you characterize the noise? (metal vibration, rubber vibration, changing noise depending on volume flow, changing noise on electrical load, etc.)

Mine is noisy too and varies with electrical and volume load. I have cracked the enclosure panel but I suspect the rubber mounting brackets are metal on metal so I hear every change in pump demand.

hacker-pschorr

I only get worried when the pump goes silent.

My stock 81 pump made noise, the S4 pump that replaced it made noise, and now my 044 pump has a nice hum to it.

928autobahndreamer

Mine does this. When I have been driving hard and the car is fully heat soaked, the fuel pump "Hums" loudly. I changed the fuel filter, in tank filter, cleaned the tank, and changed the fuel pump. No change whatsoever in the sound.

I think that these just tend to make noise. Either that, or when things heat up and expand enough, the vibratation or buzz is transmited through the adjacent structures.

namasgt

Okay, thanks for the replies.

It does it only after driving the car hard or when I get into stop and go traffic. The fuel pump is tightly mounted to the bracket. But I did see some cracking on the rubber mounts that hold the bracket. Once I even put my hand on the pump thinking that it was touching something, but the sound didn't go away and it felt hot.

The old pump would make the sound every time the fuel level was low or after only a few minutes of driving.
The new one seems to do it when the car gets to operating temperature regardless of fuel level. It makes the sound when it gets too hot.

It just bothers me that it makes noise like that, since on the box it says very quiet high quality fuel pump. The noise gets loud enough that people look at the car when I'm seating at the red light.

One thing that I just realized, I do not have my fuel rail covers on, since I put the new engine in. would that cause the fuel to get too hot and cause the pump to over heat? I know that the covers are there to reduce injector sound, but do they protect the fuel rails from engine heat? Something similar to exhaust heat shields.

SeanR

The amount of noise Ali's was making before he changed it out had me concerned, it was "Almost ready to quit working" loud. I should have listened to the new one yesterday when you were here. Didn't even think about it.

Mine are all quiet, weird.

WallyP

The pump sits on rubber mounts. If the pump, a metal line, or even a rubber line touches the sheet metal, noise will be propagated.

James Bailey

Also if the tank is not inhaling air properly you might be pulling a vacume trying to collapse the fuel tank. Which makes the pump work much harder than normal. Test by running the gas cap off( stuff a rag in the top) It will then be able to get air. Also the mesh screen "filter" inside the tank sometimes breaks free and sloshes around in the sump of the tank. When tha flat smooth round end get sucked up to the fuel outlet....it block most of the flow and the pump makes noise. Turn off the car let it sit and it may float away from the pickup.

namasgt

James

Is there more than one fuel screen in the tank?
The fuel screen on the bottom for the outlet is brand new even thought the old one was okay. I replaced it last winter.

Also, when it was making the sound with the new pump, I thought about the vacuum issue, so while the car was running I opened the filler cap. It opened easily. I will do it again just in case.

Sean is right, my original pump would get really loud, but it did not fail. I put something like 6000 miles on that pump before I changed it to solve the sound issue.

the flyin' scotsman

Ditto.

New filter with old pump, pump made noise only when it had been run for some time. Never anaylsed if the pump was getting warm/hot but it didnt make noise when cold/after sitting for a long while.

These conditions existed until the pump did its usual and quit........wouldnt say it was any louder than before.

New pump and another new filter (no intank on my car) and basically the same as before.

Pump came from Roger and is the proper unit

GregBBRD

I've seen some Chinese fuel pumps that look exactly like Bosch pumps...I had a really tough time determining the difference....I'm not saying this is the problem, but it is worth considering.

It might also be worth noting that that the cars without an "in tank" fuel pump (like your '88) require a different pump than the cars that have the intake pump ('89 and later). I'm sure that Roger knows this, but you might want to double check the Bosch numbers, on the pumps. The later pumps that are designed to have the in tank pump deliver fuel to them hate to have to suck fuel...and make lots of noise, when they are required to do so.

If all this is good, check the voltage at the pump, when it is running. Inadequate voltage would make it overheat. It is possible that the relay/feed to the relay is breaking down and you have inadequate voltage/current. If you have an amp meter, also check the current draw.

Jim Bailey is right on...the tank can have a vacuum in it....ask Rob Edwards. His tank was actually "sucking" inwards from this vacuum. If the little valve right under the thermostat (sits right on top of the water pump) goes bad, the tank will have full time vacuum through the charcoal canister and suck itself flat, which...sucks.

namasgt

Thank you for the replies.

Greg, I do remember that Roger specifically asked me that if my car is an 88 with no internal fuel pump, which it is. I will check the part number again with him tomorrow. I read the manufacturer stamp on the pump, and its not China and don't recall it to be Germany either. I have to go look again.
The pump came from the local Porsche dealer in its correct Bosch box with installation manual.

The voltage thing is interesting, I will check and report back. I changed all the "B" relays last week just in case the fuel pump one was giving me problems.

Thanks Greg

Tails

The Fuel Pump are a direct displacement pump with a rotor/impeller which has cylinderical rollers rotating around a circle stator. The stator is not on the same centreline axis as that of the armature/ pump rotor/impeller. This offset causes the cylinder roller to act as a direct displacement pump, which created the suction and delivery system within the pump stator.

As the rollers, impeller and the stator sections of the pump are all metal construction. these can generate a metalic noise if there is metal to metal contact.

This noise can be caused when the vicosity of the fuel is lowered due to higher temperature of the fuel that can allow the roller to actually touch the stator when the centrifical force of the rotating rollers overcome the viscosity of the fuel and there is metal to metal contact. The pump is self lubricating from the fuel, so there should be a thin fuel film maintained between the roller and the impeller and the roller and the stator, so they should be relatively quite (just hum along).

There can also be metal to metal contact if cavitation occurs due a restricted suction orifice, that can be caused by partially blocked inlet filter or there is insufficient head height of the fuel feeding the pump to maintain sufficient head positive pressure at the pump suction (this could be the case as the fuel is consumed and the pump head height reduces).

If cavitation occurs the fuel will vaporise and bubbles of fuel vapour can form and when these collapse they can cause noise themselves and in addition the vapour will negate the fuel lubricating film between the rollers and the stator, which can again cause metal to metal contact and noise.

Later model cars (MY '89 through to 1995) are fitted with an in-tank pump which should maintain the required positive pressure at the inlet of the pump, however, noise can become evident when the fuel temperature increases due to the fuel temperature increase as described above . This is the case with my car when the fuel temperature rises on a long trip in hot weather and it has been doing this for 10 years with not ill effects so far.

There is a fuel cooler in the fuel line cooled by the return line from the air conditioning evaporator that is meant to cool the fuel on a hot day and thereby keep the fuel in the gas tank relatively cool. If you don't have the A/C on then their is no cooling effect, so this could be another reason for generatomg the noise.

Additionally there was insulation covering the two injector fuel manifolds, which fails and falls away over time. This allows more ingress of heat from the engine to be imparted to the fuel as it circulates within the fuel system and this will gradually increase the temperature of the fuel within the fuel tank as the excess fuel returns to the fuel tank.

The effect of heat ingress in hot weather can be cumulative, hence more noise from the fuel pump.

Solution: You can either insulate all the fuel lines within the engine compartment to limit the ingress of heat or live with the noise, however, I have included more insulating in the hatch area, that iliminates the noise considerably within the cabin and the noise is only audible when the road noise deminishes.

Tails 1990 928S4 Auto

the flyin' scotsman

Tails......interesting conclusions.

If the viscosity of the fuel is decreased by heat thus causing the pump noise does this in turn cause pump failures?

Is there an additive for the fuel to maintain viscosity?.......lead?

Bill Ball

We've been having "fun" with a related issue in George Suennen's car. He's supercharged and targeting well over 600HP crank, so he has been running a higher output fuel supply. We've gone through a number of variations with high-capacity pumps and larger supply and return lines. With high-capacity pumps you have a whole lot of return to the tank from the FPR when you are just cruising. We had a number of problems that looked like pump cavitation, particularly when just cruising down the freeway at high altitude (as would make some sense). So, George installed a fuel temp sensor inside the tank, or rather, in his fuel cell. We found the temp in the fuel cell would increase rather steadily and dramatically from ambient, and when it reached 125F (on an 80F day), his Aeromotive pump would cavitate and fuel pressure would fall gradually. Once the fuel pressure dropped below 30 PSI the injectors stopped working and the car stalled. Switching to Bosch 044 pumps allowed the fuel temp to rise to 132F before the pump gave up with the same result. George insulated all the fuel lines and insulated any exhaust sections near them or the fuel cell. The final setup with one in-fuel-cell 044 and one external 044 in parallel, with a switch so either can be shut off while just cruising, has reduced the fuel heating somewhat. It still reached 122F or so on an 80F day, but that was not enough to affect the fuel pump output so much as to be noticeable. BUT either pump sure gets noisy with any fuel temp over about 109F. And it made no difference to fuel temperature whether we ran the in-tank pump or external pump when cruising for extended periods.

My 89 has never had any issues with a noisy stock fuel pump. It's still original too. Of course, I have an in-tank pump as well. This is despite a lot of hot high-desert driving at all kinds of speeds.