454 gen v down on power
 

hey guys, looking for a bit of input from fellow techs on my current drivability issue. 1995 formula 252ss, 454 magnum efi w/ b3 drive. 760 hours. All basics are covered, cap/rotor, filters, plugs, so on and so forth. No misfire. Slight starting hesitation for first start of the day when cold, nothing alarming, fuel rail could be loosing prime a bit. The boat has been down on power the second half of last season and so far this season. It will get on plain, but without the urgency it should have. Midrange operation is ok 3-4000 rpm the boat runs good. Almost no problem detected. Punch the throttle at 4K and not much happens besides the engine sounding strained. No misfire, no pops or bangs. It struggles to hit 4100. I won't do it anymore. I have a few ideas as to what the problem could be, but they are still guesses at this point. Fuel pressure 43psi with regulator unplugged. 35-37 w/ vaccum applied. Drops to 20psi and holds when key is off. Any thoughts out there? Thank you guys.

I wanted to add...
 

engine serial...0F292367
fuel screen in vs tank cleaned
injector screens cleaned
dont have rinda tool, I have not gone that route yet
Haven't checked spark strength yet, waiting on tester from amazon
havent checked compression or leak down yet.
When I clamp the fuel return line into the vs tank, fuel pressure doesn't spike...I feel like it should.

Once again, any help or input is appreciated. Thank you.

Doug

I think this is key ^^^^ ............sounds to me like its running out of fuel or timing, or just tired @760 hrs

I understand how it can seem that way, but don't buy a worn out engine yet. I'm hesitant to sway anyone's thoughts by adding my own, but here it goes. I'm chasing inop/clogged injectors. 240$ to have them tested and cleaned locally. This thing ran so well and was ripping at 59mph on gps 50 hours ago. I will do compression check today after work. But it doesn't sound like a tired engine to me. Timing could also be an issue, but I've never had an engine change timing on its own.

Do you know what the fuel pressure is running at or near WOT?

If I remember correctly, it was in the 33-37psi range.
Thanks again guys for the help.

Doug.

With manifold vacuum connected.

The lack of fuel to the engine is a likely cause. Use a Tee and a vacuum gauge to test. Also place a section of clear line and look for air bubbles (air leak). This test is done under load at WOT. Anti siphon valve has debris or they may be a screen in the siphon tube that has debris.
Yes you will still have fuel pressure in the rail just not enough volume.

The 33 could be an issue, the 37 not so much. Would be nice to know what your AFR is running at.

Anti siphon has been inspected, and replaced due to the fact that it's 16$. The old one didn't look too bad. Good thought on checking vacuum. All the vac lines have been replaced with new, clear fuel hose. I can "t" into them tonight to check. Just seems to me vac doesn't play a part in a lean condition givin low vac = more fuel pressure. As far as bubbles, are you suggesting the low pressure fuel system is sucking air? Where would I install clear hose in that system? It's a stainless line from the low pressure pump to vst.

You could remove the stainless line between low pump and VST and make one with barbed fittings for testing. Get a pressure gauge to check fuel pressure (4-7 psi). IMO, a vacuum gauge would be good for idle to check for a leak.

Ok. Next step...validating proper low pressure fuel to vst tank. The low pressure pump has 100 hrs on it. I know that doesn't mean much, but I did replace it when I purchased the boat.

I believe you answered your question.

Recheck fuel psi at WOT, it should be damn close to what it is at idle with vac removed from regulator.

And yes, with the ret line blocked off you sould see much more psi than with it not blocked. This tests to see what the pump can push against.

I will recheck fuel psi at wot, and all other scenarios. Helps to write all this down and re-read.

Thanks again.

If it is a lean condition you may of hurt the engine last year, I think you need to be closer to 40psi on the rail

As stated, verify your fuel pressure.at WOT. If it checks out good, then do a leak down. My guess is if it is not fuel related you are due for a valve job.

Just had a thought. last season, the loss of power was accompanied by undesired sounds at Higher rpm (not mechanical, but almost sounded like detonation). At the end of the season, once hauled, I found the rotor was cracked at the stem where it sits on the distributed shaft. I replaced the cap and rotor and put the boat to bed for the winter. Of course it seemed to run great on land and with no engine load. Could the ecu possibly have relearned an ignition curve and now needs to be re-calibrated? I don't mean to get off the fuel delivery topic, but this timing senerio just occurred to me. I know there could be possible engine damage, Gotta rule out the other stuff first.

no, ecu isnt self learning, mefi is pretty primitive

Diag run
 

Key on engine off primes to 25,settles to 15 starts quickly

idle with vac ref 35psi
idle with no ref 40

cruise, 33-34psi w ref
cruise 33-34psi no ref

wot 31 w ref
wot 31-32 no ref.

I obviously have a leakdown senerio occurring. Just have to figure out where.

You have low pressure and needs to be increased, should be at 37 psi, IMO your running out of gas and its lean. Even cruise your low

Agreed, but why. Injectors leaking down? No gas smell out of the exhaust upon startup. regulator leaking down, pump output weak or lack of supply. Need to install clear fuel tube to check for air on low pressure side.

Actually I think its your high pressure pump, it cannot keep up. The pump should be able to reach 60 psi and a bit more during test. The pump can be tested but if tested need to make sure the pressure does not exceed 60 psi because it can damage the regulator.

Edit: could also be the regulator

Is the high pressure pump on this the same as cool fuel ?? I know VST has the lift pump and CFuel doesnt

Is there a regulator on top of the vst? Or just the one near the test port on the rail? What is under the Allen bolt atop the vst just between the two pump leads?

Once again, thanks for the replies and help guys. Much appreciated.

This is not the cool fuel setup. This has an old school lift pump driven by the accessories feeding the vst which houses the high pressure fuel pump. Stupid vst is a giant carb bowl with an electric fuel pump in it instead of jets.

Throw that chit in the trash...LOL.......put an Aeromotive A1000, bypass regulator, return to the tank and be done with it...:drink:

I made this up to tell another how to switch from VST to cool fuel, but should help. The regulator is on the fuel rail up front under the plenum. Any over pressure is return to the VST by the regulator

Have to agree with you, my VST was working just fine, but when I repowered I changed over to cool fuel and have a steady 40 psi and return to the tank. That said, to return to the tank the rail needs to be modified to accept AN fittings

I'm 100% cool with ditching the entire system as long as i can get the rail to play with a different pump and reg. Sending return back to the tank is cake.

Not that ditching the system wouldn't be a good move, but it may be a simple issue which needs fixed. Suggest doing your low pressure test before going the route. Removed my VST and it was working fine, just wanted to go MEFI 3, cool fuel and return to tank when doing repower.

Good deal. Ordered clear line from amazon. Got work again tonight and tomorrow so back at it Saturday morning. Thanks again for the help. I love fuel injection, accept when it's from the mid 90's.

Doug.

I didn't see anywhere in this thread mentioned ignition timing being checked...

No regulator in VST.
Genuine Mercury & Mercruiser parts. VAPOR SEPARATOR TANK, SN & Up (See Note)
http://www.mercruiserparts.com/Merc/...MMON/39671.gif

Timing was a thought, but there is a known fuel issue needing corrected at this time, if the problem persists, other systems will be looked at.

This is a classic fuel issue, specifically lack of it at higher rpms. In my case it was a bad injector. Nearly identical symptoms you have. Compression and leak down might be a good idea to see if there is any residual damage from earlier operation.

Trash, you had declining fuel pressure as load increased? I'm stuck on injectors also because the low and high pressure pump have been replaced for reliability purposes. The regulator, injectors and lines have not been replaced. Mechanical fuel pumps last for decades, not hours. There shouldn't be any issue with source fuel. The pickup looks very clean and anti siphon is new.

I was under the assumption you had cleaned and flow tested the injectors. Cleaning out the injector filter baskets is not enough. The injectors must have a solvent run through them, cleaned and flow tested / balanced (i.e. all injectors flow within .X% of each other).


I had one injector working very poorly and another not far behind but didn't find out until it was too late. Symptoms were motor fell flat on it's face above 4000 rpm and an occasional miss at cruise. This miss could not be heard but I could feel it in the seat of my pants. Problem is I let it go on too long and #1 leaned out enough to command an exhaust valve seat separation. The motor disassembled itself at the most inopportune time.

Fuel pressure was not an issue in my case. From now on if any of my EFI motors are down on power I check the injectors. There is not feed back loop to the driver that an injector is going bad unless you have dual wide band at a minimum. Preferably 8 channel wide band but that is virtually non-existent on the water.

Big moves today, found a cracked o ring atop the high pressure fuel pump causing a hemerage. Fixed that. Now I have mega fuel pressure. 50-60 psi. 70 if I deadhead the pump. Drivability issues still exist. The boat gets out of the water much better, but still will not climb over 4K. I'm thinking I have injector issues also. Clogged/not functioning properly. Going to pull them and have cleaned this week.

Injectors have been dropped of for service. My logic is this, even if it doesn't fix my rpm/power issue, I can now be confident fuel distribution is proper and I won't melt any engine parts. Stay tuned.

Never hurts to have 8 known-good injectors...