6NZ runs great with low fuel level, bad with full tanks

Happy Saturday to y'all, fighting a problem with my 6NZ in a 2003 Pete... runs great when fuel is 3/8 of a tank or less....snappy, powerful, hits 39 psi boost, very little smoke....when I fill my tanks up over half tank or more, engine is doggy, smokes like a locomotive, slow to build power, and is lucky to hit 31 psi boost and I have to rev it higher to get it to pull well. You can floor it and it won't give adequate power really. Has the marine cam with stock injectors, I know not the best combo but when the engine runs well, it seems to be fine. 550 stock file, slightly bigger turbo. Fresh overhaul, brand new injectors (not reman), many sensors replaced, fuel pump to updated style, regulator, injector harness, timing sensors....whole 9 yards. has anyone had or heard of something similar to what I'm dealing with on this truck? Tried a fuel temp resister as well, acts the same pretty much, just more smoky. Thanks for any suggestions

 

I've also cleaned fuel tank vents, I run Cat fuel filters, and put on a new fuel diverter valve between the tanks

 

@spsauerland

 

That’s weird, backwards to my BXS. It runs like #### with a quarter tank or less. Low fuel like that you can burn your leg through a pair of jeans leaning on a fuel tank. I think that might be an elevated fuel temp issue. I run on the top half of the tanks, no issue. I can’t think of what would improve fuel delivery at higher temps. It should run better the cooler you can get it. I might start with the fuel transfer pump, for output pressure, and lift. And the bypass in the filter head. I’ve had so many of both of those that were bad right out of the box now I only trust gauges if there are issues. Any one of those can drive cause fuel problems. I don’t know enough about pumps to know if something might make a wonky one perform better at low/hot fuel conditions, but that’s a possibility. I’ve got 2 5EK’s and a BXS, I’ve had a few fights with that fuel system, but I’ve never had anything like what you describe.

 

Have you tried driving with fuel caps loosened a bit ? I know you cleaned vents but it really sounds like tanks are building pressure when fuel level is higher..

 

Have you installed sight glass in return fuel hose to check for bubbles from leaky injector seat?

 

Pressuring up a tank cures more problems than it causes. I prime them that way with shop air when I’ve had fuel lines off one. A blocked vent everybody can tell, you get the drool from the fuel cap ####ing up your last tank polishing. A pintle drooling in an injector you’d probably be able to hear the fuel knock in it at idle, and get a hellacious shake and maybe smoke puffs when it’s revved up. If it’s not bad enough to hear a knock, a laser thermometer on the exhaust ports will point it out. Glad I don’t have to do that with my hand anymore. That one’s a mystery to me, I’ve never had one of mine behave that way.

 

Have you tried an auxiliary fuel source and run it to see how it behaves? Run it off a five gallon can and take it for a rip. Then you’d know if the problem was in the engine, or somewhere between the tank and the engine.

 

The only thing that even begins to make sense to me is that the fuel temp sensor is not responding correctly, and the ecm is making adjustments based on inaccurate fuel temp readings. But I don't know crap about this stuff, so this is just a wild ### guess.

 

That’s the part that confuses me. I’ve got a resistor on mine so the ecu sees optimal fuel temp. Still, at low fuel levels it runs noticeably rougher from the hot fuel, but not enough to drop power much. It still boosts up and pulls. My BXS will put up 72psi of manifold empty tanks or full, just on empty tanks you can’t read the speed limit signs anymore from the shaking coming out of it when you stand on it. That truck has 120’s on it and it starts the foolishness at about a 1/4 tank, or an 1/8th apiece. I’d be curious to find out what he has going on. I’ve never seen what he describes. Unless there’s a sandwich bag or the liner from a conditioner lid in there somewhere, and that usually chokes them out altogether.