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On many boosted cars fuel pressure needs to be raised as boost is added to compensate for extra air. The 3800 FWD supercharged engines use several methods to control fueling, which render an AFPR worthless in many cases. The MAF sensor reads the airflow and tells the PCM how much air is coming in and therefore how much fuel to deliver. The MAP sensor monitors manifold pressure (vacuum or boost) and fueling is adjusted by the PCM. The ECT sensor monitors engine coolant temperature and also has some effect on fueling. And then the front oxygen sensor (B1S1) monitors overall air/fuel performance and tells the PCM how well the actual air/fuel matches the desired air/fuel.

As you begin to mod and increase the overall airflow of your engine, the MAF and MAP sensors detect the increased airflow and/or boost, and the PCM automatically lengthens your fuel injector pulsewidths to compensate. The stock fuel pressure regulator (FPR) changes fuel pressure with vacuum or boost. As your boost increases so does your fuel pressure. By changing this function by replacing the factory regulator you run the risk of making your car more difficult to tune. Using an AFPR to increase static fuel pressure may not change the maximum pressure available because the stock L67/L32 fuel pump usually can't reach 70 psi on most vehicles. So even if you might add five psi of fuel pressure at idle and cruising, you may only be adding a pound or two at WOT. And this loss of linearity can make tuning the car a nightmare.

Scott
 

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Scott, please give me a straight answer since I've heard about 2394234 different ones when it comes to our fuel pressure, but our stock FP with the vacuum tube off is supposed to be 52lbs, correct? When I changed my 104s out 2 weeks ago with new ones, all of them were white-coated and ranged in gap from about .42-.58. When I put those in, they were all gapped at .55.

If it is, why would I only be seeing 47-48lbs (at most) at idle with the vacuum tube off? It could be cold or hot outside (or coolant temp). I have your basic mods:

3.4, 180* thermo, 104s, open K&N, ubend/resonator removed, and wires were replaced with new stock ones at 40k miles (car now has 55k). Everything else is bone stock (fuel filter was replaced at 45k miles).

Now, I understand the PCM commands how much actual fuel gets sent out, but what if that fuel wasn't even getting there. If I got a FPR, I could at least bump it up to what the stock rating would be, and tune from there.......
 

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Dan,

It sounds like you have a stock FPR. IIRC, our stock FPR's only have to within something like 48 - 52 psi to be in spec. It sounds like your regulator is on the low side of spec. For most un-modded cars, that's not an issue as the PCM adapts by widening the IPW, but as we add more mods and get more power we lose some of that margin. You could try another stock FPR. I'll bet Ed has plenty laying around. Another option would be a tweak in your fueling tables. Just food for thought.

FWIW, I'm currently running a FP of 48 psi myself.

John
 
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