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.

Re: starts breaking up at 4500 rpm.
 

If it's an Aeromotive A1000 it can do either carb or EFI, with just a regulator change.
How is the fuel pump positioned and where is it relative to the cell sump? Aerated fuel is possible if the elevation of the pump has to draw the fuel out of the cell. The pump should always have a column of fuel sitting on it, otherwise the vacuum it will pull can make the pump cavitate.

With the foam removed, the return could also aerate the fuel in the cell, unless as others have stated it has a length of hose to eliminate fuel dumping above the liquid level in the cell. With the foam installed that eliminates this issue.

What pump do you have?

Sean

Re: starts breaking up at 4500 rpm.
 

thanks for all the responses. was an aeromotive 750 pump and an Mallory regulator on a 94-95 fuel rail. changed to a aeromotive regulator on the rails. then changed to 87-93 rails and a aeromotive reg on that. fuel pressure starts at 40psi and drops to 36psi. then I put a holley pump on it and it did the same thing. voltage is steady at the pump. pump is mounted right below the cell. maybe to large a return line?

Re: starts breaking up at 4500 rpm.
 

You could try installing some sort of restrictor in the return line, just to see if that's the problem again -8 seems big for return. I have an -8 return on mine but it has a -6 to -8 adapter on the regulator when I'm using the EFI setup so it has a smaller hole in that fitting. Really should not be seeing a pressure drop, either your pump isn't up to the task or for some reason your injectors are flowing way more fuel than they should be. Have you verified your tune up?

Re: starts breaking up at 4500 rpm.
 

Where are you picking up the fuel pressure for the gauge? For control of pressure on Q jet car needed larger line for return than supply or the line created Higher pressure than regulator could reduce. Obviously with injection needs that is different but maybe where you are plumbed in with gauge isn't giving pressure at correct area for operation information. (I bet on return line air creation getting in pump.) Would explain drop in pressure by bubbles in line.

Re: starts breaking up at 4500 rpm.
 

You might also check your battery/connections/grounding etc. Low voltage will cause the injector pulse to be slow and dump to much fuel per pulse.

Re: starts breaking up at 4500 rpm.
 

Are your fuel injectors sized properly for the HP you are making? Have you had all of the injectors flowed to make sure one or more of them aren't flowing too low?

Re: starts breaking up at 4500 rpm.
 

injectors were fine before I changed the tank and lines. car was running 12.30's until I had this bright idea.

Re: starts breaking up at 4500 rpm.
 

Bob, never saw a reply about what your 02 sensor does during the issue...goes dead lean, dead rich? That might shed some light on what's happening.

Re: starts breaking up at 4500 rpm.
 

sorry, starts to lean out.

Re: starts breaking up at 4500 rpm.
 

Do you have a good vent on that fuel cell ?

Re: starts breaking up at 4500 rpm.
 

May be a really stupid question but check that the return is not connected to the vent on the cell, as its a one way check. This would dead head the pump and the regulator, the check may allow some flow but I would be surprised.

Re: starts breaking up at 4500 rpm.
 

return is unblocked. -8 line for a vent line.

Re: starts breaking up at 4500 rpm.
 

Is this a Fast system ?

Re: starts breaking up at 4500 rpm.
 

nope. stock with a Anderson pms, then I hooked up my rpm extender, then I tried just my stock ecu. all acted the same.

Re: starts breaking up at 4500 rpm.
 

You might try putting some foam in the cell... I don't have a lot of experience with injection, but sure sounds like it is running out of fuel or getting air in it.
With higher pressure of injection and a lot of return at high pressure, if it is putting air in the cell, getting air in the line, it will cause headaches...

Keep working on it, you will find it.

Ken
Stock 412

Re: starts breaking up at 4500 rpm.
 

thanks. I will try that.

Re: starts breaking up at 4500 rpm.
 

Bob, does your return have a dip hose inside the tank so it's putting the fuel in below the fuel level? Personally if you were getting air in the system I would think you would be able to hear the pump surging and you would see pressure fluctuations. Have you ever looked in the tank with pump running to see if it looks like the fuel is being aerated? When you say it starts to go lean, does it go a full number or just a few points lean and if your bring it back below 4500 does the AFR come back to normal?

Re: starts breaking up at 4500 rpm.
 

Is the fuel cell outlet facing to the rear of the car?

Re: starts breaking up at 4500 rpm.
 

yes fuel outlet to the rear. return is right above it on the top of the cell. no foam so I think we have a winner with the airiation from the return. gonna put a tube in there and see how that works.

Re: starts breaking up at 4500 rpm.
 

For testing purpose dump the return line into a separate container. That would fully eliminate any aeration from the return

Re: starts breaking up at 4500 rpm.
 

got the injection back on it this weekend. put a tube in the return side of the cell, and no more breaking up! thanks everyone!!!

Re: starts breaking up at 4500 rpm.
 

back from the dyno and everything works fine! thanks again everyone !