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If the alternator and wiring are doing their job and the pump is in good condition there should be a correlation between pressure and amps.
To rule out completely the electrical you need amps (you can measure this anywhere in the line feeding the pump) and volts directly at the pump (this is not possible due to the pump being inside the tank) but you should measure voltage as close as possible to the pump. There could be a voltage drop along the hotwire kit due to a loose connector or bad relay.
Regarding the fuel pressure regulator:
Have you tried adjusting the regulator when the pressure drops to see if you get any response from the system at all?
another approach would be to insert a piece of fuel injection grade hose in the return line coming off the fuel rail and pinch it during a pressure drop to see if line pressure rises.
Agreed! Seems like a piece of fuel injection rated hose needs to be added with some good clamps. When the pressure falls off, pinch the soft rubber return line. A good pump should jump up around 100 psi. If the fuel pressure rises the regulator is the problem. If the pressure stays the same the pump and/or wiring are a problem. Starting to suspect a regulator issue.
If the alternator and wiring are doing their job and the pump is in good condition there should be a correlation between pressure and amps.
To rule out completely the electrical you need amps (you can measure this anywhere in the line feeding the pump) and volts directly at the pump (this is not possible due to the pump being inside the tank) but you should measure voltage as close as possible to the pump. There could be a voltage drop along the hotwire kit due to a loose connector or bad relay.
Regarding the fuel pressure regulator:
Have you tried adjusting the regulator when the pressure drops to see if you get any response from the system at all?
another approach would be to insert a piece of fuel injection grade hose in the return line coming off the fuel rail and pinch it during a pressure drop to see if line pressure rises.
Originally Posted by Fast355
Agreed! Seems like a piece of fuel injection rated hose needs to be added with some good clamps. When the pressure falls off, pinch the soft rubber return line. A good pump should jump up around 100 psi. If the fuel pressure rises the regulator is the problem. If the pressure stays the same the pump and/or wiring are a problem. Starting to suspect a regulator issue.
Cool I'll try that. I already have rubber hose in the return line since I added the fuel cooler so I'll just pinch that.
I went ahead and spent my afternoon troubleshooting. It wasn't warm enough for the car to lose pressure easily (only mid 80's) so I had to run the engine for 2 hours. After taking it for some spirited laps around the country roads and letting it idle in the driveway with hood closed and A/C on max to get it as hot as I could, the pressure finally started to drop. Still, it was falling very slowly. Once it hit 49 psi I squeezed the rubber line going into my fuel cooler and the pressure shot up to the 80's. I did it a handful of times just to be sure. Managed to spring a leak at the clamp on the line too, oops. So that's it then? That proves it's the FPR? I kept my old stocker just in case something like this ever happened so I can swap that back in. The real pain will be re-tuning it for the lower fuel pressure.
Sounds good.
Looking forward to see how it turns out.
Important checking fuel pressure at idle. If fuel pump is supplying too much flow you might find yourself in a situation where fuel pressure is above specs (Higher than 43-45 psi) and the regulator won't be doing its job.
I still think the 340LPH pump is more than what you need.
Finished my test drive and the car did fine. It was about 15 miles, tried to mix it up as much as possible. Idle and cruise fuel pressure settled around 39 psi, heavy acceleration and WOT hit 45 psi. Today isn't hot enough to determine whether or not the FPR change fixed the issue, it's only hitting 86. I may have to wait until next year to know for sure I was really dreading having to tune this thing again but it ended up being no work at all. I just changed injector flow rate vs kpa and the car drove like nothing ever happened, awesome!
The "rebuild" kit for the aeromotive FPR are cheap and its super easy to rebuild.
Well, if it turns out the FPR was causing the issue all these years, I'd probably go with a different brand like Holley. Don't want to risk ever dealing with this again.
Fast355, nice advice on pinching the return. I'll be putting that in the old tool box for the future. Here's hoping that puts a period on the fuel pressure issues.
Finished my test drive and the car did fine. It was about 15 miles, tried to mix it up as much as possible. Idle and cruise fuel pressure settled around 39 psi, heavy acceleration and WOT hit 45 psi. Today isn't hot enough to determine whether or not the FPR change fixed the issue, it's only hitting 86. I may have to wait until next year to know for sure I was really dreading having to tune this thing again but it ended up being no work at all. I just changed injector flow rate vs kpa and the car drove like nothing ever happened, awesome!
LUCKY to have such a cool day. My car was showing 109°F when I picked up lunch today at 3:00 PM in Haltom City and it is a fairly accurate reading. It was HOT and HUMID here in North Texas! Nearest weather station at Mecham field in Fort Worth was 106°F, 30% humidity and 0 mph wind at 2:53 PM, real feel 111*F. Glad my 220K mile 2007 Infiniti G35 still blows 40°F air out of the vents driving around in this weather after replacing the OEM compressor after the clutch coil failed this spring, lol.
Sitting outside now drinking a whiskey/coke in Hurst, TX and it is a cool 85°F and 65% humidity, with not a breathe of wind. Feels like 89°F and it feels cool.
PCMs are pretty nice like that. I have even tuned the older TBI ECMs with much higher flowing injector setups. On those a mere injector flow or BPW change and some AE/DE tuning brings them right in line without changing the VE table. The AE and DE is strictly pulse width. If the only change is injector size, usually scaling by multiplying the whole table by the percentage of flow change is all it takes to get them running well again.
Looks like we're gonna have a hot September here in central Oregon. Today was 93F with some forest fire smoke for good measure. Perfect conditions to test the car. I took it on a 2 1/2 hour drive up and back down one of the mountain passes, a drive that would have easily recreated the fuel pressure issues I've been having. While the pressure loss wasn't as bad, it still happened.
The numbers started out the same as before: 39psi idle and cruise, 45psi WOT. By the end of my drive idle and cruise were 33psi and WOT was 41psi. DFCO, coasting, and under 5% throttle I was seeing numbers as low as 31psi at times. The pressure didn't go up and down like before though, it was a slow drop over the course of the drive and eventually settled on these numbers. And WOT always increased pressure and held. I did a 4th gear pull so there was plenty of continuous WOT time to be sure. Before, pressure would bleed off with WOT. I might just replace the relay that came with the hotwire kit. That and the actual wire that came with the hotwire kit are all that's left. I'm just not 100% convinced because these are the same low numbers i was getting before: low to mid 30's. Although the behavior isn't identical it still seems like fuel volume is what's low.
Looks like we're gonna have a hot September here in central Oregon. Today was 93F with some forest fire smoke for good measure. Perfect conditions to test the car. I took it on a 2 1/2 hour drive up and back down one of the mountain passes, a drive that would have easily recreated the fuel pressure issues I've been having. While the pressure loss wasn't as bad, it still happened.
The numbers started out the same as before: 39psi idle and cruise, 45psi WOT. By the end of my drive idle and cruise were 33psi and WOT was 41psi. DFCO, coasting, and under 5% throttle I was seeing numbers as low as 31psi at times. The pressure didn't go up and down like before though, it was a slow drop over the course of the drive and eventually settled on these numbers. And WOT always increased pressure and held. I did a 4th gear pull so there was plenty of continuous WOT time to be sure. Before, pressure would bleed off with WOT. I might just replace the relay that came with the hotwire kit. That and the actual wire that came with the hotwire kit are all that's left. I'm just not 100% convinced because these are the same low numbers i was getting before: low to mid 30's. Although the behavior isn't identical it still seems like fuel volume is what's low.
Are you sure there is an actual loss? Have you looked to see if your fuel trims increase when the pressure drops. Liquid filled pressure gauges are notorious for reading low when exposed to constant high heat evironment of the engine compartment. A friend of mine chased an issue like yours on his 95 C1500 350 TBI for couple of years only to find out it was the gauge itself. I would expect some major bogging and very low power if your fuel pressure was dropping into the low 30s.
Yes the trims increased throughout the drive. LTFT's went from -4 to -6 to +3/+4. The PCM was able to compensate so I didn't notice a change in performance. I think all I've done by swapping FPR's and adjusting the tune is brought everything else down to match flow output so it's not really noticeable when the numbers are low now.
When I was running 60 psi and the pressure dropped, I did notice loss of power, rough idle, bogging etc.
I can't find this relay anywhere. I sent Racetronix a detailed email about all this, hopefully they respond. Anyone know where I can get a replacement for this relay? It's not even on Racetronix's site. I got under the car and eyeballed the power wire for the fuel pump and it looks good. Not resting on exhaust or damaged or anything.
Went ahead and replaced the jacked up portion of the fuel line. I know the union isn't in the most aesthetically pleasing area of the line right there on the bend but I primed it a couple times and there are no leaks so it's functional.
Went ahead and replaced the jacked up portion of the fuel line. I know the union isn't in the most aesthetically pleasing area of the line right there on the bend but I primed it a couple times and there are no leaks so it's functional.
I would not trust that at all. Most autoparts stores carry the correct hose barb repair fitting in the Motormite/Dorman "HELP" section to repair that. It looks like a PEX plumbing connector. They offer them for the 3/8" and 5/16" lines. Much safer and reliable repair insert.
I can't find this relay anywhere. I sent Racetronix a detailed email about all this, hopefully they respond. Anyone know where I can get a replacement for this relay? It's not even on Racetronix's site. I got under the car and eyeballed the power wire for the fuel pump and it looks good. Not resting on exhaust or damaged or anything.
I was really dreading having to tune this thing again but it ended up being no work at all. I just changed injector flow rate vs kpa and the car drove like nothing ever happened, awesome! 
