Macman

I have a 2007 Baja with the 496 mag. Last week on vacation it suddenly developed a bad misfire/detonating issue. The engine fires right up but sounds like it has a slight hiccup at idle. Once you give it gas the problem becomes very pronounced. After the boat gets up and going it runs fine unless you try and accelerate at even a halfway aggressive then it starts pinging instantly. Before I went to the lake I filled about 50 gallons of fresh fuel on top of about 25 gallons of fuel from earlier this season. The boat ran fine for a day and a half after fueling so I would like to assume that bad fuel isn't the issue. I went ahead and changed the fuel filter just to make sure it wasn't causing a restriction. The fuel pump has proper voltage and shows 44lbs of pressure. I am aware that pinging is caused by low octane, too much advance on timing and lean condition. I am leaning towards the lean condition just because it really acts up under load when the fuel demand is higher. This engine is low hours with only 165 on the meter and the boat is always stored indoors unless we are at the lake for the weekend. There are no buzzers or alarms sounding. Anyone have any suggestions on what I should check next?

BUP

iTrader: (9)

Low fuel pressure - restricted - the filters need to be changed and check for paint peeling inside the gen III cool fuel cell. Possible injectors need to be flow tested and cleaned. GM / 496 fuel rails can build up internal corrosion and alot times very badly. 2007 should have gen III cool fuel systems - there is 2 or 3 filters inside that unit. You missed at least one and that one is the most important fuel pad filter at the bottom.

If you do not have GEN III cool fuel system then you do not have a 2007 engine - Gen II cool fuel set up was 2005 and back. It all goes off engine serial number anyways. YOU need to post what it is.

Fuel pressure readings thru all rpms are ??? And it needs to be very steady thru out all rpms High pressure psi should be 43 or very close and stay there while running.

Fuel samples currently ??? What does the fuel look like - clear jar

extra possibles below

Scan the engine and look for knock faults. Possible knock sensor(s) problem and or other parameters out of spec. Coils all 8 firing with a strong spark events ??? Spark plugs condition ??? Spark plug wires ???

Engine compression testing ???

total indepth engine scan ???

Without any of this testing - pull out the dart board and thru darts at it.

Macman

I may have just found something. That previous reading was key on engine off due to the fact that didn't have a water source. Now testing it while running the pressure is about 37psi at idle and drops into the low 30's as the RPM increase (no load). The vaccume on the suction side is 2 inches and doesn't change with increased speed. According Merc this means the low pressure pump needs replaced.

Macman

I will pull it apart and look for peeling paint inside the housing as well.

monster12valve

Shouldn't the knock sensor be stopping detonation? I had same type of issue... Was bad injector. Infrared laser heat gun will give a good idea where the miss is. Or cylinder balance test on tech mate.