Thread: Interesting issue I cannot seem to figure out....

Originally Posted by agrazela

Yes, the FP on my topswapped 98 LeSabre just got weaker and weaker, instead of dying all at once. And it did just what yours is doing: cause massive KR when I punched it and max the IPW's because it was trying to pump fuel but couldn't because the pressure was tapering off at high load.

Fuel pressure values at idle were just at the low end of spec, but one tell was that the FP after keyoff would continue to drop gradually over time instead of settling down and holding. (Kind of like it sounds yours is doing.)

I went through all the same diagnostics, checking all the lines for leaks, replaced the FPR, etc. Ultimately, I replaced the pump (in my case with a TRE-340), and all my fueling problems went away.

I suspect the checkvalve / backflap (or whatever the heck it's officially called) in the pump went bad / stopped sealing well. The pump could flow, but it just wouldn't get up to or hold high pressure.

Good post! So it is a possibilty, interesting. For my leak down test, how long did you do yours for? I think i'll perform the same test on mine too. Let me know...

With mechanical gauge: my keyon FP was ~47psi. About 5 min after keyoff it had dropped to ~40psi. After an hour it was at like 30 psi. After overnight there was no pressure left on the gauge. It should hold some pressure even overnight.

Another thing it was doing was, idle FP was ~41 psi (a tad low); when I'd rev it by hand it would go up to ~45-ish, then when I let off it would drop down to 32-33 psi or so before returning back to 40-41.

Then I got an electronic FP sensor; here's what it was doing at WOT, before and after the pump swap:


This data is all in WOT (MAP ~100), collected through a/d converter on the PowrTuner. Note also that this data was collected (and the pump replaced) before I topswapped (i.e., L36); WOT fuel pressure is higher now, of course with an L67 FPR seeing boost.

As RPM (thus MAF, thus fuel demand) went up, the original pump was petering out badly on fuel pressure. With a new pump, fuel pressure stays solid all the way across. WOT KR went away, and IPW's backed off from near-static.

Originally Posted by agrazela

With mechanical gauge: my keyon FP was ~47psi. About 5 min after keyoff it had dropped to ~40psi. After an hour it was at like 30 psi. After overnight there was no pressure left on the gauge. It should hold some pressure even overnight.

I'll try and do a longer test tonight and post up results.

Another thing it was doing was, idle FP was ~41 psi (a tad low); when I'd rev it by hand it would go up to ~45-ish, then when I let off it would drop down to 32-33 psi or so before returning back to 40-41.

Mine don't do that. If I rev mine it raises to close to 50 psi and goes bad to 44 psi. No sudden dips, no nothing.

Then I got an electronic FP sensor; here's what it was doing at WOT, before and after the pump swap:


This data is all in WOT (MAP ~100), collected through a/d converter on the PowrTuner. Note also that this data was collected (and the pump replaced) before I topswapped (i.e., L36); WOT fuel pressure is higher now, of course with an L67 FPR seeing boost.

As RPM (thus MAF, thus fuel demand) went up, the original pump was petering out badly on fuel pressure. With a new pump, fuel pressure stays solid all the way across. WOT KR went away, and IPW's backed off from near-static.

Thanks for the info. I wish I could do the same test but I have no way of doing so unfortuneitly.