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Working on a Fuel Pressure (I think) problem with a friend. Recent rebuild of his motor and complete R/R of subframe as background. 24V LS car with Q runners.
FP measures barely 40psi at idle, and drops heavily under engine speed (30 ish PSI at 4500 /5K rpm). Car dies under real load at higher RPM (5K, nothing near limiter), seems to go lean (like not enough fuel-- throws 1223). No vacuum leaks that we can find, did brake cleaner trick everywhere and brand new intake hose. Tried new FP regulator, no go. Tried spare fuel pump assembly. R/R fuel filter. No Change. Can get to 58 PSI when clamping off return line, gets there quickly. Vacuum is there from air box. We;ve isolated the Fuel Pump by jumpering the relay -- no change. Fuel pump is getting 13 ish volts and the ground is solid to chassis (~ 1 ohm). I think we ruled out motronic by jumpering the relay and we still see the pressure drop/car go leaned out to the point where it dies (5K ish RPM).
I expect a reasonably steady fuel pressure even under higher RPM -- FP should not drop by 1/3 or 1/2 under higher RPM, in fact it should go higher per the manual (vaccum increases, FP regulator increases pressure). Don;t think its coil packs or other causes since the FP drops so much.
Im left with thinking its a clog somewhere in the supply line thus limiting the fuel delivery to the rail. As soon as engine demand (higher RPM) overloads supply (from the rail) the pressure has to drop and things lean out. At least thats the working hypothesis.
Few questions.
1. Would you expect FP to drop so much at higher RPM? To me this seems like the prima facie evidence for lean out and eventual fuel starvation.
2. Anyone seen anything like this at all? Before we tear out the fuel lines and try to find a blockage.....???
3. I am thinking that we have to solve the FP issue first before we do anything else.
TIA
FP measures barely 40psi at idle, and drops heavily under engine speed (30 ish PSI at 4500 /5K rpm). Car dies under real load at higher RPM (5K, nothing near limiter), seems to go lean (like not enough fuel-- throws 1223). No vacuum leaks that we can find, did brake cleaner trick everywhere and brand new intake hose. Tried new FP regulator, no go. Tried spare fuel pump assembly. R/R fuel filter. No Change. Can get to 58 PSI when clamping off return line, gets there quickly. Vacuum is there from air box. We;ve isolated the Fuel Pump by jumpering the relay -- no change. Fuel pump is getting 13 ish volts and the ground is solid to chassis (~ 1 ohm). I think we ruled out motronic by jumpering the relay and we still see the pressure drop/car go leaned out to the point where it dies (5K ish RPM).
I expect a reasonably steady fuel pressure even under higher RPM -- FP should not drop by 1/3 or 1/2 under higher RPM, in fact it should go higher per the manual (vaccum increases, FP regulator increases pressure). Don;t think its coil packs or other causes since the FP drops so much.
Im left with thinking its a clog somewhere in the supply line thus limiting the fuel delivery to the rail. As soon as engine demand (higher RPM) overloads supply (from the rail) the pressure has to drop and things lean out. At least thats the working hypothesis.
Few questions.
1. Would you expect FP to drop so much at higher RPM? To me this seems like the prima facie evidence for lean out and eventual fuel starvation.
2. Anyone seen anything like this at all? Before we tear out the fuel lines and try to find a blockage.....???
3. I am thinking that we have to solve the FP issue first before we do anything else.
TIA