HarleyHauler

I don't know if anyone has experienced this or not so here goes.

I installed the GDP Fuel Boss mechanical pump about 18 months ago. I have the Commander monitoring FP (with the sending unit in place) and have not had any issues until this weekend. I am usually seeing 17 PSI regardless of the RPMs.

This weekend the pressure started varying, dropping as low as 2 PSI at idle, but not staying there - just bouncing around. Once I get up over 1700 RPM it goes back steadily to 16-17 PSI. I tightened the drive belt a bit and changed the fuel filter. It was OK for about 1 day, then started doing the same thing all over again.

I placed a call to GDP, but no reply yet. Has anyone seen this before? I really don't know how to validate what component is causing the problem (sending unit or pump) without having to install a mechanical pressure gauge and then monitoring.

All suggestions are welcome!

HarleyHauler

Come on folks! 30 views and no insight? Someone has got to have some suggestions?!

xtoyz17

First thing is first, need to verify with another gauge. It's the cheapest, easiest test you can do. My guess it the sending unit bad, I can't think of a single thing that would cause a mechanical pump to have fluctuation pressure unless your engine is somehow completely stopping and starting again.

-PowerWagon-

Here is what you do..... Take the bypass valve apart and clean it.
I fought this for a while and usually found a small piece of debry holding
the valve open....it doesn't take much. I would just pinch the fuel line before and after the valve so I didn't take a diesel bath. A good idea is to take notes
on how you take the valve apart. Watch the spring and shims!

You said you see 17 psi..... even at WOT? Mine idles at 9 psi and jumps to 30+ psi if i just look at the go pedal.


Great choice in bikes....I have an 85 FXR and my Dad has an 88!

HarleyHauler

Thanks for the info - I will definitely try that out first. I am a detail-oriented guy (engineer by schooling) and will photo-document everything to avoid messing it up.

I only see a max of 17-18 PSI WOT and at idle (typically). Since I installed it there has never been a drop below 14 PSI except when first installed (small issue corrected through GDP). Now it seems to be RPM sensitive which it never was before.

As for the bikes, Cool connection! I 'inherited' my '87 from my brother-in-law when he passed away. Numbered bike (389 / 750) due to the Custom from the factory paint - all tin and frame the same blue color.

Thanks again for the suggestions. I will post the findings!

HarleyHauler

I agree, but my concern is having another gauge (in the cab) to really see what is happening as I ride down the road.

The only thing I can think of with the fluctuation in a mechanical pump is slop in the gearing that becomes more sensitive at lower RPMs.

TORQUE BEAST

Have you updated the commander with the new fuel pressure sender?

AH64ID

I would guess the pressure regulator is bad over the gauge sender... But a test gauge would tell you for sure.

HarleyHauler

When you say a 'new' fuel pressure sender, how new is new? I got it when I bought the Commander about 2 years ago.

HarleyHauler

As an update for anyone who wishes to add some more thought to the matter.

I disassembled and cleaned the bypass valve but that did not seem to change anything. I have driven about 150 miles since this started and still there is low pressure readings (less than 4 PSI) seen at lower RPMs (less than 1500) and pressure stays pretty constant at about 16-17 PSI while turning higher RPMs (1800 or so).

There has NEVER been a stall / sense of no fuel available even though the pressure will sometimes go to zero on the gauge. In addition, sometimes the pressure readout says Zero and then goes to 52 or higher values!

I am considering taking the sending unit out and cleaning it (maybe debris in the sensor?) but I am open for other thoughts. I am down for two weeks due to travel so it will take some time to try the next set of solutions.

Thanks for the assistance!

Sixpack73

those older quadzilla FP kits have been known to have problems. Quad now has a new design out that corrects the issue. More than likely, that's what the problem is. I got my FP kit about a year ago and its having the same problem but I have not put up the cheddar to get the new one yet.