Burning valves and melting spark plugs 5.7 efi

[Cdn20valve]

1996 5.7 efi 0F831652 TBI
Hey guys. I'm having a problem with my port engine. for two years now I've been trying to fix this. I've had three heads rebuilt. and the problem keeps popping up. At low speed the boat can travel all day with no issue. but any planing speed for 45min or longer results in engine loss.

Issue: At sustained planing speeds 3000 rpm for 45m or more, without warning, no overheat, no alarm. the spark plug in either cyl 8 or cyl 5 melts and pops out. leaving the threaded portion in the head. and the porcelin floating in the bilge. I do a comp test and the affected cylinder is at 0 psi. So far it has affected cyl 8 twice, and cyl 5 once. (both heads affected). each time this has happened the gauge does not show overheat. i can still put my hands on the risers etc...

This time it's cyl 8 again, and I need to find the underlying cause. I started investigating and found a bad kink in the fuel hose between the lift pump and the VST(high pressure) pump. the fuel hose is approx 50% restricted and very soft. once i get the head off i'll report back. also i will do a fuel pressure test once the head is checked and reinstalled. Hopefully just a head gasket, but last time was a burned valve.

I now suspect a lean condition at high rpm. let me know your thoughts please.

thanks Lou

 

[Jeff J]

I agree with your thoughts about it going lean at high rpm. The kink in the fuel line could easily cause it. I don’t think a water temperature gauge or warning system would catch it (depends on where the sender is located). Oil temp might spike but maybe too late. Only a cylinder head temperature gauge or warning system would give a timely indication something is wrong in this situation.

 

[kenny nunez]

One other place to check. Look inside the intake manifold runners just to be sure there are no restrictions.
There may be a rust or dirt build up in the fuel rail to the cylinders giving the problem. Try blowing out each part of the rail.
Have the fuel injectors flow tested.

 

[alldodge]

Agree, sounds like lean condition
Unable to locate your serial number but being a 1996 350 TBI I would have the injectors cleaned and flow tested. Make sure to keep the 2 injectors identified for which side they came from to confirm

 

[Bondo]

Ayuh,..... I agree as well, but I'd start at the fuel tank, 'n inspect/ clean everything, all the way through the fuel system,.....

 

[nola mike]

Can you monitor rail pressure on this model electronically or does a gauge need to be connected? That could tell you if the problem is with injectors or before. Also, any common connection between the port/stbd fuel system would indicate your problem is downstream of that.

 

[Bt Doctur]

I also had a plug blowout .The final cause was the clip inside the boot was missing and the plug wire was arcing to the top of the plug.

 

[Cdn20valve]

I took the heads off today, both of them to look everything over. cyl 8 has a tuliped intake valve. block looks ok. cylinder walls ok. no cracks in the head visible. i did notice the water jackets in the block were full of rust . they are only 1/4" holes, and they were plugged up around cyl 8. 4 out of 6. i dont know how i missed that before. must have been working late on the boat and overlooked it. cyl 7 had a couple plugged jackets but not as bad as 8. I will run compressed air through the block tomoro while holding a shop vac and get all that corrosion out of there.

The plugs dont look like lean burn, they have some carbon on them, and are a light tan colour. Is there anything you guys want pics of while everything it out?