backfire at wot

[tim10034]

hi,,, i have a 1996 efi 7.4 volvo..today when i went out for a test run it gave me a little problem....it ran fine for about one hour..then i let it sit in the water for about two hours...when i went to start it up again and tried to get on plane it backfired around 3000 rpm for a couple of seconds ...this went away in as soon as i let off the gas...well i decided it was time to go in and i didnt have any backfire problems until i went to wot (around 4600 rpm)..its a popping sound..but if i let off the gas to 4400 and below im fine...just replaced the low pressure fuel pump...checked lines to pump last time and replaced fuel filter last trip...filter did have rust in it..thanks

 

[cjflanagan]

Re: backfire at wot

Is this happening while on plane, or is it on the way up while under load? Is the fuel left over from last year? Have you looked at your compression lately?

 

[fireship1]

Re: backfire at wot

Hello, You might have a lean misfire. It's usually caused by low fuel pressure or sticky or clogged injectors. Did you check fuel pressure after you replaced your low pressure pump? If you left stale gas in the tank for a while without running you might have gummed up one or more of the fuel injectors. You also need to check your plugs, as they are an excellent indication of what's happening inside your engine. Injectors need to be flow tested with pressure behind them. You will need a pressure gage that taps in to the fuel rail and a "pulser unit" a breakout box that energizes the injectors for test purposes. If you find any clogged you can try to power flush them with injector cleaner. Otherwise you have to replace any that won't clean up. Good luck....keep posting!

 

[tim10034]

Re: backfire at wot

this happens on plane and when i push it to full throtle...gas is new ...and i dont know what the compression is...when i replaced the low pressure pump ..it didn't sit like the old one ...but i ran it before the last time and didn't have any problems....

 

[COLDWATER]

Re: backfire at wot

The possibilit of clogged injectors or low fuel press is not your prob you must have fuel to create a backfire fuel injection shuold rule out predetination so we have unburned fuel escaping through the exhuast system and igniting in the exhuast manifold. this is caused from exactly what you have described. During the pull to top end theres always more fuel dumped than can be burned unless its equiped with a mass air flow SENSOR like the ones on most injected vehicles like vortex chevys ect. the MAFS allows only the correct amont of fuel/gas to be injected so that it is burnable this all falls under volumetric efficence any way thats a diff story. So If it is ecm controlled your problem could be in this area. If not you could have a sticking exhaust valve or one with a little trash or carbon in the valve seat allowing gas to escape periodically during throttle up and on top end due to of course the engine is running a higher rpm and the engine rpm is increased to the point at which the stick valve can't close fast enough to seal out the wasted gas that as i said earlier is normal during these times of operation full and in a hard pull. At lower rpms the fuel is burned and the problem is not apparent. could also be a weak valve spring. I would run some seafoam cleaner through it and clean it out but a lean misfire is usaully encountered when the engine is cold and at start up as well as higher rpms and not as bad at higher temps. If your system was clogged it would misfire at any rpm been there done that. I would pull my rocker box covers and simply check the valve spring tension. But hey if has sit up for a while run it awhile and givi it a chance before you grab a wrench. some of the rust in the filter may have escaped and be bouncing around. good luck

 

[hotmerc]

Re: backfire at wot

i had this problem once. check your risers. i had a hole the size of a dime in one of them and water was getting down into the cylinder and foulded out the plug.

 

[tim10034]

Re: backfire at wot

funny thing boats are.....i took it out today and ran it for about 45min....no problems....no more backfire...engine idled fine....could it be that i have to run it for awhile and let it sit for about an hour???? i dont know...it will probaly give me problems when im running for the day with the family...lol...where can i buy that seafoam at in Louisianna?

 

[Darian F.]

Re: backfire at wot

Try hooking a fuel pressure gauge on the engine and check it at idle planing speed and wot.<br />problem could also related to heat soak, in the igniton side (coil or module) also check base timing and the timing advance