2007 Gxi-G 5.0 Rough Idle Issue

[bruceb58]

Hi. I’m a bit confused how a gxi engine with multi point injection and not tbi injection would manage to dump fuel into the throttle body. Are you sure this is what’s happened ?

A multiport fuel injection system still has a throttle body.

 

[Tail_Gunner]

Hi. I’m a bit confused how a gxi engine with multi point injection and not tbi injection would manage to dump fuel into the throttle body. Are you sure this is what’s happened ?

well both TBI and MPI do have throttle bodies, but to the point your making it does bear mentioning..

 

[QBhoy]

well both TBI and MPI do have throttle bodies, but to the point your making it does bear mentioning..

Of course...But how would fuel get into the throttle body, on Mpi ?

 

[QBhoy]

A multiport fuel injection system still has a throttle body.

Of course. But how would it get full of fuel on an mpi ? It would surely be a serious and catastrophic event that would cause that. Litres of fuel it would take and literally turning into a bloody bomb.

 

[bruceb58]

Of course. But how would it get full of fuel on an mpi ? It would surely be a serious and catastrophic event that would cause that. Litres of fuel it would take and literally turning into a bloody bomb.

The same way it would on a TBI! No difference at all. Both would have fuel dumping into the intake sytem bypassing any injectors!!! A TBI system still uses injectors just like an MPI...they are just located in the TB itself. Dumping fuel into the TB just gets burnt up by the engine.

Same thing happens on a carburated engine when a mechanical fuel pump fails.

 

[ggc]

Just my theory. I dont think the fuel is being dumped into the throttle body. If the float and needle valve in the fuel cell reservoir are stuck, fuel from the fuel regulator fills the return hose. When that happens, the regulator cant bleed off excess fuel to control the pressure. At less than full throttle, the pressure regulator cant regulate properly so the fuel pressure in the rail is too high. That causes too much fuel to be injected into the cylinders. At full or near full throttle, the pressure in the fuel rail is lower because more fuel is being jetted into the cylinders. Engine runs ok because the regulator does not need to reduce fuel pressure.

 

[QBhoy]

The same way it would on a TBI! No difference at all. Both would have fuel dumping into the intake sytem bypassing any injectors!!! A TBI system still uses injectors just like an MPI...they are just located in the TB itself. Dumping fuel into the TB just gets burnt up by the engine.

Same thing happens on a carburated engine when a mechanical fuel pump fails.

Bruce. This is exactly my point. A tbi I can totally understand dumping fuel into the throttle body. But an mpi has injectors much further down to each individual intake. For fuel to be filling the whole manifold enough to come back up to the throttle body...Jesus Christ. Now that’s something. Surely you haven’t seen this happen. Let’s get real here. Not a chance. It’s never going to happen. If it did...it would be a first surely. And not to mention, literally a bomb waiting to explode. There would need to be litres and litres of fuel filling up the whole thing. I just don’t believe we are even considering such a thing. Totally ridiculous

 

[bruceb58]

Bruce. This is exactly my point. A tbi I can totally understand dumping fuel into the throttle body. But an mpi has injectors much further down to each individual intake. For fuel to be filling the whole manifold enough to come back up to the throttle body...Jesus Christ. Now that’s something. Surely you haven’t seen this happen. Let’s get real here. Not a chance. It’s never going to happen. If it did...it would be a first surely. And not to mention, literally a bomb waiting to explode. There would need to be litres and litres of fuel filling up the whole thing. I just don’t believe we are even considering such a thing. Totally ridiculous

My point is that it doesn't matter if its TBI or MPI. Doesn't matter WHERE the injectors are. That fact doesn't matter. If you can't see that, I can't help you.

 

[QBhoy]

My point is that it doesn't matter if its TBI or MPI. Doesn't matter WHERE the injectors are. That fact doesn't matter. If you can't see that, I can't help you.

I don’t think you’re getting this at all. I can’t explain any further. I asked how that could possibly happen. No one has told me an answer. The question of whether an mpi or a tbi has a throttle body was never asked or in question. We all know it does. The question was...how the hell would fuel fill up that much in an mpi to get to the level of the throttle body. There are no injectors there on an mpi. Simple really.

 

[ripjmk]

Just to clarify to the purpose of this float valve and vent to the "inlet manifold". It is a vapor/air vent to ensure the fuel cell reservoir stays full of liquid (fuel). If air (during initial start) or gas vapor (during a hot start) should accumulate in the reservoir the float drops and opens the small line to the inlet manifold. The vapors are vented thru this line during the priming sequence and as the liquid level rises the float valve shuts and stays shut during normal operation keeping the cell at LP regulator pressure (small relief valve in the reservoir spills back the LP pump suction at approx 5 psi) . If the float valve does not seat it will continuously pump fuel at LP fuel pump pressure to the inlet manifold. That is why it ran fine with the LP pump disconnected.

 

[bruceb58]

I don’t think you’re getting this at all. I can’t explain any further. I asked how that could possibly happen. No one has told me an answer. The question of whether an mpi or a tbi has a throttle body was never asked or in question. We all know it does. The question was...how the hell would fuel fill up that much in an mpi to get to the level of the throttle body. There are no injectors there on an mpi. Simple really.

LOL Likewise.

The fuel pump assembly fails. Instead of dumping fuel into the bilge, it sends the fuel to the TB assembly through a hose so the engine will burn it instead.

 

[QBhoy]

LOL Likewise.

The fuel pump assembly fails. Instead of dumping fuel into the bilge, it sends the fuel to the TB assembly through a hose so the engine will burn it

LOL Likewise.

The fuel pump assembly fails. Instead of dumping fuel into the bilge, it sends the fuel to the TB assembly through a hose so the engine will burn it instead.

that doesn’t dump fuel into the throttle body. Surprised you have said that Bruce. That goes into the manifold.

 

[FFR]

I think what he is describing is the float and needle valve at the top of the Volvo fuel pump cell reservoir between the low pressure pump and before the high pressure pump. It functions like a check valve and prevents the fuel being pumped into the reservoir by the low pressure pump from backing up into the hose returning excess fuel to the reservoir from the pressure regulator on the fuel rail. There are youtube videos on this (Man Cave Mechanic) if you are interested.

Exactly.

 

[FFR]

Just to clarify to the purpose of this float valve and vent to the "inlet manifold". It is a vapor/air vent to ensure the fuel cell reservoir stays full of liquid (fuel). If air (during initial start) or gas vapor (during a hot start) should accumulate in the reservoir the float drops and opens the small line to the inlet manifold. The vapors are vented thru this line during the priming sequence and as the liquid level rises the float valve shuts and stays shut during normal operation keeping the cell at LP regulator pressure (small relief valve in the reservoir spills back the LP pump suction at approx 5 psi) . If the float valve does not seat it will continuously pump fuel at LP fuel pump pressure to the inlet manifold. That is why it ran fine with the LP pump disconnected.

An even better description.