Going crazy on my Mercruiser 496, need Help!
 

Hi guys, I am new to the forum and need your expertise.

Have and Sea Ray 340 and have repowered from Twin Mercruiser 6,2L to 496 HO engines, Hurth V-drive and new ACME 4-blade CNC-cut propellers.
Engines from 2001, 400h

Have owned the boat for two summers but one engine is causing me problems....
Symptoms:
Engines starts fine and runs great at idle. If I only go slow, 1200 rpm I can travel a long time without problems
But when I take the boat up to plane it works perfect at first, strong and easy up from the water.
Regardsless of rpm (3000-4500) can I only drive the boat for a approx 4minutes!, then the starboard engine start to cut/bog down, rpm jumping, loose power and pop. Then stop....
Easy to restart but will cough and stop if I go over 1000rpm.

I have read a lot about problems with these engines, but nothings helps...=(. No beeps and no stored alarms when hooking up computer.

I have
-Changed water separating filters many times
-Changed both fuel pumps (fuelboost and inside cool fuel)
-Emptied the gastank och refilled with new fuel.
-New fuel lines
-New sparkplugs and cables
-Cleaned injectors and fuelrail
-Changed regulator inside cool fuel module (Gen2)
-New IAC
-Lifted the engine during the winter and sent it to Mercruiser Service, they sad that some cables to the injectors where in wrong order and vacuum hoses where connected wrong, but no alarms. Testrunned the engine
indoors and sad was OK, but I still have the same problem now when I am back in the water.....


Tearing my hair, can anyone help me?? Can´t continue to run the boat in 5 knots....what can I do/test?

Have you done a compression and leak down test?

most fuel tank pickups have a small screen in them that can become stopped up .How much trouble to swap hoses at pickups and see if problem follows swap.. You didn't say you had run boat with a fuel pressure gauge attached,,, have you?

Sydwayz: Yes I did, within range, think it was 160-165psi on all cylinders. Pulled all sparks when did the test.

motor: Got the tip to check the 90 degrees elbows that connect the fuelline to the tank. There where wide open....Have also taken away the fuel shut offs.
My next step is to try and external tank and connect to the engine. Have also ordered a fuel pressure gauge that I can connect on the fuelrail. So I can se if the pressure drops when this occur.

Good call.

Maybe even (safely) rig up a test tank in the cockpit and take it out for a run. You could do this with a cheap 6 gallon tank from WalMart, some fuel line, and appropriate outboard fittings for the tank.

ck fuel pressure 1st under load..see what it does when falls down

The early models had a problem with the crank position sensors. you could swap from one motor to the other to see if it makes any difference. They are easy to get to, right on the back of the motor behind the intake. Check the temps on both exhaust risers for overheating.
Last year my friends 496 had a similar problem and we changed the sensor, but still had the problem. We scanned everything and didn't find any real issue other than one exhaust coolant temp was higher than the other. 1 was running 120 and the other was 150-160. We found out that manifold was way hotter by the touch, and that's where the problem was at. We flushed it out and it ran, but it still was hot. I finally pulled the bottom hose off the manifold while reving the motor upto about 2000rpm, and it finally blew out a bunch of sand.

Yours may not be sand, but could be blocked by any number of reasons. The point is that it had low water flow to it and we couldn't see any valid alarm indicating this was the problem, but the ECU was shutting it down regardless. I hope you get if figured out, sounds very frustrating.

could be crank sensor like mentioned but usually they wint start...it sounds like the fuel tube has a crack in it,,if the fuel tank has aluminum pick up tubes check the threaded part that threads into the 90* fitting that screws into the fuel tank..i have replaced alot of those...if their nylon tubes then you have the replacement's if not check and replace both..

What your discribing is sounds like fuel pressure problem,,also if you have a fuel pressure gauge that would be a good diagnostic tool for you ..

After rereading this, your first thing (as stated) should be to check fuel pressure while running.

Thanks so far for your replies. Did a fuel pressure test. Had 31 psi at idle....to low right? Took the boat for a run, couln't see the pressure drop when the engine stalled. Was about 35psi under run. Have know taken away the check valve in pressure tester to see if this changes the result. Will also try external tank to rule out that factor. If I only have 31 at idle, does at mean I still have problem with the fuel boost?

I believe those injectors need 44 psi to cycle.

If fuel pumps are new may need to check regulator. Will fuel system hold pressure when engi e is off. A slow bleed off is normal but an instant fall off isn't.
edit
Assuming fuel plimbing is routed like my 350 mags..

Time for an update, has been on vaccation so there haven´t been time to experiment with the boat.

-Took away the check valve in pressure tester to see if this change the result. It did!, I now have the right pressure on idle and under run, about 40-41psi. Within range...
When the boat (Engine) stalls/bog down the fuel pressure drops...U can see it clearly on the pressure gauge.

Connected the Engine to an external tank with a long fuel line nearby the helm, the Engine runned perfectly!! So I thought it must be the fuel tank that is causing the Engine to loose pressure...have checked the inside with an underwater camera an also checked the tanks airvent. Nothing remarkably.

So I cross wired the fuel lines to the tank to see if the problem moved to the other Engine, It didn´t. Still Starboard Engine stalls after 1 minute at plane.

Used the external tank again, no problems. There are differt noises on the fuel boost between the Engine when getting ready to start, Could the fuelboost still be broken/weak and can´t keep up under high load.
Thinking that the exernal tank creates a siphon effect that helps the pump when beeing near up the helm...Do anyone have any other theories?????

Trying to follow along here are you saying a portable fuel tank added help the motor clear up of its running problems? Always try to eliminate which side of the boat your problems are. Boat side or motor side so you done a good thing with the portable fuel tank, fuel line and fresh fuel.

It is wise to add a outboard primer bulb on your portable test tank fuel line to add more fuel to the motor side to see if that helps also as this could tell you of a fuel delivery problem.

Next another test is checking for fuel restrictions with a vacuum gauge inline between the fuel line from the gas tank to your fuel water sep filter if you have the Gen II cool fuel. Gen III cool fuel system is a totally different system and does not have the spin on fuel water sep filter. Anyways a digital vacuum gauge works best for this test - Mercruiser specs are 2 in. Hg (inches of mercury) max of vacuum. Most of the time anything higher than that spec will cause running problems and vapor lock.

Next if you have the Gen II cool fuel system there a small screen under the fuel pressure regulator that very easlly becomes clogged. It can cause a big fuel restriction. It will need to be cleaned. Next you could have a fuel pressure regulator problem.

I think you posted you have a 2001 496 if so this should be the Gen II cool fuel system. But as always I have seen boats that should have what it should have but do not. Example a lot people on this board do not have stock set ups so just saying. If you have Gen III cool system you need to post what you have because that system is totally different to diagnose. No matter what cool fuel system you have you can still do the vacuum test. Good luck

-To clearify. Engine pressure drops under load after about 1 minute on plane, now problem to restart.
-Have new fuel line and Connection between the fueltank and Engine.
-Connected starboard Engine too port gastank, still problems with the Engine....bogs down and looses fuel pressure. (Regulator is new inside the cool fuel module)
-Used an external tank without bulb and the Engine runned fine. Had the fuel tank up at the helm about 80cm above the fuel inlet on the Engine. That what I ment about
a siphon effect that helps the pump get more fuel.....just an theory....

You are right my Engines are from 2001 and Gen II cool fuel system.
Have ordered a new fuel boost again and will check the inline filter on the old one as well.

Summer is over, I think I finally found the solution to my problem. I ran out of fuel on my port engine that I havent had any problems with. After refueling it would not run smoothly. Removed the fuel boost and the filter was full with crap almost like mud. So hooked up two external fuel tanks and the boat ran fine, both engines!
So this must be problem, must empty the tanks, clean them and install extra fuelfilters. Anyone clean their tanks and have any good advice?

Believe it or not ethanol is a great cleaner. Fill your tanks with that almost full, let that sit and shake up for a while. Then completely drain your tanks, if your tanks are really bad you might need to redo that process.

If you have rust in your fuel tanks that's a different story and might not be able to get 100 % cleaned out. If your fuel tanks are aluminum, aluminum oxide / rust will lead into fuel tank leaks over time. We have seen this before, tiny pin hole leaks in aluminum fuel tanks especially in Center Console boats that had saltwater time on them.

Also do not add any extra filters on top of the filters Mercruiser already designed your engines to run on. Install new ones to replace your old ones but again do not install anymore added fuel filters. That will cause a fuel restriction and cause running problems along with lean running conditions..

2004 sea ray sundancer 340 twin 8.1s
 

I have the same problem im dealing with now omg

2004 sea ray sundancer 340 twin 8.1s
 

Idles great smooth can bring it up to 1200 1500 no problem i can even pour on the coals to it the power is there once it gets to about 3000 rpm it starts cutting in and out i can keep it running like that all day it seems but i back it off to about 2800 2900 and she steadys i dont get it if i push it above 3000 she will bog down bad i have to back it off

Same issue
 

Hi was wondering what the final outcome was IAM having the same issue