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Fuel pressure dropping on ‘demand’

Off Topic Discussion

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Hi guys, I am currently struggling with the fuel pump on my Grasstrack sidecar. It is dropping pressure under high rpm. I have compared the lph flow rate (133lph) of the fuel pump to the amount of fuel my injectors require (about 78lph at full throttle). Does anybody have any suggestions as to why this may be occurring? The fuel pump, and fuel pressure regulator are brand new. Please see attached video. Thanks, Tom

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Is that a return-style regulator? Any chance it's not plumbed correctly? What size are the fuel lines -- they look very small.

Can you do a test to see if the pump can deliver 78 lph at 4-bar (that would be 0.325 liters in 15 seconds).

It is a return style system yes. I am beginning to question the fuel lines/plumbing myself, however my tuner said he did not struggle with this issue which is confusing me. I will be in the workshop again this coming Thursday so I will post some pictures of the plumbing then for you to have a look at. The fuel lines are what match the outlet from the fuel pump; 5.6mm ID. It’s the join pieces that I am querying as their internal diameter is tiny (about 3mm ID), and they are also at a poor angle which is going to disrupt flow. Thank you for replying

I think you're on to it. 3mm ID will not hurt pressure, but it will hurt fuel volume and that looks like what your issue is.

If the ECU monitors fuel pressure, your tune may be compensating for the fuel pressure drop. And if you fix it, it won't have to do the compensation. If however, the ECU doesn't monitor fuel pressure and the tuner has "baked" this error into the tune, then you may need to revisit the fuel map when you've corrected the problem.

I had an issue with an adapter fitting on a high-volume pump and while it made pressure, it couldn't produce the volume that another identical pump (with a different adapter) did. The final difference was the fitting.

Pull the return hose off, put it in a jug and run the pump for exactly 1 minute, then you know how much the system can flow at maximum pressure rather than guessing.

Hi guys, sorry for the delay I haven’t been in the workshop till today. I have taken a video of fuel line routing which I will attach. I have calculated how fast my fuel pump should pump 5 litres (as that is max capacity of fuel tank). Which is 2 minutes 15 seconds (using the 133lph flow rate advertised). When I tested this via the fuel return line from the system it only took 1minute 47 seconds. So essentially my pump is over achieving, so to speak. I also checked the internal diameters of all fuel line attachments and the smallest is 4mm ID at the entrance to the secondary fuel rail. Now this could be causing a restriction introducing fuel to the secondary fuel rail but...

The bike has two maps on it; one to run it on petrol to warm the bike up and run up after a race meeting has finished, and another map for methanol (hence the need for the secondary fuel injectors which weren’t originally on the bike). Now initially I only noticed the fuel pressure drop on the methanol map, which is what led me to believe that there was an issue either with fuel pump, pressure regulator or the fuel line plumbing itself. However today whilst warming it up on the petrol map, when I was blipping the throttle I also noticed a fuel pressure drop, which I thought was unusual considering the petrol map requires far far less fuel. Could this be because of a battery limitation? As in not enough current being able to be provided to power all ancillaries? Could there possibly be a need to have a secondary battery? Or could I just have a clapped out battery (it does measure 13.7 volts). Any opinions will be greatly appreciated as I am currently at a crossroads with what I think is the issue. Cheers, Tom

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Update: it’s not the battery. Re doing fuel lines and seeing if that makes a difference