Lean Condition - Third Generation F-Body Message Boards

slrvette · 08-31-2009, 05:44 PM

Ran the dyno this weekend in Carlisle
Had to stop the run at 4000 RPM in third gear due to my A/F was measuring 15.99. So it was lean.

FYI- Xram, 2 inch TB's,90lb injectors

So as I'm searching for either a vac or fuel psi problem I noticed the following. fuel pressure was at 14psi. When i checked it today it was only 12.5
So I tried to adjust I couldn't. Even tried a different spring. No matter what It will not adjust. So thats one problem. Then it got me to thinkin that just a few weeks ago all of a sudden my blm's were running higher than normal in different operating conditions.
Question 1- is it possible that since my fuel pressure is now lower that my BPC-BPC-Vac is not set correctly? I had it set at 99 using a psi of 14. And in turn is why my BLM's seem to be running higher.

Question 2 - PE commanded- is set at 800 RPM- 11.8 and 6000 RPM- 12.8.
What will happen if I set these values different.

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RBob · 08-31-2009, 08:08 PM

Fuel delivery issue. Either the pump is bad, the filter is clogged, or the FPR has debris under the seat. With the lower fuel pressure the BLMs will increase to compensate.

It isn't that the BPC table is off, it is really because the fuel pressure dropped off. I've BTDT with the fuel pressure gig. With EFI setups the fuel pressure is paramount. It really has to be dead on with lots of flow and delivery.

The best way to look at injector flow is to start with what the engine requires to make the HP that the engine is capable of. Then size the injectors and the fuel pressure to deliver that volume of fuel. The BPC is then set to that parameter (the true injector flow rate taking into account the fuel pressure).

Note that if you end up replacing the fuel pump, do not go too large. At the lower pressure of TBI setups a 255 lph pump will easily outflow the stock TB, regulator, and return line. And you won't be able to lower the fuel pressure to the required level.

Check my site for the TBI Fueling article under the information menu. Has some insight for fueling TBI based engines.

RBob.

slrvette · 08-31-2009, 08:46 PM

Thanks, according to your site i need to run a pressure of 15.11, so at 12.5 this I am assuming is my whole problem. I did replace the fuel pump last year with an 85 type pump. So at this point I will assume its ok. First i will check the fuel filter and FPR before going after the pump.
thanks again for your help.

Ronny · 09-01-2009, 10:41 AM

My experience with an underhood gauge is that they are not accurrate when heat sunk. Mine on a cold engine is accurrate. After a ride it is way off. Unfortunately mine is liquid filled. I hear non liquid is way to go. Summit has both.

I just read the 1985 pump is different that later TPI models and may be better. I dont know why.

slrvette · 09-01-2009, 10:50 AM

Ron I am using a non liquid filled guage and have tried 2 different ones. Also last year I did install a 85 TPI pump.
I got to thinking that may my front tb may have a blockage. I'm gonna take it apart and check it out. But the pressure won't adjust either way. I also picked up a filter this morning. Who knows maybe even the sock is plugged up at the pump. I have tendency not to let the gas tank ever get below half way, but in the past few weeks it did get lower than normal.