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Leaning out after new fuel pump

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My stock fuel pump was making a hell of a noise so I got a Walbro GCA3386-2 (255LPH)

The mounting is different from stock. With this the strainer attaches directly to the pump while the stock pump sits about half way down the mounting bracket with a pipe to the strainer at the bottom. So with this the pump has to be mounted at the bottom of the bracket. 

Not sure what is the cause but on boost the car is running very lean. AFR is showing 18:1

On normal driving its fine.

The car has a Haltech fitted. Am I supposed to get it remapped after fitting the pump?

Do i need to change the FPR from the stock one?

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Was it a stock pump that was removed? if you have gone to a smaller pump that will cause the lean issue.
The walbro also require a wiring & relay upgrade as they draw a lot more current than the standard pump.

No need for retune after a pump swap but if its going lean check vac line to fuel reg, pump part numbers and that the filter on the pump isn't blocked.

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Thanks Rob. I didnt know it requires a wiring and relay upgrade. Is it to do with the "relay mod" that some other threads mention?

Can you give any more detail on the wiring upgrade?

The vacuum line from the intake mani to the FPR  is fine. 

The new pump came with its own strainer. So that is new. I forgot to change the fuel filter in the engine bay. Will change that too.


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10 hours ago, Die$eL said:

Can you give any more detail on the wiring upgrade?

It's just a case of giving the pump a decently thick power feed (and decently thick earth to go with it!) that is no longer passing through the original fuel pump controller. The relay allows you to switch the power to the pump however you like/need (ie, possibly still with the fuel pump controller) but the relay makes the power path direct from a main 12V +ve to the pump. Fatter wires to carry the current without burning up or causing voltage drop problems.

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15 hours ago, Die$eL said:


Can you give any more detail on the wiring upgrade?


Generally I use a 50 amp relay, 40amp fuse with 5mm wire for positive and 3mm for negative. Either use an uninsulated crimp and pot the factory cap connection or use a new bulk head



Most big pumps like the walbro 460l will draw 22-27amps at full load so I like to make sure it's up to the task. Even the old 255l will draw 14amp at idle and 22amps at full load. 

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Thanks for the detailed info. This really helps.

I will upgrade the wiring, relay and change fuel filter then report back. 

Edited by Die$eL
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A Big thank you for all of the advice. 

I've upgrade the wiring and this has resolved the leaning out issue. 

I've done a video detailing what I did and bits used etc. Hopefully it can help others who experience the same issue.


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On 9/26/2020 at 11:42 AM, White GTS-T said:

Did you change the fuel filter at the same time @Die$eL? Because if you didn't, there's a fair chance that is your problem. The filter will be full off contamination, including brass from the failed pump.

I've decided to run the car for a little before changing the filter just incase there was any contamination which got in while i was working in there. 

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