rangerrick63

99 454 mag mpi, starts & idles just fine. Problem is, after running hard for a while, (wot), it just drops back to idle, it doesn't die, but it struggles to get over 3000 rpms, really cuts out. After sitting for a couple of hours at the island, I only pushed it to 3500 rpms on the way home & it ran fine.
I just replaced the map sensor after a code 34. It now seems to be much "snappier" on the throttle, & it revs just fine, but this issue is still there when under a full load.
I haven't checked the fuel pressure, but I do plan on doing that tomorrow.
Does this seem like it could be a fuel pressure problem? I did check the pump to see if it seemed to be over-heating, but it felt normal to the touch.
Where is the best place to check the pressure?
Any help will be greatly appreciated....

rangerrick63

After doing a search on fuel pressure issues, & reading many posts, I'm really thinking that my troubles is definetly fuel related. I do have to ask, if I had either hi-pressure, or low-pressure, wouldn't I have a trouble code indicating that? Or does it take something else to set that code? This is very nerve racking, I feel as though I'm "chasing my tail" with this. One other thing I noticed right about the same time this happened. The engine labored to get to 4800 rpms, & my top speed dropped down to 55. I had figured at the time I had 4 adults, coolers, & full fuel, so I just ignored it. Yesterday, it was just myself & wife, & about 3/4 fuel, still only 55, & then it falls off....I'm really baffled with this whole thing...Thanks for any input with this.

rangerrick63

Ok, here is what I found this evening....Changed fuel filter, fuel was clean & no sign of water or anything. TPS checked out perfect, IAC checked out perfect. Checked all fuel lines, & hooked a pressure gauge to the schrader valve on front of the engine. Turned the key on, 22 psi, repeated this several times & each time pressure went down, finally bottomed out at 10 psi. I tried to crank the engine, hoping to get it to at least prime back up (new filter). The engine hit a few times but that was it. So, I have pretty much accepted that I have narrowed it down to the fuel pump. Before I order a $300 fuel pump & cool fuel (only way I mercruiser sells it), I have to ask, did I miss something on the repriming the engine with a new filter. I've always just let the pump cycle a few times & the system is primed. If there is a different way, please let me in on it, but I'm thinking with the extreme low pressures, it has to be the fuel pump, someone chime in if I've missed something with this.....

Dave_N

I would continue to try to get the system primed and recheck pressure. It's probably the pump, but it could just be the regulator.

Invest in a factory Mercruiser service manual, the troubleshooting procedures are good and will usually result in the fastest, least expensive and least frustrating repair.

Dave

speedreeder

Fuel pressure, too high or too low will never set a code. No need for expensive scan tool. you really need to run it under load for a while to check accurate fuel pressure. Check with a local dealer on what your fuel pressure should be. Good Luck

rangerrick63

Thanks for the input Dave, I will try to pre-fill the filter & see if that works. I've not had to do that in the past, & I don't think I have a line issue, because what pressure it does build up, it holds solid. The regulator was just replaced about a month ago..(go figure)...I discovered a rust hole in the side of it, so I replaced it (should've replaced the pump then...LOL).

rangerrick63

The fact that I was not getting a "pressure code" was puzzling, so thanks for clearing that up. I just hate to drop that $300 for a pump & cool fuel & that not be the problem...Thanks

rangerrick63

Ok, here's the update....Still nothing definitive...
I managed to get it to prime & start, with gauge hooked up & vaccum hooked to regulator, I had 35 psi & the needle was very eratic. I ran it up to 2000 rpms, pressure dropped to 30, & needle was still eratic. I then removed & plugged off the vaccum line, pressure went to 42 & eratic, I ran it up to 2000 rpms, & no change, but needle was eratic.
Shouldn't the needle be stable, & not eratic? By the way, the regulator is new, but I was reading thru the service manual, & it refered to a damper. I've not been able to locate a damper, does this engine have one (99 454 mag mpi) ?
Additionally, I removed the pick-up tube from the tank, & had planned on checking the anti-siphon valve. Well, there is no anti-siphon valve, just a small shut-off valve, & the pick-up tube is the nylon type, & it's not stopped up or restricted, & the shut-off valve is working fine, no leaks.
So, what does all this add up to? Being that the issue is so inconsistant, could this just be an issue of a pump either going out, or just having a "bad spot" so to speak? In which case, I should just bite the bullet & replace the pump ?
I do appreciate everyone's help on this matter, it's always good to hear from folks with much more knowledge than myself, it does help....

kennyo

I had a pump that would hold idle pressure fine but could not supply the volume. It seems your pump is the problem.

jeff1000man

Sounds like a pump. You might be able to find aftermarket and save some cash.

Good Luck.