Twin Walbros, one pump louder than the other
 

I have Twin 255's in my miata. One was given to me, brand new, by a very generous person on this forum back in 2008 and has been in the car ever since. I was told it's a Walbro 255HP. I have tested this pump and it will make over 100 PSI with only 12.0V, so I'm pretty sure it's a 255HP.

Today, I installed a new Walbro 255HP as well to have twin pumps as I needed more fuel.

Well got it all done, and the new pump is about 1/3 to 1/2 the volume of the old pump. Old pump is loud and high-pitched whining compared to the new pump. This was tested at ~11.5V since the car was off and I haven't drove it in a while so battery is a bit weak. But they sound way different.

Any idea why? Is my old pump going out, or maybe the new pump isn't worn in?

Are they both mounted exactly the same?

Fake Walbro 255s are definitely out there. If you didn't buy them both from reputable vendors (i.e. not eBay), you might have one fake.

And not amazon. I'm getting a lot of fake junk in official boxes, because Amazon return policy.

Perhaps not, maybe that's the reason actually. See picture. One pump is in stock spot, but clamped to the metal arm beside it with a piece of rubber between the pump and the metal (no metal/metal contact at all, as stock). This is the original loud pump.

Second pump is just clamped to the first pump right beside it.

Attachment 183865

I bought it from Amazon. This pump:
The description at the top said "Walbro GSS342 Fuel Pump" and by "Walbro". So I thought it was directly from Walbro. However it's actually from HFP Fuel systems.

Funny enough the first pump was physically damaged (broken fuel outlet right out of the box) and I had to return it. I've emailed the guy at HFP Fuel Systems a few times and he was very nice. But I suppose it could be a fake. It looked physically identical in every way (metal walbro stamped into case too), but I dunno how to spot a fake from a real.

Question: How does a fake pump suck vs an original? I know at 12.0 V, the old pump will kick out over 100 PSI and scream very loudly doing it. Wouldn't be hard to do same test on new pump to see what it will do. Each pump has separate wiring so not hard to shut one off and turn on the other.

A little off topic and I've never done a dual pump setup so this maybe way in left field and subject to ratification by experts but:

Both outputs feed into the one line which is going to restrict flow volume unless you've increased the diameter.
That tee is going to allow pressure from one pump output to bleed back through the other pump when one of them is off.
Any differential in output between the pumps is going to compromise the weaker pump.

Maybe the difference in sound you hear is caused by back pressure on the weaker pump. :dunno:

Both pumps have a 1 way valve so fuel can't bleed between the pumps. Also both pumps run at the same time normally. I just could hear how different they sounded so I unplugged one and then the other to listen to the difference.

Will test again with 14V and probably do a fuel pressure test on each pump just to verify they can both kick out fuel at 100 PSI. As long as the second pump boost my fuel delivery by about 20% I should be ok, pretty sure fuel pressure was falling off causing me to max out my ID 1000s.