Fuel pressure----ODD
Thread Starter
Heel & Toe
Joined: Nov 2012
Posts: 23
Likes: 0
From: Greenville SC
Fuel pressure----ODD
So i have been suspecting my fuel pressure regulator of going bad and i would just like your thoughts. its a 93 LT1.When the gas tank is full it takes 2 or 3 tries for the engine to fire. As soon as i get close to half tank it always starts fine. I think its a pressure issue. The injectors where replaced about 3K miles ago by the last owner. Does this seem right? 
Thanks,
Thanks,
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
sounds like coincidence.
How can a full tank be detrimental to start up?
This is a pump issue. If the regulator were a problem, once broken they pretty much stay broken. They may leak inside but there is ALWAYS evidence. Besides, the true symptom of a bad reg is massive flooding. You wouldn;t be getting it started even if you towed it...it would gush raw gasoline into the vac system and intake and you;d know it !
Do this,
pull the vac hose off the regulator AFTER a start up attempt. See if there is wet fuel INSIDE the vac hose. If there is, the regulator is bad. If its dry, test fuel pressure. You have filters between the pump and injectors that are more likely to be at fault. Without knowing what the pressure is, ANYTHING is simply a guess. Diagnose with fact, not speculation. Thats too costly in time and parts.
You say its harder to start with a full tank? do you mean when its cold? or 1st attempt each day?
Hard cold starts after new inj are installed is a common complaint. It almost always comes back to a problem in fuel pressure, flow or a sensor issue that was undiagnosed before. After the crappy performance is cured with the new injectors the true tuning problems become more obvious and have to be addressed. One theory is that these bosch-III injectors bleed off over night and the first start of the day has to purge air from the rails and fuel system...so it takes 2-3 seconds of cranking to get it to fire. Mine will lite off immediately IF I turn the key to ON, allow the pump to cycle its 2 second run then shut off, turn key off and THEN crank it. Fires instantly every time. However, if I simply crank it and hold the key in cranking position it must crank for several seconds before firing. Every other start that day is flawless.
This is purging the air from the line and allowing pressure to build in the system again. A bad pump will also cause this IF it allows fuel to flow back into the tank thru the pump. Anyway, the extended cranking is often just the time it takes the fuel pump to replenish the volume and pressure needed to fire the engine. These systems won;t even think about firing under 25 psi. Your pressure HAS to be 40 when you turn the key otherwise THAT is what is causing the delay in starting. The regulator either works or it does not. Not alot of inbetween there. Its most often the pump unable to supply or build pressure. A good pump should be able to send much more pressure than you see on the test gauge. The regulator holds it down, not up. As long as the pump is good.I'd look at the filter and the pump. Both are normal wear parts on a Corvette.
How can a full tank be detrimental to start up?
This is a pump issue. If the regulator were a problem, once broken they pretty much stay broken. They may leak inside but there is ALWAYS evidence. Besides, the true symptom of a bad reg is massive flooding. You wouldn;t be getting it started even if you towed it...it would gush raw gasoline into the vac system and intake and you;d know it !
Do this,
pull the vac hose off the regulator AFTER a start up attempt. See if there is wet fuel INSIDE the vac hose. If there is, the regulator is bad. If its dry, test fuel pressure. You have filters between the pump and injectors that are more likely to be at fault. Without knowing what the pressure is, ANYTHING is simply a guess. Diagnose with fact, not speculation. Thats too costly in time and parts.
You say its harder to start with a full tank? do you mean when its cold? or 1st attempt each day?
Hard cold starts after new inj are installed is a common complaint. It almost always comes back to a problem in fuel pressure, flow or a sensor issue that was undiagnosed before. After the crappy performance is cured with the new injectors the true tuning problems become more obvious and have to be addressed. One theory is that these bosch-III injectors bleed off over night and the first start of the day has to purge air from the rails and fuel system...so it takes 2-3 seconds of cranking to get it to fire. Mine will lite off immediately IF I turn the key to ON, allow the pump to cycle its 2 second run then shut off, turn key off and THEN crank it. Fires instantly every time. However, if I simply crank it and hold the key in cranking position it must crank for several seconds before firing. Every other start that day is flawless.
This is purging the air from the line and allowing pressure to build in the system again. A bad pump will also cause this IF it allows fuel to flow back into the tank thru the pump. Anyway, the extended cranking is often just the time it takes the fuel pump to replenish the volume and pressure needed to fire the engine. These systems won;t even think about firing under 25 psi. Your pressure HAS to be 40 when you turn the key otherwise THAT is what is causing the delay in starting. The regulator either works or it does not. Not alot of inbetween there. Its most often the pump unable to supply or build pressure. A good pump should be able to send much more pressure than you see on the test gauge. The regulator holds it down, not up. As long as the pump is good.I'd look at the filter and the pump. Both are normal wear parts on a Corvette.
