Help Identifying Cause of Low Fuel Pressure

I could use some help with a fuel issue I'm having in my 2001 SAN (Here is the background on the boat). I took it out for the first time and the boat struggled to get over 1800 RPM and can't get on plane. It sounds decent in idle and revs up out of gear. It just doesn't have hardly any power under load, runs rough, etc. I immediately replaced the rotor / cap / wires / plugs as this hadn't been done in a while.

Before taking it back out to the lake to test to see if the ignition tune-up resolved the issue, I put a fuel pressure gauge on the fuel rail and ran the boat off a hose. To my surprise, I'm getting 10 PSI off the schrader valve on the fuel rail and the HP pump. I talked with the prior owner and he has a video of 40 PSI when running the engine out of the boat. He also just replaced the LP and HP pumps, filter, injectors, etc. I have absolutely no reason not to trust him, he's been very helpful giving me more ideas to try and even offered to send me a new pump.

Here's what I've done so far (w/ references to Service Manual):

• 3H-1: Visually inspected everything from tank to injectors and can't see any obvious issues to my somewhat untrained eye
• 3H-2: Fuel pressure is only 10 PSI when idling / under throttle. Does not budge when giving throttle.
• 3H-3: Ran both pumps for 30 seconds using STO jumper and the pressure goes to ~8 PSI. It immediately drops to 0 PSI when pulling ground. Per the service manual, it should hold within 3 PSI for 3 minutes, but this might be only in the case you are getting ~40 PSI? I'm not sure on this one.
• 3H-11: Ran pumps by grounding the STO connector with key on, engine off and I can hear / feel both pumps running
• 3H-12: Checked voltage at pump, power is getting to both HP and LP pumps (12.5V)
• 3H-13: Checked fuel pump ground, getting about .3 to .5 ohms so good here.
• 3H-14: Filter has only ~2 hours of runtime.
• Swapped relays, no change
• Pulled fuel hose off anti-siphon valve and inserted end of hose straight into tank (via opening for fuel sensor). Still getting 10 PSI.
• Pulled fuel hose off connection from LP to HP tank and ran pump using STO ground and fuel flowed very well into a bucket (I think I saw somewhere they suggested 1 qt per 15 seconds and that seems about what I got)

Based on the above, it seems like the logical next steps would be to:

• Pull the FCC and inspect the filter and pickup tube (PO just replaced them though)
• Replace the fuel pressure regulator

What else should I try?? THANKS!!

(Note: I cross posted this on PN -- hope that is OK. Let me know if that is a party foul and which site is a better place for these sort of questions!)

Thanks Mark! Really appreciate your help. I've got the GT40 Pro Boss. The fuel is pretty new. The previous owner pulled the tank and replaced the fuel, he put in enough to run the engine himself before handing it off to me. So max a couple months old. Still I picked up a bottle of Seafoam and will definitely run it through the system.

FYI section 3H-15 is on page 3H-7 in the GT40 manual

Woohoo, the tube inside the FCC was totally shot. Not sure how that happened but the outer coating essentially disintegrated. Plenty of cracks and holes. Definitely the cause.

Do I need to order the tube or is it something generic I could buy locally?

Sure sems like if the PO had just replaced the HP fuel pump he would have notice the condition of that hose, based on your picture on PN????

The inside doesn't exactly look too good either 

Making progress, but still having issues and I could use some help with where to go from here.

I replaced the tube in the FCC and immediately got 38 psi fuel pressure (had 10 before). But, the engine is now hard to start when cold. I have to put throttle at almost WOT while turning key to get it to start, whereas before fixing the FCC tube it fired right up without needing any throttle. It also wants to die when I return to idle until its warm..

I then took it out for a lake test and I'm also still unable to get over about 1600ish RPMs. The engine does sound better. It accelerates smoothly to about 1400ish RPMs, then it levels off at 1600. Pushing the throttle past this point makes it almost sound worse. I did run a quick key on engine off test and got code 53 which is TPS out of range.

After the disappointing lake test, I came home and pulled the fuel rails and injectors. They were all in good shape, couldn't find any debris. Flushed with lots of carb cleaner. I manually ran the injectors off a battery with an air compressor and the spray pattern looked good. Tested resistance and they were all about 15 ohms. I haven't tested it on a hose yet, but I don't have much hope that this cleaning did anything.

Also, I was curious and pulled all my almost new spark plugs to inspect them. 7 of the 8 plugs were covered in heavy carbon deposits, suggesting to me they are running rich maybe? Not sure why 1 was clean though? Could my formerly lean engine that now has great fuel pressure be overcompensating?

Next steps:
-I'm thinking the TPS could potentially be malfunctioning? I'll try to test that tomorrow.
-Inspect IAC / spark arrestor
-???

Thanks again for the help!

Hmm, I know for sure I didnt unplug it, but the prior owner definitely could have. I forgot to mention that after getting code 53, I did clear the codes and run the boat for about 15 minutes on the water and retested them afterwards...no codes. Not sure if I gave it enough time for the code 53 to come back or if as you suspect that was an old irrelevant code.

Yeah I just was running short on time last night, going to check the TPS this morning. I may have just had a breakthrough though, I looked up the injectors and they are 42 lb, looks like they should be 24 lb. They were replaced recently by the prior owner. My theory is 42 lb injector + 10 psi fuel pressure was a workable combo. But now 42 lb injector + 40 psi fuel pressure is running incredibly rich.

Thanks! I'm sure I am going to have plenty more surprises at this rate haha.

To the sellers credit, he has been super responsive and without any prompting sent me a generous refund on a portion of the purchase price.

The seller sounds like an OK guy

Update: Installed the correct injectors today. Engine fires right up and sounds much better, but I still have a rolling idle. Pulled the IAC valve and cleaned it, but it didn't look too dirty. Just to verify though, the solenoid fired when I turned the key on / engine off to open the valve, and then it closed a couple seconds later. Is that what should have happened, or should the valve have stayed open with KOEO?

I'm planning to check the vacuum hoses for a leak in the morning. Then I hope to do a lake test in the afternoon to see how the engine does under load. Fingers crossed the injectors / FCC tube solved my top end power issues!

I took the boat out today and the power under load was significantly better, but it's still not running great. Whereas before I couldn't even plane, now I'm topping out around 32 mph / 2800 RPMs. It also hesitates when accelerating and just generally runs rough / surges. The idle is also still rough but that isn't as big of a deal to me right now.

I gotta admit I'm getting frustrated and could use some help with where to look next as it feels like I'm running short on options. Maybe replace the coil? Maybe the full distributor? I'm also gonna open up the main wiring harness and start tracing all the wiring to make sure everything is correct.

A few more details:
-It sounds a bit like a misfire at WOT, but I can't say for sure. I also think I hear a tinny clicking noise too
-About the last 3-4" of travel on the throttle doesn't increase RPM.
-I disabled the oil pressure switch / water temp switch to bypass limp
-I don't think it's a fuel issue (fuel pressure was 39 at WOT, all new injectors, new fcc tube, new filter, checked anti siphon, new fuel pressure regulator)
-I couldn't find any vacuum leaks
-New MAP sensor
-New rotor, cap, wires, plugs
-All the wires are making a spark

Now the other direction I do have to consider is more serious mechanical issues. I did another compression test with the engine warm (but dry) and got 100-115-115-115-110-95-100-110. Not great numbers, but if you adjust for my elevation they would be about 15-20 higher at sea level so more like 120-145 range. Is that a huge red flag? Think its worth a leak down test on the 95 cylinder?

Thanks for helping me stay sane!!

It could be a bad baffle in centek muffler causing back pressure and loss of top end power.  My story linked below.