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.