Fuel Pressure Fluctuations

[cocoaspud]

I've just completed installation on an RCC stage 1 Turbo kit. I've started the bike but haven't ridden it yet or had it tuned. The PCIII has the RCC map in it.

I added a DynoTune electronic fuel pressure gauge to the bike today and the fuel pressure is fluctuating a bit.

At idle, the pressure will read 43.5 and as the gauge updates every second or so the pressure will hover around 43 give or take 0.5. But every so often, say around 4-6 updates, it will drop down into the 10-25 range for a single update before going back to around 43.

I held the throttle at 2000 and then 3000 rpm and saw about the same behavior, except that values in the 17-25 range happen a little more frequently. Sometimes the pressure holds steady at 43ish for 15-20 seconds before jumping around a bit.

I don't see any kinked fuel lines and the bike is idling ok.

If I turn on the ignition and watch the gauge during the few seconds the fuel pump runs, the pressure goes up to 36 and then drops way down after the pump stops.

This is my first turbo and I'm not sure what is typical for fuel pressure. Is this kind of fluctuation normal?

2005, stock injectors, basically all stock other than the RCC kit.

 

[OZBooster]

New kit ?
does the pump sound like its load is varying, or is the noise fairly consistant

maybe try an analog gauge

 

[cocoaspud]

It's a brand new kit. The kit came with a mechanical boost gauge and for awhile I had an AutoMeter mechanical fuel guage connected up. I used the mechanical gauge to set the initial pressure to 43.

After reading here about needle fluctuations and short lifespan for mechanicals, I decided to switch to the electronic gauge as recommended by someone here.

To my recollection, the mechanical gauge was jumpy but basically centered around 43.

I can't hear the pump while the bike is running, but it sounds steady when I turn the ignition on for the few seconds it runs.

 

[busataz]

I just got my rcc stage one kit on and done and it holds dead at 42 psi all the time.

 

[cocoaspud]

Thanks Busataz, I guess that answers the question of whether this is normal

For power, I wired the gauge to that accessory light plug on the right side under the front cowl. Both power and ground, if that matters.

 

[POWERHOUSE]

I would hang another gauge on and run both at the same time. If both gauges reflect your previous results, I would say something is not right. If not, I would say your electronic gauge is not right . . .

 

[cocoaspud]

Thanks for the suggestion. I'll try that. I just wanted to make sure this was really a problem before I took off the fairings again.

 

[RYC1966]

Are all the lines unobstructed? From the pump to up front and the return? That drove me nuts for an hour or so...and the cause was a line that would get kinked when I pushed down on the tank.

 

[cocoaspud]

Last I checked all lines were unkinked, no sharp bends even with the tank down. I'll double check though, the tank has been up and down several times during various tinker sessions.

 

[cocoaspud]

Raised the tank, visually checked fuel lines and there were no problems that I could see. Started the bike, same problem.

Disconnected the electronic gauge sending unit and connected a mechanical gauge, and the needle barely fluctuates. The needle looks like it's vibrating even if I hold the gauge in my hand (not hard mounted to the bike). Still, I'd say the mechanical gauge indicates consistent fuel pressure. If the pressure dropped to 10-25 every few seconds even for a brief moment, I'm assuming I'd still notice a variation of the needle.

I guess I have an electrical connection issue, or a defective unit.

 

[cocoaspud]

I connected the gauge power/ground directly to the battery and now the pressure is steady.

Thanks for the suggestions guys. I guess I have a wiring problem.

 

[RYC1966]

I am just curious to what type of gage you have...or what is available. I have a pro comp liquid deal mechanical...with the elec gages do you have a pos / neg + the wires from the sensor fitting? it seems that you would have to power the gage...I am considering switching since the gage readout is a smaller size.

 

[cocoaspud]

As recommended by someone else here (can't recall who) I'm using this gauge:

square fuel pressure Gauge, velcro anywhere, size of a keychain remote. Digital fuel pressure gauges in a square package will read fuel pressure in PSI. - square fuel pressure Gauge, velcro anywhere, size of a keychain remote

The gauge arrives in two pieces, the sending unit and the gauge. Both pieces have 4 wires with ample cable length for a bike install. All 4 wires from both cables get joined together (white-to-white, black-to-black, etc), and two of them also go to pos/neg.

The sending unit screwed right in to the fuel rail insert provided in the RCC kit, no adapter needed.

 

[POWERHOUSE]

As recommended by someone else here (can't recall who) I'm using this gauge:

square fuel pressure Gauge, velcro anywhere, size of a keychain remote. Digital fuel pressure gauges in a square package will read fuel pressure in PSI. - square fuel pressure Gauge, velcro anywhere, size of a keychain remote

The gauge arrives in two pieces, the sending unit and the gauge. Both pieces have 4 wires with ample cable length for a bike install. All 4 wires from both cables get joined together (white-to-white, black-to-black, etc), and two of them also go to pos/neg.

The sending unit screwed right in to the fuel rail insert provided in the RCC kit, no adapter needed.

They have a nice digital boost gauge to match . . . .

 

[cocoaspud]

I just wanted to add a follow up for my gauge problem.

After some searches here and on sh.org I found two other posters with the same problem (ie: fluctuating pressure readings using a digital gauge). One of them mentioned that a smoothing capacitor was needed to fix the problem. In my case, hooking power up to the battery worked fine since the battery apparently acts like a giant capacitor. Hooking power directly to the accessory plug in the front cowl gave me the fluctuating readings.

Since I wanted the gauges to switch on/off with the ignition, I ran power from the battery to a relay with an inline fuse and used the accessory plug in the front cowl to trigger the relay. At some point I'll probably add a fuse block so that I can plug in other things (cell charger, etc).

The fuel pressure gauge works fine now and it fits nicely up under the cowl next to the BoostBySmith shift light