I haven't done anything too extreme to my engine other than lowering the compression to 9:1 and boring it .030 over. It's otherwise fairly stock other than the blower. I had 24# injectors and an FMU on it, went to the dyno, but it was running 15:1 A/F. So I put 36# injectors in it, had a friend burn a chip with the constant changed for the injectors, and put on an FMU that had a faster rate of gain. It's still running 15:1.

How can it still be that lean? Am I missing something? Can any particular sensor cause this? I'm not getting any check engine light and no codes in the computer.

Any ideas or suggestions are appreciated.

 

Sounds like your fuel pump isn’t keeping up, do you have a fuel pressure guage? Does the fuel pressure drop at WOT?

 

That's what I thought at first too, but I do have a fuel pressure gauge and the fuel pressure never drops.

 

TTT, any other ideas?

 

With 36# injectors that should be pig rich. I had a more heavily modded combo and got away with 30's. Whats your pressure at at WOT. Also when you had the old procharger FMU was the bleed screw turned in all the way to the right(ie screwed in clockwise). ?

 

I agree, it should be pig rich. With the 24#s, it would peg out around 90-95 psi. With these 36#s and the new FMU the fuel pressure starts out (wherever I want it to) around 40psi, hits about 60psi within the first few psi of boost, and just keeps climbing from there. I'm afraid to stay in it for too long with it running so lean, plus they are only GTP injectors so I know too much pressure could lock them up.

 

This may be basic, but you are sure you don't have any exhaust leaks? Any false air entering the exhaust will cause an inaccurate reading at the WB.

Have you done a flow test on the fuel system? It is possible to have normal pressure, and still have low flow depending on what the issue is with the system.

 

Anybody else think that fuel pressure is too high? If I remember correctly procharger says with 24's you should see 40psi @ idle and it should increase to around 65-70. Also when was the last time you changed your fuel filter?

Are you checking pressure at the rail? How many pounds of boost are you seeing. On the FMU procharger supplied was the screw turned all the way to the right. What does the dyno sheet look like, meaning is it 15:1 as soon as you put your foot into it or does it start out with enough fuel and fall on its face up top.

You are either running out of pump, your friend burned the prom wrong, or you flat out need bigger injectors. I seriously doubt its the injectors.

 

No exhaust leaks, but good idea. I haven't done any flow tests. How would I go about doing that?

I am checking the fuel pressure at the rail. I wanted to know the exact pressure the injectors are seeing. On the Dyno, it starts out @ 15:1 and never richens up.

I really appreciate all the ideas guys!

 

You would just disconnect the feed line from the pump and stick it in a graduated container. Then kick on the pump and monitor the amount of time it takes to reach a certain fluid level. I don't recall what the specs are, but for a "stock" car it's something like one pint in 15 or 30 seconds. You could probably get your specific specs from whoever made your pump.

 

Thanks onebinky. I'm gonna have a friend of mine help me check that. (He used to work for Aeromotive.)

 

UPDATE: I changed the fuel filter just because it hadn't been changed for about a year. I found out that the injectors weren't 38# like I originally thought. Turns out they are only 36# injectors. These should still be more than enough. I had my friend burn me a new chip with the injector constant changed to 36. I hooked the wide-band up to the car and went for a drive. The A/F ratio is looking better @ 13:1, but it's still not good enough. Does this mean anything to you guys. I'm still stumped. Would lowering the injector constant more help?

 

As fuel pressure increases, fuel volume decreases.
Thats one issue.

Second, playing with the injector constant is changing everything accross the board. Set the constant to 36, and then adjust ve. Then start playing with PE fuel until you get the correct AFR.

I'd look into a high pressure walbro pump if your gonna continue use of the FMU. In a perfect world, you would only rise 1:1 with boost, and use injector pulse for enrichment. But since your not burning your own chips, doing your own tuning, that information really wont help you.

-- Joe

 

I really need to start burning my own chips anyways. I need the same hardware for my car as I do to burn the chips for my wife's GN anyways.

 

If I were you. i'd:

1) Get a walbro 255hp
2) Get a memcal adapter from Craig
3) Get a 2-bar map sensor from autozone
4) Remove FMU and BTM (if you have them)
5) Burn a chip with $60 code, and tune

-- Joe

 

Thanks. I'm considering that route. I have a little $ left over from my tax refund, so that's probably what I'll do.

Meanwhile, shouldn't the current setup be making my car overly rich?