Backfire under load

[trailgypsy]

Have a 99 centurion with 350 mag mpi ski. Took the boat out last week ran fine all day until last half hour on the water. Backfired and lost power. Turned it off, started back up and drove to the dock with no problems. Then I took it out 2 days ago. Ran fine all day no problems. Then I took it out today and after 30 minutes it started to backfire and lose power under load. When in neutral no backfiring and it sounds normal but rpm jumps around a bit when you leave throttle in one position (will lose rpm then pick back up). Boat will drive under 2k rpm maybe 5-10 mph at most but as soon as you get on the throttle power dissappears and backfires.

 

[achris]

Quick WAG.... Sounds like a fuel problem. Without using diagnostic software and a fuel rail pressure guage, here's what I would check first.

Check the fuel tank vent isn't clogged. Try running with the fuel tank filler cap loosened or off.
Check the fuel filter (ie, replace it and carefully drain the old one and look for water and/or excess dirt).
Check the fuel lines aren't pinched or kinked.

Next I'd look at ignition system.
Check (or replace) the crab-style distributor cap.
Check the spark plugs and their leads.
Check for a good spark from the coil.

Chris........

 

[frobotz]

Sounds like ignition timing to me. Just a hunch.

 

[achris]

Sounds like ignition timing to me. Just a hunch.

MPI engine. Timing is done by the ECU, even the initial timing....

 

[trailgypsy]

Quick WAG.... Sounds like a fuel problem. Without using diagnostic software and a fuel rail pressure guage, here's what I would check first.

Check the fuel tank vent isn't clogged. Try running with the fuel tank filler cap loosened or off.
Check the fuel filter (ie, replace it and carefully drain the old one and look for water and/or excess dirt).
Check the fuel lines aren't pinched or kinked.

Next I'd look at ignition system.
Check (or replace) the crab-style distributor cap.
Check the spark plugs and their leads.
Check for a good spark from the coil.

Chris........

Found a bad spark plug cable. They all look like **** but one had a small in the back of connector and would arc if u touch it with a screwdriver. 2 plugs look a little burnt as well. Going to replace all plugs and cables tommorow. Also going to replace cap and rotor. I'll update tommorow after I see if it changes.

Backfire I thought is from "unused" fuel. Which would lead me to believe I'm getting plenty of fuel. Also I would expect it to die and not be able to start if it weren't getting fuel. Definitely not a boat mechanic so I'm not saying that theroy is right

 

[achris]

'Back firing' is usually a sign of a lean/weak mixture, or a weak spark... It is 'unused' fuel, but not in a 'rich mixture' sense. It's 'unused' because it didn't fire at the right time.

 

[frobotz]

MPI engine. Timing is done by the ECU, even the initial timing....

Correct. Still doesn't eliminate it completely..

Sounds like it might have been a dead spark plug wire.. Still ignition related. Not carb.

 

[MBAKER]

I would do a compression check on all cylinders before chasing too much electrical or fuel. At least that way you know mechanically its sound or not. Or at the very least put a vacuum guage on it and see if you got strong steady vacuum.

We had similar issue a few years ago while spending a week on the lake, low speed was fine but put a load on it and open it up and it would backfire through the carb erratically. Did the usual change fuel filter and chase the electrical but kept getting worse. Finally at home did comp check and found a dead cylinder and another very weak one. Ended up being intake valve problems where the valve had warped and would let let combustion gas back into intake whenever that hole tried to fire. It started out at high rpm but started to creep down as the valve got worse.

 

[harleyman1975]

Im leaning towords the plug wires fixing it.

 

[trailgypsy]

Changed cap rotor and the bad plug cable. No change. If anything its worse today. Now it won't stay running. After a minute or 2 it dies

 

[trailgypsy]

Quick WAG.... Sounds like a fuel problem. Without using diagnostic software and a fuel rail pressure guage, here's what I would check first.

Check the fuel tank vent isn't clogged. Try running with the fuel tank filler cap loosened or off.
Check the fuel filter (ie, replace it and carefully drain the old one and look for water and/or excess dirt).
Check the fuel lines aren't pinched or kinked.

Next I'd look at ignition system.
Check (or replace) the crab-style distributor cap.
Check the spark plugs and their leads.
Check for a good spark from the coil.

Chris........

Is there more than one filter I should change. Can only find the fuel/water separator filter

 

[Bondo]

Can only find the fuel/water separator filter

Ayuh,.... Welcome Aboard,.... That's what yer lookin' for,....

Examine it's Contents, for anything but clean fresh gasoline,....

 

[trailgypsy]

So there was almost no pressure in the fuel rail. Pump was making noise but no suction. Mechanic had a theroy that it wasn't getting enough voltage. Hooked up a bigger fresher battery and the pump started actually sucking fuel. Seemed to fix the problem (in the driveway not under load in the water yet). So it appears the alternator is no good causing the engine to have not enough voltage. Alternator shows up wed. Will update after I put it in

 

[trailgypsy]

Fuel filter was a little dirty. I know the boat had been sitting a while and not maintained very well so I expected that. Going to flush out the tank and buy a new line from the tank to the filter

 

[trailgypsy]

So the alternator fixed my voltage and light issues but not the actual loss of power and backfire issue. I checked the fuel rail pressure and there is nothing there. It gets enough gas to start and idle but that's about it. When you push the pressure valve there is no pressure and just a small dribble of gas comes out. Not sure if it is a pump or regulator issue. Pump makes noise and sounds like it is working so I'm leaning towards regulator

 

[wellcraft-classic210]

Have you considered checking for fuel leaving the pump ? As in checking pressure and or flow ?

 

[trailgypsy]

Learned of the anti syphon valve. Thought I should check that before replacing pump or regulator. Replaced the valve and fuel line. Getting pressure and fuel on the fuel rail now. Seems to be back to normal in the driveway but not convinced until I can get it on the water today or friday