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Unexpected loss of fuel

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Hi, I am tuning the fuel right now, and as I'm doing a WOT pull my AFR quickly reaches the target of 11 at about 3500 rpm and stays there up to 4500. But as I move on after this point, the AFRs quickly start going up to almost 14. I couldn't figure out what's causing it, so any help would be great. Attached is a log of a few pulls done in the same day.

Attached Files

Looks like your fuel delivery is struggling. When was your filter last replaced and how old is your pump?

What kind of pump is it?

Not sure about the filter, but the previous owner mentioned the pump being old. It's a walbro 255.

I've ended up adding a fuel pressure sensor to my dyno because of all the aging/copy pumps on the market, 2 last week alone needed real/new pumps

Here's a classic example of a failing pump, manifold pressure vs fuel pressure before and after pump replacement.

Attached Files

I'll go ahead and order another walbro. Thanks!

I agree with Chris, it sounds like fuel delivery is the probable issue. Any time you're adding fuel in the map and seeing no change in AFR (provided your injectors aren't maxed out), that's a good indication the fuel isn't there. As well as a potential dead pump, also check the voltage getting to the pump. I've seen many situations where the factory wiring might only provide the pump with 10-11 volts and hence the flow isn't what the pump is rated to. You should see full battery voltage of 13.8-14.2 volts at high rpm.

Replaced the fuel pump and everything works great now!

Really glad to hear it, like I said, I'm seeing it all to often now. Glad it was an easy fix

Yes, I would say this is the number 1 issue I come across when tuning cars. It's nice when you can log fuel pressure because every owner thinks their car is in 100% great condition and the car leaning out is simply a tuning issue.