Looked at a log from the other day and battery stays above 13 during a pull....EDIT....and that was with 10 psi of fuel pressure drop. My thought on the alternator was too much stop and go/ start and stop driving was slowly draining the battery because I wasn't at a high enough room constantly to keep the battery charged for all the electronics I have in the car. No idea on the history of the alternator and the car has 165k miles.

 

Really....even the old alternators are rated at 105A.

The chances of you ever maxing that out during normal driving is slim.

Even on my car when I tested load a few years back, I was only getting up to around 70A or so. And that was with full lights, interior heater fan, engine cooling fan, wipers, and obviously all engine related functions running. It was wet when driving and I was not driving hard, so there is probably scope for another 10-15A of load there too
Everything I could turn on in the car was turned on for the load test, just wasnt able to drive the engine hard. So pumps, injectors, coils would all have been working less.

I'm not saying not to upgrade the alternator, but if voltage is staying stable I would say the current one is fine.

 

I did see a voltage drop at the pump fr the beginning to tht end of a drive but I didn't think .4v was bad. I guess next 2 things would be to A) let it idle for awhile and see if that drops the battery voltage more and B) go for a spirited drive and see if that drops fuel pressure.

 

If pump voltage only drops .4V, that is not going to cause a good pump to drop 30psi. If pressure drops due to engine bay heat and not pump heat, that's a big clue. If pump can supply the volume to maintain base pressure, it sounds like the reg is not capable of regulating set pressure.

 

Sounds like time to quit hunting for zebras on a horse farm, and just change out your bad FPR.

 

​​​​​​​Sounds like you 2 didn't read the last couple of updates where the regulator regulates just fine pump on engine off and where a 45 min cruise regulated just fine.

 

45 minute cruise and no drop? What does it take to make the problem happen?

 

I noticed it at the end of street tuning it the other day. Lot of stop and go driving, idling, and start/stop to flash the pcm. The 2 lines of though are either A) weak charging system that can't keep up when doing a lot of idle and stop/start, or B) the heat from the turbo under the tank is heating the fuel causing a pressure drop. I've hotwired the pump to run for 20 min with the engine off and fuel pressure was perfect so it's not the pump heating the fuel.

 

Have you duplicated the problem so you can effectively diagnose? If so, what is the pump voltage when problem is occurring compared when problem not occurring? How does pump voltage compare to battery voltage? Side note - fuel would have to begin to boil/vaporize to no longer pump and cause drop in pressure. Do you think the fuel is that hot when problem occurs? IR temp gun is a great tool to diagnose temp related issues.

 

I believe I've got my smoking gun......I let the car idle in the garage this morning with a meter on the pump wires and the scanner logging fuel pressure. Cold start battery voltage was ~13.6VDC and FP was ~51 PSI. After 12 min of idling, battery voltage dropped to 13.1VDC and FP dropped to 40 PSI. I can put my hand on the shield above the turbo between it and the fuel tank and its warm but not "HOT". Seems similar to what a muffler would be. The FPR is also cold to the touch while running that long so I'm ruling out a FPR issue. Sure sounds like weak charging at idle to me. Thoughts?

 

.5V drop should not cause 10psi drop. What is fuel pressure with key on engine off? (i.e. battery voltage is likely around 12.5V) I suspect it stays at 40psi and doesn't drop to 30psi. Can you cap return and see what pump dead head pressure is?

 

And if you rev it a little to increase voltage, does FP increase ?

If the FPR is cold, and lines cold....then I'd be happy that the fuel and pump are cold too.

Still surprising such a small voltage change is showing such a dramatic FP change.

Battery voltage has dropped to 13.1V. Can you easily probe the connectors to see what voltage is close to the pump ?

Where is the ground from for the pump ?

 

IMPORTANT - I would not assume battery voltage is fuel pump voltage. Log pump voltage or use DVM with VDC min/max capability and compare with battery voltage. Lastly, measure pump voltage with POS and NEG connected to pump supply wires. Using another ground as reference to measure pump voltage may lead you astray if problem is on ground side.

 

on just the 2 second prime it gets to 18 or 20, priming it 3 times gets it to ~50 psi that I would see on a normal cold start.

Im measuring pump voltage at the ground stud of the chassis that the fuel pump return wire is tied to, to the hot wire of the actual pump at the tank via the trap door. It looked like on the stock pump wiring that the factory ground wire is Y'd to the stock fuel pump return and to the fuel pump hanger bracket which is tied to the chassis. I just tied the return wire of the 450 to the hanger assembly to the chassis since I have a metal 98 tank.

See responses above related to where voltage measurements are taken and grounding. Perhaps the ground side is my issue?

A rev of the motor doesn't change fuel pressure.

 

But does increasing idle/rpm change voltage ?

Is that "ground stud" an OEM clean bare metal chassis ground, or other ?

And you've then linked this somehow to the wiring directly at the fuel pump ?

You could measure for volt drop between as close to the pump as you can get, and to that earth stud...but it would seem unlikely.

Really...if it looks good, it probably is good.

You could waste forever doing a plethora of tests. You could buy a cheap voltage stabiliser/regulator if you can get one for around 15A....and slap it in front of the pump. At least that would eliminate any voltage changes to the pump.

 

The talk of grounding is sparking a thought.....if the hanger assembly is tied to the circular shaped clamp that fastens the hanger to the top of the tank, and the ground wire is from this clamp to the chassis, then that would mean I'm only getting ground through metal to metal contact of the hanger assembly to the clamp and not a secure bolt-down solution. Surely I'll need better than that for 10+ amps of a 450. I'll have to runs dedicated ground wire from the pump to the chassis directly and bypass the hanger assembly.

 

I can't picture what you're doing.

But yes there should be a good and proper live and ground directly to the pump. With suitable and save connections throughout

 

Think of the top of the factory tank as having a circular opening that the entire fuel pump hanger assembly drops into. There is a circular collar that actually secures this hanger assembly to the tank....but it just sandwiches the hanger assembly to the tank, it's not truly fastened.....that may be causing crap voltage at the actual pump. I'm using a chassis stud as my DMM ground reference which is falsely giving me a better voltage at the pump. Or at least that's the theory lol. It still doesn't explain why a cold start shows better voltage unless the impedance of the pieces of metal at the tank is lesser until everything warms up.

 

So you do not have a proper wiring connection from pump to ground ?

Surely any "hotwire kit" has proper connections throughout ?

 

Im thinking the ground is part of the issue. All a hotwire kit is is a thicker AWG with a fuse/relay for direct power and a ground wire to the chassis. I made my own, but neglected to ground to the correct spot. Or at least that's my hope since it's an easy fix.