496 Fuel Pressure
 

When I turn the ignition on and the boost pump runs for a few seconds I have about 44 psi at the fuel rail. When the pump turns off the pressure starts dropping fast. All the way down to 10, holds there for a few seconds and eventually all the way to zero. Once the engine starts I have a consistant 42 psi and runs fine at all rpm range and no codes.

What prompted me to check the pressure was that occasionally when sitting for a half hour or so after running hard I have trouble starting. It will either start slow and sputter for a second before the rpms come up or I will stop cranking wait a second hit it again and it will fire up like normal. Almost feels like a vapor lock. Only happens when the engine is hot. Never from a cold start. It also sounds like the boost pump is laboring alot more when it is hot than what it sounds like when it is cold.

Should my fuel pressure drop that fast when the boost pump cycles off? I have in the merc inline filter before the pump. I havn't pulled the boost pump out yet to check the inlet screen. I don't think it's clogged since it runs good once started. I just think my boost pump is ready to take a sh!t.
Just want to make sure I'm not overlooking something like the fuel pressure regulator,etc before I order a new boost pump.

Thanks

Rob

Rob,

I noticed that after running hard with mine, it starts WAY faster (actually perfect) if I turn on the key and wait until you hear the kool fuel pump turn off. I have not checked my pressure and have not really had a reason to do so. Perhaps you have already tried what I mentioned but if not, give it a shot. I have usually always waited for the pump to turn off but lately for some reason I've hit the key a little too soon. I dunno..

Yeah, I try to wait a few seconds as well, but when everyone is firing up around you and the hatch is closed its tough to hear it shut off. The pump you hear running when you turn the key on is not the cool fuel pump, its the boost pump. I'm thinking that once my boost pump cyles off it should maintain pressure. I also pulled mine off this afternoon and the inlet screen off the boost pump had no crap on it. Perfectly clean.

Hopefully Reckless 32 will chime in here. He seems to be the resident expert on 496 boost pumps. LOL

Some of the questions answered here will depend on the year of your 496 Merc engine. On pre-2004 Mercury was still using coolfuel 2 whch had the two fuel pumps in different places and a seperate fuel water seperator. On 2004 and later Merc wnt to Coolfuel 3 which has both pumps and the fuel water seperator in one housing with just one pressure line up tot the engine fuel rail. The earlier systems have more system to prime and sometimes when the motor has been sitting a long time without starting it will take longer in the older coolfuel system to fully prime the system and build the 42-44 lbs of fuel pressure . On the 496 setups the pumps are on a relay which when the key is turned to the run position turns them on for about 5 seconds from the ECM signal and then they will shut off unless the motor is started immediatly after or during the beep. If you wait to crank the motor with the ignition still on the motor will try and start with the pressure present which can be low due to normal dropoff and it can make the motor harder to start.
My suggestion is to start the motor as soon as you turn on the ignition or if not then turn the key off, wait about 10 seconds and then try restarting. The fuel pressures you list are good and as they should be so pressure on sart is not a problem. If you have an active fault code or a problem with a cam sensor fault it can take the motor longer to start and if the cam sensor circut is bad it sometimes won't start at all. If you have fuel pump or filter problems it can show up as hard starting but its easily recognizable because the motor just won't pull full power under load and will show obvious signs of fuel starvation and stalling.
Hope this helps.

Thanks Ray. I'll try hitting it will the boost pump is still running. Mine is an 03' an coolfuel 2. Why does the presure drop after the boost pump shuts down? Shouldn't it hold for a while?

Also no cam sensor or crank sensor codes active or logged.

I have the Cool Fuel 2 system and my pressure will hold the high pressure for a day or more. To leak down completely it will take like a week. Sounds like you have a check valve or other internal leak bleeding off the pressure that fast. My 496HO engine has never been hard to start. 200+ hours.

Thats what i was wondering if the fuel pressure is supposed to hold or not when the pump cycles off. Every other efi system I have ever seen holds pressure in the system.
As far as I know there are no check valves in the system. There is not fuel leaking from anywhere so it's either leaking out through a bad injector into the manifold or back feeding through the boost pump once it shuts off? If it was a bad injector I would think that would show up in other symptoms?

If leaking injector, washing down the oil from the piston walls in the cylinder where it is leaking and then into the crankcase oil is one issue that comes to mind. Not good for ring wear.

Pull the fuel rail from the manifold, then ignition on to pressurize it and see if any one injector leaks. Of course take precautions in case one or more do leak so nothing improtant gets wetted down with fuel, like a source of ignition.

I don't think its that deep. The reason I say that is because it runs too good to have a bad injector. And I just changed the plugs and they all looked good. If I had a bad injector or even 8 bad injectors for that matter that lost pressure that quick and dumped that much fuel that quickly it would never start.

The one thing I forgot to mention was that i purposly waited for the pressure to drop all the way to zero after the pump cycled off and it started perfectly normal when it's cold. Somethings not adding up here.

Person on another forum was going thru same thing and couldn't fix it until he switched gas brands. After switching he supposedly has no more problems.

Keep in mind that as soon as you turn the ignition key, that energizes the fuel pumps which repressurize the fuel rail. You can hear them pumping and then quit when the set pressure is reached.

Unless, the PCM555 is different, most ECUs prime for a preset time allotment such as 4 seconds vs a pressure threshold.
Dave

That is good to know and coincides with my limited experience. I thought that the pumps were just making up the difference between my slightly lower static fuel pressure when the engine is not running than when it is running.

I've been teking Ray's advise and cranking while the boost pumps are still running and has seemed to work, no hard starts lately. I decided to go ahead and replace the boost pump and I still get the same result where the pressure starts drop as soon as the boost pump shuts off the pressure drops fast down to 10 psi eventually 0. Cool Fuel 2 system (03') Engine still starts fine even at below 10 psi??? No fuel leaks or anything. Fuel pressure is solid at 43 psi at idle on the hose. At 2500 RPM it seems to drop a bit down to 40 psi. Seems to run alright, no stalling, missing etc. It did seems to labor a bit getting onto plane, but I had a heavy load.

Where could the presuure be going to? Leaky injector, think that would show more of a performance issue though. It runs rich,(Transom needs to be washed at the end of the day) but all stock 496 around that vintage seem too.
Any ideas?

Contact the Mercruiser Tech Line (405-743-6566) with your engine serial number and symptoms. Sometines they can be helpful.

496 problems
 

THanks Ray.

I have a problem, possibly fuel related. (2002 Twin 496 HO's)
Running at 4000 RPM+ the starboard motor just quits. Like if I turn the key off. Most of the time it starts right back up, so I guess it does not actualy dies. But... running at 60 MPH+ and one of the motors shuts off is like droping an anchor in the water.
Totally unsafe starboard turn. Possibly going in "guardian mode". I wonder why and only the starboard motor does it.

IT's one think that a motor all of the sudden does not run well or misc. problems but shutting off like this.. GGGRRRRRRRRRRRRR I almost flipped.

HELP !!!

Thx .
P

Paradox:
First thing you need to do is find out whether engine is going into a Guardian strategy, ie codes or alarms. If this is what has been happening, these guardiian power reductions will be recorded in the ECM history and they will point directly most of the time to your problem. If no codes are found in recent history to coincide with this shut off, then I would suspect you have a wiring harness power or bad or intermittent electronics -electrical conections problem.
If this is the case, on that motor, start disconnecting and reconnecting all the sensors and connectors one by one, look for partial disconnects, corroded or wet connections or just loose or patial connections. Make sure you undo and check the 10 pin or 14 pin main harness connector also. Also make sure all ground and battery connections are corrosion free and tight both at the battery switches, batteries and at the motor. A little contact non-flamable contact cleaner may be helpful. I suspect that one of these two scenarios is your problem Hope this helps,

Regards,
Ray @ Raylar.

Raylar

Thanks Ray

All connections are OK. I will have the computer check out the history. The last time the starboard motor quit, it did infact died. Started right up but I heared a small low picth klinking, knocking noise in the rear. Random, intermittent. I did not use the starboard after that. Put... put to the docks was not fun with one motor. If it is a lifetr or cam issue, then it's time for the Raylar 525 HP kit anyways. I need a great price and a deal Ray..

The symtoms your explaining point to a cam or crank sensor. There is another recent thread on here stating that Merc told the particular shop that the crank sensor will not throw a code but will inhibit to 90%.
We have experienced the exact same symtoms as you are explaining on another boat. The boat shut down just like someone shut off the key at 3500rpm. Started right back up and then quit and progressively got worse. It stored some cam sensor errors but was told those are common (and they are as I've seen them on my own and I haven't had any problems) but did not show as active until I got it running crappy on the hose. It then showed as an active code. We replaced the cam sensor and it fired up and ran like new. For a week or two anyway. The problem recurred and at that point we changed out both the crank and cam sensor and it has run with no issues for a year.
Got to love these electronics!!

Dave

Am still trying to figure out where my fuel pressure is going after the the ignition is turned on and the pump cyles off.
I replaced the boost pump, not it.
I pulled the fuel rail and injectors out of the intake manifold to make sure one wasn't leaking and they are all dry as a bone under presure as the pumps are running.
No fuel leaks anywhere. I did notice that under closer inspection the pressure actually goes up to like 47 or so while the pumps are running. Once they cycle off it drops fast.
Can hear no signs of an air leak once the pumps shut off.
has perfect 43 psi at idle on hose and runs good. Just starts hard occasionally when hot.
What else is left? Pressure regulator on top of the fuel cooler?

Makes sense since that is a pontential fuel poressure leak down point.

had the same problems and it was the crank sensor. changed it 2 weeks ago and all is running good now. I went ahead and purchased the cam sensor but will have to disassemble some seats to get to it.

I guess I'm not positive my occasional hard start is fuel pressure related but there are no codes active or logged for crank, cam or any other sensor.

I have no clue if the fuel pressure leak down id related to the hard start issue but the fast leak down is not normal and may be indicative of other maladies with the system that is. The fuel pressure regulator could be the faulty item as suggested.

Yes, That fuel pressure leak down will be the regulater on the cool fuel pump @ the bottom of the motor. I know no fun to get to..

Yeah, the motormount bracket off the stringer is right in the way. The plastic cover is even tough to get off. Any tricks to it?

Do you have to take the fuel cooler out completly to remove the pressure regulator or can you get to it while its in place?

Did you get your oil pressure problem figured out?

Turnernd, Yes just loosen the two nuts that hold the cool fuel to the block and it will pull out enough to get that cover off. You can change out the reg. without taking it all the way out . ..Yes figured out loss of oil press. on 496?....Motors where not "pickeled" as said they were. Alot of junk on pickup screen.

What kind of junk?

Trash in the pickup,mud?,A little rust particles??, Sludge....Partially submerged motor.

Changed fuel pressure regulator (Real fun to get to)! Not it. Pressure still drops after the pump shuts off. Must be that the pressure is backfeeding through the main fuel pump. Am still occassionaly experiencing a hard start after sitting for a while. What I've been doing is letting the pump cycle once without attempting to start then start it during the second cycle. It sounds like the pump(s) are really laboring and almost cavitating the first time as they cycle. Sounds much better on the second cycle. Runs great once the engine starts. The only thing that hasn't been changed is the main fuel pump.

One interesting thing I saw on the code history with Diacom hooked up was an occurence for a "fuel pump" something? I almost overlooked it as the std codes that always come up "fuel level chkd hi", pitot, trim, steering, etc. Everything else looked normal on that occurence. I forgot to save it before I cleared the history or I would post. I didn't think Smartcraft was tied into the fuel system at all????

It's really not a big deal other than it never used to do this until the middle of this year. Any expert advise? I'm tired of replacing expensive non returnable parts.

I had the same type errors where it was giving me crank sensor and cam sensor errors. my real issue was the trim limit sending unit portside (NOT the gauge sending unit stbside). the limit sender is tied into the 5 volt reference system and when it is dirty or failing, it can cause really strange symtoms and trigger false hard codes. one thing i remember is that it totally acted like it was starving for fuel and it also totally lost spark at random times with no warning. just for elimination, it wouldn't cost you anything to disconnect that 3 wire unit in order to make sure.

i realize that you're probably past this stage but sometimes it's the simple things that can throw ya when you think you're looking for a "bigger" problem. good luck!

How About A Baf Fuel Pressure Regulator Or A Bad
Check Valve In The Tank Mounted Fuel Pump

Changed fuel pressure reg. No tank mounted pump, boost pump has een changed.

I'd put my bet on either the high pressure pump or a leaky injector. The pressure loss may cause the fuel to vapourize which in turn creates a hard starting situation.
Do you have a fuel shut off valve that you can close to prevent fuel from re-entering the tank backwards? Try that and check pressure at the same time ...

Not a leaky injector. Pulled the rail and checked that while pressurized. No leaks.
That thought crossed my mind with the fuel shut off. I will try shuting the manual valve between the tank and boost pump as soon as the pumps cycle aff and see if it traps the pressure. I will probably try pulling fuel from a 5 gal can to see if that eliminates anything.

Why does merc no longer offer just the high pressure cool fuel gen 2 pump, pn 861156A1? They want you to get the pump and fuel cooler together. pn 861156AO2.
Based on the service bulletin the fuel cooler looks exactly the same as it is now.
http://www.mercruiserparts.com/bulle...2004/EN_05.PDF
Unless the cooler and the pump are one piece now and get away from that stupid 90 deg elbow w/ o rings I can't see the advantage.
Does anyone know how to get just the pump?

Changed my high pressure pump (lots of fun) and that was it! Now the fuel rail maintains 40 psi after the pump shuts off. Haven't had a chance to take it for a ride, but I bed it surely takes care of my hot start problem. It was weird because it made full pressure when running, must have had a bad internal check valve or something like that. Hopefuly this helps someone else down the road.

P.S I have a brand new fuel cooler to get rid of that merc makes you buy with the pump . I still don't get it, its exactly the same as the original. If anyone needs one let me know.

The check valves in those pumps are not at all fail safe. Running dry or water in the fuel increases the risk of a failure.

Regarding the fuel pump/cooler kit I think it came alive due to an upgrade that I seem to have forgotten the details about right now. If the pump fails (again) after this upgrade you'll once more be forced to buy the complete upgrade kit. Can't understand why the pump only is not available ...