I have a 75 280

 

I've just fitted Throttle bodies and mapped injection along with a whole lot of engine work/mods

 

A success pushing 245 bhp but every so often (well more often than not)) it holds back momentarily and surges back and forth as if it's losing fuel pressure. However it runs fine on the rolling road when the car is stationary

So....What is in the tank that could possibly be interrupting the fuel delivery? Something come lose maybe?

Mine stumbled up until 2,500rpm. A potentiometer in the coolant sensor wires fixed that for me. $5 from radio shack? Google that for more info.

Could also be the throttle position switch. Black box under the throttle body. It kicks on or off in between 2,000 and 3,500rpms, rough guess. Google that too.

Good luck.

Thanks for your suggestion but its not running any of the standard factory system

It's all after market modern stuff fully mapped.

Process of ellimination but I have to start in the tank I thing

1 hour ago, Vipergts said:

every so often (well more often than not)) it holds back momentarily and surges back and forth as if it's losing fuel pressure. However it runs fine on the rolling road when the car is stationary

So....What is in the tank that could possibly be interrupting the fuel delivery? Something come lose maybe?

"Mapped injection" implies fuel injection  although I've never heard it described like that.  So you have a high pressure fuel system.  I would put a gauge on the system and see what fuel the pressure does.  Don't assume it's a tank problem

And I would not sit still on a rolling road.  It sounds dangerous.

Seriously though, what are you running?  Throttle body injection with Megasquirt?  

Edit - actually I meant individual throttle bodies.  ITB's.  Since EFI systems generally have at least one throttle body.

Edited March 14, 20168 yr by Zed Head

I assume by "Mapped" you mean you are running a MAP sensor? Do you have an Idle Air Control valve? Closed loop or open loop?

Getting too technical for me talking loops etc   I'd have to ask my engine man

 

The car has ITB's  Jenvey 42mm and a link management system. Adjustable fuel pressure regulator

The way I would define loops, is open loop runs on a program based on throttle position and sensor feedback. Closed loop runs on a program but uses an o2 sensor to modulate fuel. When my suburban is warming up it runs on open loop and surges some. When the truck heats up and switches to closed loop the surging goes away.

Zed is probably right, fuel pressure would be a good place to start. If it bounces around the car will surge because the ECU is trying to prevent stalling then it catches, back and forth...If it was mine I would plumb in a fuel gauge in a permanent location. On the fuel rail or somewhere under the hood...

Edited March 15, 20168 yr by Patcon

Yes I'll hook up a gauge but I'm in no doubt the pressure is dropping causing the power loss but I need to find out why it's momentarily losing the fuel

 

3 hours ago, Vipergts said:

Yes I'll hook up a gauge but I'm in no doubt the pressure is dropping causing the power loss but I need to find out why it's momentarily losing the fuel

You're assuming that the pressure is dropping.  But engines can surge with no fuel pressure change.  You have an electric fuel pump, powered by the battery, not connected to what the engine is doing except for the vacuum reference (if Jenvey even uses one).  That's why measuring pressure is important.  Disconnect the vacuum reference when you're measuring to be very sure.  The engine will run richer but it will remove any questions for you.  Edit - actually it may not.  You might have a surge caused by the air-fuel ratio.  Still though, the numbers will tell you something.

More detail than your description would help a lot also.  A video would be great.

" it holds back momentarily and surges back and forth "

Here's some Jenvey stuff - http://www.jenvey.co.uk/products

And Link - http://www.linkecu.com/

Edited March 16, 20168 yr by Zed Head

OK

 

I'll keep you posted with developments but does anyone know what the fuel pick up looks like inside the tank?

 

Is there a swirl pot inside the tank for example?

Yes, the EFI tanks have "baffle" or "swirl" (I've never heard that term ) type pot to keep fuel around.