would refer you to the thread that i ran some weeks back where went thru exact same thing... and did all the same stuff.

run perfect one minute, flat on its face the next. in my case it was one bad injector...bad as in shorted internally. you might want to look at that thread as an idea of what was done in what sequence... went thru the whole fuel presure thing because acted just like you say but bottom line was that motors run fine with a whole range of fuel pressures... and by fine i mean that the differences you observe are not enough to make the motor run " badly" not perfect fuel curve for sure but would run cleanly on 8. in my case it became apparent that as the injection pulses 4 injectors at a time that the shorted one was electrically affecting the pulse width of the other 3 on the same circuit.... enough to make the motor run just like you say ... except when it didn't.

the merc tool has an injector pulse function. i took the injectors out, pressurized them and pulsed them and found the bad one... replaced it and the motor has been perfect since....

am simply saying that symptom from THIS failure had me checking and testing EVERYTHING else .... just like you are...

If you blow up this picture, it clearly shows the fuel direction arrow (i) going the wrong direction... maybe that's the problem, fuel goes from the rail into the pump, then to the return line back to the filter head.... ya's just gotta wonder how it gets to the rail in the first place.... doncha just luv flawed drawings.

Jim

Steve, or someone can you tell me where to find your thread on the fuel injection problem, that could be possible, but im not sure how to test the injectors, as for the whole computer thing, we have had a "MERC" computer hooked up to the boat and all apears to be fine according to the computer. thats what blows my mind, fellows this really sucks, i never get the simple problems, its always the long drawn out problems, its beginning to get costly as well!

just hooking it up and it says all is fine is not the end of the story did you run the boat on the water with it hooked up ????? did you check fuel press while on the water, did you check history and guardian ect there are many factors that can make a 496 run as you have stated, is the tech very familiar with the 496...

this is not an easy mtr to diag without hands on and can get very expensive if the tech does not know what he is doing.... if you email me Ill send you a trouble shooting flow chart that will give you the steps in logial order to check and a work sheet (it will help) did the tech who put it on the comp do a work sheet???

thats whats wrong with these forums they end up with "throw enough sh*t against the wall something will stick" by working the flow chart it goes back to basics and will save you time and money... any good tech here could do it without it because we have been there but for those who dont do this every day just start at square one!!!!!

ill find it and get you a thread # .... all you have to do is get the injectors out in your hand... on my motors that was less than 20 mins.... then i just ose clamped the high pressure line to it and cycled the pump a few times to buld pressure and then used the scan tool injector test circuit to pulse them... once out takes about 5 mins per injector.

let me tell you.... i checked EVERYTHING... spent 3 full days or more on every concieveable thing... fortunately mine is a twin engine boat so i had another of " everything" to swap out so i could confirm good or pad parts...

amazing thing was that motor did not go on a clean 7 with the bad injector... ran SO bad and flat...just like yours that i convinced myself it was elsewhere and systemic... not cylinder specific... but thats exactly what it was.

If your going to be correcting people you might want to check your spelling, it's "grammatically"

By the way, credit to Dixie Doug who helped me with this issue a couple of years ago. It will take 15 minutes, pull the pump and look. Either solve your problem or eliminate it before you do all the crap I did trying to chase it. Like doug said it wont show up as a code, I tried that. Good luck, JC

well sounds good, the guy im working with talked to merc, and from what i gathered we werent doing are fuel pressure test correct, i wasnt pulling the vacuum tube off which would mess the results up, so merc said full sounds fine, however they also agree that it sounds like either a spark,ignition problem or fuel injector, and told him how to test these things useing the new merc computer, we'll see how it goes, i just know, i pray i hope i dont have to replace all the spark plug wires, big, big money$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ $$$$$$$$$$$$$$$$$$$$$$$$$$, is all i see!

P.S i hear there about $350.00

We have two sets of brand new sparkplug wire sets for the 496 engine and we will sell them for $120.00 per set plus UPS, if anybody is interested. Call Us. By the way, I don't think plug wires are your problem here. It sounds like a few others here have expierenced a similar problem on the 496 with some fuel system pump and filter problems. You are right this won't show up as a code problem on a scan tool, but it is not difficult to check the screens and filters to eliminate that problem. As Goodt has said until you run the boat under load with the scan tool hooked in and someone qualifed reading the tool for both current outputs,codes , guardian modes and history, you are going to be trying to find a needle in a hay stack.

Ray @ Raylar

Raylar i will know more today but might go ahead and just get them plug wires, will post later today or give you a call, but i agree, i need to look much further into this. can someone tell me where to find a blown up sketch or diagram of the fuel filter/fuel pump area and where them screens are, i dont have anything shown me even where to start?