Loud fuel pump when it’s hot outside

[JaykReynolds]

Strange phenomena with my ‘82 911SC. I replaced the fuel pump last year and it worked great until it got hot out. As it got into summer heat, stop and go traffic let the car get pretty toasty, and the fuel pump suddenly started buzzing. Almost like it was vibrating a bit from not being tight in the bracket, and then reverberating through the car. Worse hearing it from outside the car. When I’d come to a stop, it buzzed very audibly.
So I replaced the pump again late last summer even though it had maybe 2000 miles on it. Worked great again.. quiet and no buzzing. I assumed it must have been a bad fuel pump since was tight in the clamp and not vibrating. I drove it through the fall, some winter drives when the roads were clear, and through the early spring. It has been perfectly quiet. I put maybe 1000 miles on the pump so far. But now as it warms up, on the days it hits about 80 degrees outside or hotter, the fuel pump buzzes again.

Thoughts or ideas?

Thanks in advance!
Jayk

[JaykReynolds]

I should note.. these are brand new Bosch pumps from Pelican. No cheap off brand or non-standard install. And the car runs great whether the pump is buzzing or not. Never any drivability concerns or low power that I can detect. But again, only does it when it gets hot. Put most of those 1000 miles on this latest pump from late fall, some winter drives when roads were clear, and the early spring. No buzzing.

Thanks again
JR

[High Life]

Im not sure if this helps but I have had the Bosch Buzz for years.. I have put several thousand miles on mine with it buzzing. Some of these trips are 250 miles + from home. I always have carried a spare because I worry - but as of late if I hear the buzz, I just drive on.

Here is my initial post a few years ago (same pump in the car)..

https://forums.pelicanparts.com/porsche-911-technical-forum/1029795-bosch-fuel-pump-noise-whine.html

['78 SC]

Has the in-tank fuel filter (screen) been cleaned? If it is clogged the inlet flow to the pump is restricted and the pump can cavitate, which makes a surprisingly loud noise. Warm fuel would vaporize more easily and cause cavitation.

While the filter is out, check the passages leading to the pump.

[JaykReynolds]

Thanks High Life and ‘78 SC for your thoughts here.
High Life, I’ll read your thread, but it sounds like this may just be a common issue that fortunately doesn’t mean my pump is on the verge or crapping out. I kept that first one I took out as a spare just in case, so I suppose I’ll drive on!
‘78 SC.. good questions. I actually went through the entire fuel system at the same time just to make sure everything was clean. I pulled the tank, flushed it, did some electrolysis to clean out any existing light surface rust, and replaced the in-tank filter. I also did replace the filter and accumulator in the engine compartment. Then i ran it like that for a few hundred miles before pulling the tank to flush it again and reclean that new filter just to make sure nothing else was hiding in a nook or cranny inside the tank. Everything looked good. Car starts on the first or second revolution almost every time. Even when it’s hot and I shut it off, it fires up immediately. It runs great. I do have some new injectors I was going to throw in, but haven’t yet.
So for now, unless someone has the magic repair, it sounds like maybe it’s just a common problem with these pumps.
Thanks to you both for the replies, and hopefully it just keeps on running!

[911MANN]

Jayk- Yes, I would say common with the newer fuel pumps. Same happened to me with my new pump, but I notice TWO variables that cause the loudness. First: hot temperatures, as you noticed. Second: low fuel tank. With mine, anything about 1/4 tank and lower triggers it too. But a fill-up usually solves it unless very hot outside. I live in Central Florida.

[JaykReynolds]

Thank you for the response rtilden. I hadn’t thought about whether the tank had more than a 1/4 tank or not. I’ll fill it up and give that a try to see if it helps. Otherwise, just sounds like that’s the way these pumps are!

I appreciate the reply, thank you!
JR

['78 SC]

Originally Posted by JaykReynolds

Thanks High Life and ‘78 SC for your thoughts here.
... ‘78 SC.. good questions. I actually went through the entire fuel system at the same time just to make sure everything was clean. I pulled the tank, flushed it, did some electrolysis to clean out any existing light surface rust, and replaced the in-tank filter. ...

That's really thorough. Not likely there's any restriction on the pump feed.

Originally Posted by rtilden

Jayk- ... but I notice TWO variables that cause the loudness. First: hot temperatures, as you noticed. Second: low fuel tank. With mine, anything about 1/4 tank and lower triggers it too. But a fill-up usually solves it unless very hot outside. I live in Central Florida.

Maybe JaykReynolds can try an experiment to test out theses observations. Run the tank down on a warm day so the noise returns, then fill the tank and see if the noise subsides. Even in a warm climate the fuel stored underground should be cool.

[reclino]

Big Mercedes V8 cis engines of the same era have a fuel cooler built into the AC system. The CIS system pumps a lot of excess fuel through the fuel distributor and back to the tank. Each time around it gets a little warmer. Now imagine your on a road trip in the desert, and the 130 deg F pavement is warming up the bottom of your fuel tank.
https://www.onallcylinders.com/2016/12/15/cavitation-explanation-what-is-fuel-pump-cavitation-and-how-can-you-avoid-it/

The 911 tank is really restricted for the high flow CIS and I suspect the pump inlet is near cavitation, warm the fuel up and it cavitates at the pump, causing early failure.
Mercedes engineered around this by cooling the fuel on the return line if you were running cabin AC, I guess they figured if the people in the car were warm the fuel was also getting warm.