Fuel Pressure Woes

Terminator2

Terminator2

Site Supporter
Joined
Jun 27, 2016
Messages
20
Age
70
Location
Auburn California
Bike
2006 ST1300
Looking for some input. Smelling some fuel vapor (unable to locate source). Riding the bike up a mountain, attempt full throttle and bike starves at 6800 rpm. At home, I warm up bike, able to rev to redline. Idles properly. Installed fuel pressure gauge. Key on....58psi. Engine start/run...50-55psi with fuel gauge bouncy needle. Key off, immediately drops to 45psi. Quantum fuel pump has 30k on it now-5 years old. Volts to fuel pump 0.5 volt less than at battery engine running. On the freeway she'll rock n roll.

I am only a 'B' student, so all of you A+ feel free to chime in :). Appreciate your input. Hope all your riding days are going as planned. Thanks, Rick
 
Since you installed a Quantum fuel pump the first thing that comes to mind is the issue of the inferior in-tank fuel lines restricting fuel flow that has been reported. Did you install the quantum fuel lines?
The fuel pressure is within specification but that doesn't mean that the volume being delivered is. The specs for that test are in the manual.
Based on your fuel pressure readings it doesn't appear to be a fuel pressure problem. It seems to be more of an engine management issue. It can be as simple as a dirty air filter. At higher elevation more air volume is needed to achieve the same result. The effects of a badly restricted air filter become more evident if that extra volume can't be fully realized. It can progress from there to anything that affects the ECM's ability to correctly read and adjust for current circumstances like a blocked 5-way Tee, a failing map sensor, blocked fuel vent lines, charcoal canister vent blocked, etc.. You didn't state anything about trouble codes so my first recommendation would be to check to see if there are any trouble codes stored if you haven't already.
 
Fuel lines, as Andrew mentioned, are worth a look... we've seen other aftermarket fuel pumps with non-fuel-submersible-rated rubber hoses.
 
I always suggest when testing pressure to leave the tester hooked up to monitor drop over a period of time, like an hour or 2, with the engine off. This is to ensure there is no leak down at the injectors or pressure bleed off in the regulator. Does the bike fire up right away or does it have to crank for a bit after sitting for a while?
 
Bmacleod said:
I always suggest when testing pressure to leave the tester hooked up to monitor drop over a period of time, like an hour or 2, with the engine off. This is to ensure there is no leak down at the injectors or pressure bleed off in the regulator. Does the bike fire up right away or does it have to crank for a bit after sitting for a while?
Click to expand...
Requires a little crank time. The fuel pressure leakdown seems normal - a couple hours to drop 20psi. The FI light came on running up the mountain and stayed on until I parked for couple hours. Since leaving the top of the mountain last week the FI light has not come back on. The FI light has come on twice in 10 years, both in the last month.
 
Terminator2 said:
Requires a little crank time. The fuel pressure leakdown seems normal - a couple hours to drop 20psi. The FI light came on running up the mountain and stayed on until I parked for couple hours. Since leaving the top of the mountain last week the FI light has not come back on. The FI light has come on twice in 10 years, both in the last month.
Click to expand...
When you say " a couple of hours to drop 20p.s.i." do you mean it stops at 20psi drop? does it drop lower overnight? 20psi drop is acceptable. When you turn the key on, the pump only runs a few seconds. If that few seconds isnt enough to bring the pressure back up to spec, it could make the bike hard to start. Try turning the key on until the pump stops and then turn the key off, wait a few seconds and key on again to prime the fuel system a second time and see if it starts quicker/easier. If you have your pressure gauge on, take note of the readings at each prime. There are 3 places where pressure can bleed off... sticky injector (check for gas in the oil), the regulator or the pump (it contains a check valve to maintain pressure). As mentioned earlier, the quantum pump kits have been known to have defective hoses included with new pumps and the hoses degrade and leak. This can cause leakdown at the pump as well as fuel starvation, starting under load and getting worse as it degrades more. You mention bad running under load. If it were me, I would be starting to look at the quantum pump and possibly the hoses.
 
April 8 2025. Me and Spock return to the flight deck (oh-nevermind). I thought I had the situation licked, however, within 2 blocks of a fuel fill, the FI light came on. I rode 18 miles up the mountain, parked for 10 minutes, start engine FI light off. Sheesh. When I installed the Quantum fuel pump I used Oem fuel hoses, filter, gasket. This one has me beat boyz. Thanks.
 
I wonder if you have a fuel venting issue with the tank. When it acts up, try opening the cap and see if that helps.
 
Did you disconnect and plug the vacuum line from the fuel pressure regulator when you checked fuel pressure?

Honda specifies 50 psi with the line off and plugged. This simulates wide open throttle fuel pressure.

If you checked with the line connected, your pressures seem to be higher than normal and I would look for a vacuum leak in the lines from the regulator to the throttle body. There are 3 hoses in total.
 
Terminator2 said:
April 8 2025. Me and Spock return to the flight deck (oh-nevermind). I thought I had the situation licked, however, within 2 blocks of a fuel fill, the FI light came on. I rode 18 miles up the mountain, parked for 10 minutes, start engine FI light off. Sheesh. When I installed the Quantum fuel pump I used Oem fuel hoses, filter, gasket. This one has me beat boyz. Thanks.
Click to expand...
The FI light has nothing to do with the fuel pressure as there is no fuel pressure sensor. The FI light is telling you that the computer is missing an input from a sensor or the input is out of range.

You need to use the diagnostic procedure to read the fault code. An assumption would be that you have dodgy wire connector or leaking vacuum hose that is throwing the fault code and then the FI system is moving to the default (rather than optimised) settings, and that caused the mountain climbing issues.

https://www.st-owners.com/forums/th...-mil-malfunction-indicator-lamp.133950/unread
 
Last edited:
Terminator2 said:
Requires a little crank time. The fuel pressure leakdown seems normal - a couple hours to drop 20psi. The FI light came on running up the mountain and stayed on until I parked for couple hours. Since leaving the top of the mountain last week the FI light has not come back on. The FI light has come on twice in 10 years, both in the last month.
Click to expand...
Fuel pressure seems normal even after 1 hour but once it get to 20psi just turn the key on. The pressure should immediately hit 50 psi . Some malfunctioning fuel pumps would take longer to get to 50 psi but this would also be accompanied with a long crank on start ups. You need to check on the FI light. For giggles how old is the fuel filter?
 
Al st1100 said:
Fuel pressure seems normal even after 1 hour but once it get to 20psi just turn the key on. The pressure should immediately hit 50 psi . Some malfunctioning fuel pumps would take longer to get to 50 psi but this would also be accompanied with a long crank on start ups. You need to check on the FI light. For giggles how old is the fuel filter?
Click to expand...
Fuel filter oem same age as Quantum fuel pump 4.8 years-30k miles.
 
You must log in or register to reply here.
Back
Top Bottom