View Full Version : 496HO Mayday: Low fuel pressure, poor idle, surgin

09-22-2010, 03:08 PM

I have an 2003 donzi with a 496 ho giving me some trouble. Idle rpms are fluctuating as the IAC sounds like it is contantly opening and closing (loud hiss). Fuel Pressure at the Schrader valve with key off is only 24 pounds, motor off. When I turn key to on, fuel psi spikes to 30 and drops to 24 where it generally remains, even when I start engine and romp on it. When I give it a little gas to around 1800 rpms in neutral, the engine will surge to 1800 then it will drop to around 1400 and flucuate between 1400 and 1550 rpms. At WOT I've only been able to achieve 4400 rpm when I typically get around 4900. Any ideas?

-NO alarms, no scanner codes triggered, no bad history reported on scanner. I used carb spray to search for vacuum leaks and found none. (mechanic verified this, but he also "said" fuel pressure was in spec and it clearly is not @ 24 pounds). Spark Plugs are fresh. Crank position sensor changed.

This is a 2003 engine, so fuel cooler is gen. II. I've replaced the water separator/filter, but not the inline filter before the boost pump, which I plan to do this weekend. Could a clogged in line filter (before low pressure boost pump) be the culprit or is it more likely something with the high pressure boost pump? Spec fuel pressure should be 43 psi. While I'm getting only 24psi, I'm still able to get 4300 rpms. Could problem be something else? Boat has about 200 hours on it.

posting videos of problems below.

Any help would be appreciated.



09-22-2010, 04:00 PM
I thought we already went through this..Not to be a dic!!!

Again the fuel pressure is to low for your engine,did you try a second gauge to verify fuel pressure?find someone with a more accurate gauge..preferably a snap on digital one and not some cheapo one that craftsman sells.

did you check for miss in the engine by either unplugging the injectors or the coil packs to verify which cylinder it is?

look for a miss first, it don't sound rite in the video at idle.

rev the engine to 2k and start unpluging the injectors or coil packs and listen for a change in the rpms..if you find one that makes no change then that would be the problem cylinder,where it goes from their you will have to figure out if its the injector,coil, sparkplug,injector signal from the pcm..

09-22-2010, 08:36 PM
I know it sounds barbaric but carefully spray a good amount of Gumout (which is compressed gasoline in a can) (never, never ether) down the intake at 1500ish rpms, if it clears out and runs perfectly for a few seconds then you KNOW it is a fuel supply problem. If not then it is an ignition problem. This very old trick even works on EFI.

09-22-2010, 11:25 PM
YOu could certainly have a clogged fuel filter. The pump may be going bad also. I agree with spraying fuel down the throat. If it changes, you have a fuel problem. This also sounds like a vacuum problem on a motor with a carburetor.

09-22-2010, 11:30 PM
We could hear the motor much better if you closed the hood. All I can hear is the air sucking into the intake.

Elaborate on what exactly the motor is doing and when. Does it run better or worse the more you push on the throttle? Does it run better or worse under a load (ie: driving slow or fast or even accelerating)?

Details, and we might be able to help figure this thing out, or point you in the right direction.

09-23-2010, 12:09 AM
if the injector is plugged it will make a hissing because the ecm is trying to correct the idle so it will always be telling the iac to compensate..if you have access at a scanner go to the live data and fine the cylinder misfire data,if your scanner don't have it then look at the map sensor data.. it should be reading somewhere around 128 hg if its not then you have a injector problem..you can also take a long screwdriver and hold it against the injector body and put your ear to the handle and you should hear the injector clicking..also if you pull the spark plugs and if all but one looks black the that would be the cylinder that's not firing..

When fast tucker bought his boat he had some issues with less then 50 hr's on the engine, his mechanic replaced the fuel pump and 1 injector and now his boat runs great..

Mr X
09-23-2010, 08:16 AM
So, I guess you did not like our answers the first time you posted this??:confused:

09-23-2010, 11:28 AM

Yes, this is the second time around with this problem. I'm sorry for the inconvenience. Unfortunately, since last post, I discovered that my mechanic lied to me when he said fuel pressure was normal. Turns out he never checked it, which I discovered when I checked it myself. Fuel pressure guage I used was brand new Actron unit. I don't have access to boat on weekends, but I will try using digital one this weekend. As for the injectors, I didn't unplug them, but I did put a stethoscope up to each one, and each one sounds normal (clicking, etc). I've sprayed carb spray around injectors and intake manifold in search for vacuum leaks and have found none. Fuel pressure is only 24 pounds, and that holds whether the vacuum line is on or off the regulator. I changed the spark plugs and nothing has changed. I'm changing the inline filter this weekend. I am also planning to replace the regulator on the fuel rail and the regulator on the fuel cooler this weekend. Under load, starting from idle, if I give it a little gas, the engine will surge to about 1900 rpm, then drop to 1400 or so where it will jump around about 100-150 rpm. Boat still comes out of the whole pretty well. Midrange is good. Over 4000, it begins to struggle. I'm lucky if I get 4300 rpm. Problem is most noticable at low rpm. I was idling thru a no-wake zone last weekend and the engine kept "wanting" to go faster. It would lurch up to 1100 rpms and then back down another 200 rpms. Engine would do this in a consistent repetitive pattern. I'd say it lurch up to 1100 and back to 900 or so every 7-10 seconds (under load). It really may be running rich. If I pinch off air supply going to IAC, hissing stops completely but rpm drops as well. At this point, I assume it's a fuel problem, and I fear Mr. X may be right. If it is a fuel problem, where should I start?

09-23-2010, 01:29 PM
We changed the low and high side fuel pump.I would do that.It was$203.33 for the low and $255.09 for the high side.He also replaced two fuel filters.one injector.8-plugs.

Mr X
09-23-2010, 11:39 PM
Take the top cover off the fuel pump filter/sump.
It should look perfectly clean...like new inside. If there is any corrosion
visible inside, then the coating has come off and turned into goo...
and is now everywhere inside your fuel rail and injector system.
The symptoms you are describing are exactly what this condition will
Please post your findings on here.

09-24-2010, 11:18 AM
Mr. X,

I am going to work on the boat tomorrow. The motor is a 2003 so the fuel cooler is gen II, which doesn't have a filter or sump. I do plan on taking the fuel cooler apart and looking for any crap before the high pressure pump. I'll also replace both regulators, one on the cooler and one on the rail. I'll post what I find.

Thanks for the advice

09-27-2010, 09:50 AM

Looks like the high pressure fuel pump is going bad. I didn't find trash anywhere in the fuel system, so I'm pretty sure it's the fuel pump. I'll let everyone know how it goes once the pump is replaced.


09-27-2010, 01:38 PM

Looks like the high pressure fuel pump is going bad. I didn't find trash anywhere in the fuel system, so I'm pretty sure it's the fuel pump. I'll let everyone know how it goes once the pump is replaced.

Looks like I was right again..........Its hard being the :kingme: