starts breaking up at 4500 rpm.
 

here we go. 89 mustang 5.0 stocker. changed from the stock tank and lines to a fuel cell and an external pump. starts breaking up at 4500 ever since. the only thing I haven't replaced is the mass air flow meter and the cell. it's an 4 gallon aluminum cell. no foam. --10 line from tank to the pump. -8 line up and back. I change over to a 4bbl intake and carb and it runs great. any ideas. i'm to the point where i'm ready to put the stock tank back in.

Re: starts breaking up at 4500 rpm.
 

Check fuel delivery first. Line out of regulator into a jug, how long to pump a gallon?
Make sure return is not putting air into pickup area?
Good voltage to pump at RPM?
Once you are sure you are getting fuel into carb, then start down other paths.

Good luck.
Ken

Stock 412

Re: starts breaking up at 4500 rpm.
 

This came with injection correct?
It breaks up with the injection but runs OK with the carb may be that it is running out of fuel. What is the pressure? Had the mass airflow sensor fail on my Buick and it would be ok under light throttle but was lost accelerating under load. It was fine one day and bad the next. At first I thought it was the gas I'd purchased but quickly discovered new fuel wasn't the answer. I'd think fuel first as Ken said. Does it break up even in neutral?
I'm sure the other guys here know way more on this than me and can assist. My niece had a Mustang Cobra but it has been a few years since that was sold.

Re: starts breaking up at 4500 rpm.
 

The fuel returning to the tank can really aerate the fuel in the tank if you just let it dump back in. Try extending the tanks return fitting so the returning fuel is submegered....like a pick up tube in reverse.

Re: starts breaking up at 4500 rpm.
 

Just for grins, check your high end chip. I've seen this happen twice in recent years.

Re: starts breaking up at 4500 rpm.
 

Doesn't fuel injection take a higher pressure pump than a carb car or is there just a difference in the regulator. I was under the impression that fuel injection needs a lot more pressure than a carb

Re: starts breaking up at 4500 rpm.
 

Agree - injectors need a lot more fuel pressure to work properly, and it needs to stay high under load.

Re: starts breaking up at 4500 rpm.
 

40 psi for efi vs. 7 for carb.

Re: starts breaking up at 4500 rpm.
 

As others have suggested, have you verified fuel pressure? Is the regulator the correct one for EFI? An -8 return seems big but should not be an issue if the regulator is working correctly (possibly returning too much fuel). What does the O2 sensor read during the event?

Re: starts breaking up at 4500 rpm.
 

Are you changing pump setups between the 2 systems? Your pump my not be putting out enough flow at the higher pressure 40 and above, but put enough flow at 7 psi for a carb. I have run as fast as 11.60 with all the stock fuel lines in my car with just one size up pump. This is also a 5.0 mustang.