Have a 2004 4.8lsx swapped into a obs truck.
Behind a 5 speed.
Got everything up and running and im having a bit of an issue.
(Computer has all emission stuff and what knot deleted. Throttle is NOT drive-by wire)

In first or second, when I call on it, it pulls SO HARD< but then around 4 or 4500 (no tach yet) the engine literally cuts off. Its hard to explain but if feels like I let off the gass and no later than half a second the engine comes back to life and pulls to the rev limiter.

It is wierd. And sometimes it would do it twice, once in the 4's and once i 5's im guessing.
Its not like the timming is being retarded but literally like it just cuts off.
Half throttle or 1/3 and it pulls all the way through, but anything more than half and it does that limp thing.

Checked codes and one if low knock senser voltage (going to fix)
and p0507 which should be engine idle speed higher than exspected.

Tried crank senser releard procedure and it said that a
"Nuetral" signal was not found (as in transmission)

No vacumm leaks (did the whole smoke machine thing)

So what is causing the ecu to freak out and fall slap on its face?
Does the ecu think i am in park with an auto trans and when im driving it is just me revving the engine while not moving?
Also I do not think the ecu has a vehicle speed signal. That be a problem?

 

abuse mode . may be your problem. no VSS. i had this problem with mine. had it bench tuned.

 

So do I need to feed it a VSS?
My transmission is a nv3500.
Any thoughts on how to?
Im pretty sure the signal goes from the transmission straight to the speedo.
Can I tap into it?

 

bumpity bump

 

Update:
Got VSS into the ecu and it still happens. No difference.
Hooked up fuel pressure guage and watched it during 1st and 2nd gear pulls and when the engine 'cuts off' the pressure drops to about twenty.

I literally put in a new pump 5 days ago, so im thinking its the old filter?
Anyone have any other Ideas?

 

could be your fuel regulator

 

Tank and or cap vented properly?

 

Both the regular and the vap system are ok.
replaced the fuel filter and that didnt do anything.
Fed the ECU 'drive' signals from a fake transmission and that didn't do anything but make it worse.
Swapped fuel pump relays and still nothing.
I suppose it must be the ECU/
Something is causing it to cut off.
It also idles LIKE it has a vacumm leak, (up and down and up and down rpm wise) but it has been checked and confirmed no leaks)
MAF? Ran the engine with the MAP only and it did EVEN WORSE....

It has GOT to be software, all hardware has been checked (I think).
Any other Ideas?
Was looking forward to driving it tomorrow for a special occasion but it looks like I wont be....

 

"Bump" this over to the tune/diagnostics forum?? likely to get more response[s].

 

I really can't think of anything in the tune doing it. The the pcm shuts off fuel for over rev or abuse, I'm pretty sure it just cuts injector pulse, not the pump itself.

I would check all grounds and power wires to the pump. If they look good, I would jumper the relay to the pump and test drive. If it drives fine with the pump hard wired then maybe a PCM problem. If it still doesn't run fine, then I would suspect your fuel system is inadequate.

Also thinking a bad PCM ground could cause this, I would check those out as well.

 

How do I check PCM ground? look at the pin layout and find ground on th epin layout and make sure its good?
And how do i force the power on to the pump?

 

Yes, that is probably the best way, there are several grounds though so you would have to check them all.

For the pump, you would either have to bypass the relay altogether or power the relay such that is is on all the time. Hard to say the bast way for you to do it without seeing your setup.

 

'Hot wired' My fuel pump and it still cuts off.
So it has to be something in the fuel system.
Regulator is fine.
So the pump is either bad or having a voltage problem.
Will do a voltage-drop test on the lines to confirm either or.

 

I think you have a dead spot on the TPS..

 

Bingo!!!

What truck and pump? If it's a TBI pump it is inadequate, new or otherwise.

 

1995 regular cab long bed.
Already did the fuel pump replacement to a walbro 255 or whatever the genaric universal one is for non-tbi fuel injection. 65psi or something like that it is suppose to be at.

Still in class but will do the voltage drop test and check all grounds when I get home. Will re-securre the ground thats beside the tank (clean the connection and the conductor)

Any other advice?
Hopefully this will fix it!!!!

 

Pretty sure if that was the case, the fuel pressure wouldn't drop 40 pounds.

 

It sounds like you are on the right track. If you have good pressure when you turn the key on (don't start)(~58psi), I would assume that the pump connections are good. I would more suspect a restriction in the line somewhere, kinked line, pinched hose, plugged filter, something of that sort.

 

Well thats the thing. I replaced all of the lines and the filter. all with new stuff.
So My guessing would be that the ground or the connection on the pump isn't that great. Its good enough to work under light load but when the FPR see's low vacumm when I hold it wide open, and closes as much as it can, the pump has increased resistance and building more pressure, and therefore takes up more amperage, which can be restricted if voltage drop is present because of bad conductors or connections.

The only thing that I did not replace would be the metal line in the fuel tank assembley that goes from pump > top of tank > filter. The rest is brand spanking new.

But of course I will triple check.,

Oh my goodness though, can't wait to get this fixed and the ECU retuned. It already pulls like a raped ape and its not even running 'correct'.

 

No help, but that sounds an awful lot like the issue I have with the 6.0 in my Diamond T. I'm working doing the same things you are...

Brian