Truck accelerates like a rocket just before running out of fuel

[Full Monte]

Here's a great brain teaser for some of you geniuses. Yesterday I was making a run to the dump with the '88 IDI (truck 2 in signature). I knew I was almost out of fuel in the rear tank, but didn't switch it in time. I was going up a grade inside the dump at about 25mph with constant foot pressure on the pedal when the truck accelerated VERY quickly for about 5 seconds just before it ran out of fuel. I switched tanks and all was cool. Since then, I've been wondering why I got that rocket-in-the-seat-or-the-pants feeling just as it was running out of fuel. What caused it? Can I reproduce the performance when I'm not running out of fuel? I'm baffled.

 

[camo85]

This subject has been up before, none can seem to come up with a logical reason. I have no idea myself, but I know my '85 idi does the same when it runs out of fuel, and a few mounths ago when I let the fuel filter go too long it seem to have a kind of the same effect. I think maybe something with fuel air ratio, but who knows!

 

[smoking89IDI]

IIRC the pumps timing is affected by inlet fuel pressure, when you run out pressure drops and timing advances.

 

[camo85]

IIRC the pumps timing is affected by inlet fuel pressure, when you run out pressure drops and timing advances.

Never thought of that.... very logical possibility!!!

 

[94f450sd]

IIRC the pumps timing is affected by inlet fuel pressure, when you run out pressure drops and timing advances.

when the pressure drops at the pump it also drops at the injectors.so it will take a few extra turns of the pump to make up for the pressure loss before the injectore will pop open,retarding the timing.damn i hope that made sence

 

[seawalkersee]

I thought retarding the timing would work backwards and you would lose power. It is my understanding that the afr would make it go low on power when the fule is/was dropped out of the equasion.

SWS

 

[typ4]

IIRC the pumps timing is affected by inlet fuel pressure, when you run out pressure drops and timing advances.

This is the correct answer, If you look in the last diesel power about the db2 pump you will see how the advance works.
Once the case pressure goes away it advances the rotor, Now I really wonder how much power we are leaving on the table.


MEL any input on this.

 

[FordGuy100]

This is the correct answer, If you look in the last diesel power about the db2 pump you will see how the advance works.
Once the case pressure goes away it advances the rotor, Now I really wonder how much power we are leaving on the table.


MEL any input on this.

I've felt it to when my truck almost ran out of fuel once.

And running the pump out of fuel like that might help power, but isnt it bad for the pump?

 

[DeepRoots]

wouldn't this be a governing issue?
on detroits when fuel is cut off the injectors and rack go full throttle in order to try to maintain governed speed.
course I dunno if our ip has flywheights and whatnot

 

[typ4]

wouldn't this be a governing issue?
on detroits when fuel is cut off the injectors and rack go full throttle in order to try to maintain governed speed.
course I dunno if our ip has flywheights and whatnot

Completely different animal. But good thinkin. I can go into detail if need be. Dont have much time right now.

 

[Agnem]

The phenomena is indeed due to critical advance because of lack of inlet presure. There is no change to the fueling at the injectors. The amount of advance is past tolerances, hence while it does go like hell, the forces endured are destructive and would not result in an engine that lasted very long. You may have noticed there is a good bit of hammering to go with that brief burst of power.

 

[Full Monte]

Very interesting answers! There was no hammering at all....just a sensation like a rice-burner motorcycle taking off. As usual, you geniuses have answered the question very well. Thanks!

 

[seawalkersee]

I thought about this too. When you ask about the availability of power there, you must think, if we pushed (taking into consideration Agnem's response of it already being too far) the pump to that level, then there was a problem of fuel cavatation or something to that effect, you would be playing with something bad. I am not 100% on how the IP works but I think that the slapping of fuel, air, fuel, air, fuel, etc, you may cause more monitary problems than the power is worth.

BTW, I assume the fluid inside the pump keeps the pump timing as it has to push it. Leaving the air in the line causes the timing to advance right?

SWS

 

[subway]

i have had this happen to and it didnt sound bad at all just pulled HARD. is there a way to change the advanced curve in the injection pump like you can with weights inside a gasser distributor?

 

[Agnem]

The advance at higher speeds is internally regulated. It would take some plumbing and machining to get a pump to the point where you could tweak it manually.