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So now I truly believe it's losing pressure, not just a bad sender.
I'm leaning towards a bad butt splice in tank. I personally like solder.
I think I read earlier you have 2 pumps. Is it possible to static test 1 pump at a time, them both?
That was precisely my plan.. disconnect the 4pin connector at the tank and hotwire each individual pump one at a time... possibly also crank the FPR pressure up to simulate load and see what the resulting fuel pressures are.
Take your boost, bring it down low, say 3 PSI. That way, the motor will only make lets say 550 HP. 1 pump will support that much HP. I am suggesting this because you need to minimize variables and create a documented test method that is repeatable.
Step 1, isolate a pump and label it. Run it AS IS with low boost, document a baseline with existing wiring but only 1 pump. Probably have to unwire the second pump somehow.
Step 2, repeat with the other pump (#2), but don't run pump "#1" in this test, only 1 pump at a time.
Step 3, re-run test 1 but with big wire directly to the battery. Hot wire that sucker so it is running before you even hit the key! Get in, log data on low boost again, compare data.
Step 4, repeat test 2 with pump #2.
Now, you should have 5 sets of logged data that show a baseline, pump#1, pump#1 hot wired, pump#2, pump #2 hotwired.
This is what I did and there was a difference between how pump 1 and 2 ran. Also, separating them helped eliminate variables in my head. It can also show if there is anything funny with plumbing or a bad check valve.
What I learned when I did this was also that 1 pump on low boost was at the limits of my injector duty cycle. But that is a whole separate discussion. In the end, the new wiring brought my fuel pressure from looking EXACTLY like yours to a perfect climbing graph line due to better power supply. I was floored by how much these pumps rely on high amp/volt supply. They are hungry electric motors for sure.
Take your boost, bring it down low, say 3 PSI. That way, the motor will only make lets say 550 HP. 1 pump will support that much HP. I am suggesting this because you need to minimize variables and create a documented test method that is repeatable.
Step 1, isolate a pump and label it. Run it AS IS with low boost, document a baseline with existing wiring but only 1 pump. Probably have to unwire the second pump somehow.
Step 2, repeat with the other pump (#2), but don't run pump "#1" in this test, only 1 pump at a time.
Step 3, re-run test 1 but with big wire directly to the battery. Hot wire that sucker so it is running before you even hit the key! Get in, log data on low boost again, compare data.
Step 4, repeat test 2 with pump #2.
Now, you should have 5 sets of logged data that show a baseline, pump#1, pump#1 hot wired, pump#2, pump #2 hotwired.
This is what I did and there was a difference between how pump 1 and 2 ran. Also, separating them helped eliminate variables in my head. It can also show if there is anything funny with plumbing or a bad check valve.
What I learned when I did this was also that 1 pump on low boost was at the limits of my injector duty cycle. But that is a whole separate discussion. In the end, the new wiring brought my fuel pressure from looking EXACTLY like yours to a perfect climbing graph line due to better power supply. I was floored by how much these pumps rely on high amp/volt supply. They are hungry electric motors for sure.
Thanks for all that info/advice! I'm heading out of town this weekend for a couple weeks so this won't get done until December.. hopefully I can find some dry weather to do these isolated tests.
The 4-pin Deutsche connector is pretty easy to access between the tank and the rear diff if I have the car chocked with the back wheels up on 4"x4" wood or something like that.. I think I will just pull the Pump #2's power pin out of the tank-side connector for Pump #1 test, and vice versa remove Pump #1 pin for Pump #2's test. Then make a second dummy temporary plug harness to hotwire both pump power pins direct to the battery with 10ga. I'll also make sure to hotwire the pumps' grounds to the battery as well, to ensure the best electrical signal to the pumps.
No updates.. haven't had the time or the dry weather to crawl under the back of the car and test-wire some robust fuel pump wiring direct to the battery and then take it for some boosty test runs. Hoping March is when the weather starts having several-day-long dry spells!
I have been fiddling with different shifter mechanism mods while it's been parked though, and making a shifter boot for is (black seude with red stitching to match steering wheel & seats)... I may make an update with those details once it's complete.
No real progress other than just some neat pics of the car. I originally thought that having the car at my apartment would make taking it into work for some after hours tinkering easier, but I feel less inclined to work on it if I have to drive it, then work on it at my work, then have it back to a state to drive it home after. I still need to investigate the fuel pump wiring, and desperately need stiff front springs as it rubs the fenders horrendously near full lock now, with the extra weight from the TT build.
To me it really does sound like a cigar boat type exhaust note...
Thanks guys. Attention to detail where it matters... maybe I'll eventually get around to completing the little details like installing all the bumper bolts, aligning the front trim including adding the bumper-grill trim plate, wax and buff it, etc etc. But those things don't make it go faster!!
Love all the new pics! It's great to step back and see overalls of your car now after spending years looking closely at details under the hood. All the details add up beautifully. No doubt it'll get featured in a magazine or e-zine soon.
I look forward to seeing more of the car in different places doing cool things, along with whatever projects you tackle next.
Love seeing these long-term projects finally on and shaking the ground. you + clint have also inspired me to finally get at aligning (and maybe washing/waxing) my body panels.
My panels need alignment work too. We should all just meet up and get this done in one place. I can't make LS Fest West this year due to lack of a man pedal and overdrive, but can we target next year? I think Joe has the longest distance to travel but maybe he can tow some of that with the Ram. I've got to source a 6 speed by the end of the year. I want to road trip my car!
Don't waste your time buffing and waxing. Only suckers get caught up in that sisyphean nonsense.
Even a socal trip would be epic. My buddy (LS70SS) is finishing a 408 in his '70, and I'm growing more confident with each drive that mine can make a longer trip. We could do this!
I'll try to get Bandit-level documentation for my panel alignment.
This weekend I made progress on "inspecting" the fuel pump wiring. I figured if I was going thru the effort of jacking up the car to climb under it, and most likely also dropping the tank to inspect wiring, I might as well just update the whole setup... whoops.
I have always had some fuel starvation below 1/4-full tank, so I kinda want to get rid of that at same time as troubleshooting the fuel pressure drop issue.. Plan is to try the Holley Hydramat setup, and reorient the pumps to allow some more room for an update to the fuel level sender as well, to go from a stock float type to a capacitance type from SpeedHut (same company as gauges, so 100% compatible.
First step was to inspect. I never liked that all the wiring and solenoid is under the car, if there was a failure other than pump-related, so I plan to move the wiring up to next to the battery in the trunk.
Feed from the battery is a single 10ga (by my eye-approximation), which I'll be upsizing.
The ground between the solenoid and the tank was frayed at the mounting point on the solenoid body. This solenoid is supposedly self-grounding, however I originally added this ground to the tank just to be sure. There is a separate/dedicated ground from both pumps directly to chassis, and then another wire from that same location on the chassis to the battery negative (this ground is separate from the 4ga main chassis ground to battery negative). I'll replace with pump+tank ground directly to battery negative.
Out with the old stuff...
Below two pics show the wiring sizing on the battery for fuel pump stuff:
- 2 x 10ga power wires that merge together near the underside pump solenoid to a single 10ga cable on the solenoid
- 1 x 14ga ground wire from chassis near tank to the battery negative
Removal of old design to inspect. The fuel hose that "isn't designed to be submerged" appeared fine inside and outside after spending ~7years submerged in gas. No weird increase in pliability or any degradation to the material itself.
What's interesting and so clearly wrong to me now, is the dual 5/16" pump outlets T-merge into a single 5/16" hose that then fits to a 3/8"-hose-barb-to-8AN fitting on the bulkhead! What the hell! A 5/16" hole is 40% the size of a 1/2" hole (just talking ideally, not considering how the fittings are actually drilled)... this clearly would be a choke point if kept unchanged, if it wasn't already!
There is interesting discoloration on the wires at the Deutsch connector terminal crimps.. looks to me to be heat discoloration, I'm going to have to research high-amperage alternatives to this connector/pin style, if possible.
Fabricated bracket all cut apart, to use top plate as a template for a new bulkhead plate.
Do connect the in-tank pumps to remote-located Hydramat filters, I'm using these neat adapters from Z1 Motorsports which convert the hole-and-stud mount of a pump to a 3/8" hose barb.
New top bulkhead plate.. ahhhhhh.. a blank canvas.
Fits! Here is the top of the new SpeedHut capacitance fuel level sender.
Arranging the various bulkheads and required clearances to cutout shape in tank as well as tool/sockets.
Test assembled.
1) -8AN 90* bulkhead fittings for fuel supply to engine bay
2) -6AN 90* bulkhead fitting for fuel return to tank
3) -6AN 90* bulkhead fitting for fuel tank breather line
4) 2 X electrical pass-thru bulkhead fittings
