C-15 6NZ stumbles / pops / power drops

swaan · Apr 27, 2020

Read the last post. Fuel return line plugged

Trucknodie77 · Jun 4, 2020

That's nice of you to come back with an update on your fix. My truck is doing the same exact thing, I already changed the two diff, the two speed sensors, the fuel filters. I do have a Freightliner though. I am going to try what you explained on it.
Thanks for updating.

Tcollins · Mar 14, 2022

truck 307 said: ↑

Thanks agin for everyones replies in effort to help resolve my one year (and counting) problem.
Many times i changed all fuel filters, dumping each into a seperate ziploc bag and hanging it on the wall in my shop to see if anything, any sediment at all settles in bottom of these clear ziploc bags, but nope.
One time when this all first started, i built a fuel line, stuck one end in the fill cap on drivers side tank, and the other end directly connected to the primary fuel filter housing so all fuel would be properly filtered. Went truckin with this hose drapped on the top of drivers side tank, over my drivers step and to filter head. No difference. CAT even changed the primary fuel filter head on one of my visits there, and checked everywhere for air, but no air was getting to the head at all.
Another thing i forgot to mention. I hear "TICK...TICK...TICK...TICK" coming from engine compartment when it gets to popping and stumbling really bad. The same kind of ticking you get from a bad injector. Strange is that if i get on a highway from a ramp, foot to floor to get speed up to highway speed (i pull 5 axle trailers, 140,000 LBS all day long) engine pops and stumbles through 1400 rpm, but therafter (1500 to 1650 rpm) it clears up, and purrs as it should. When i engage my PTO to dump, i rev engine to 1300 ( + - ) and engine runs great, as it always has when not under load. Id swear its sucking air and then, due to faster engine and sucking more fuel from tanks, the faster fuel travelling slams the door on the opening where it sucks air through, i dont know, it sure computes that way when i think what the hell is going on. All i do is think,,,think,,,think, is it the ECM,,,,is it sucking air....is it my alternator not converting the A/C power (which is what alternators initially create before converting to D/C current) to D/C power.
Any ideas anyone may have please please forward them, its all so greatly appreciated guys.
Click to expand...

Did you ever figure out what was wrong with your 3406e when it stumbled at low rpm and cleared up at high rpm

T-Cat · Mar 14, 2025

maggard359 said: ↑

When I was getting combustion in fuel rail on mine the fuel was turning black and acting like what you are describing. You are sucking air somewhere whether injector seat, loose connection on hose, or even a cracked pickup tube in tank. I would do like Swaan said and run a clear hose or at least a pressure gauge tied to wipers while driving. Also dump your filter out and see if it has a black shade to it

maggard359 said: ↑

When I was getting combustion in fuel rail on mine the fuel was turning black and acting like what you are describing. You are sucking air somewhere whether injector seat, loose connection on hose, or even a cracked pickup tube in tank. I would do like Swaan said and run a clear hose or at least a pressure gauge tied to wipers while driving. Also dump your filter out and see if it has a black shade to it.
Click to expand...

I know this is a very old post but I have the exact same prob with mine.. how to fix this prob if there’s a black shade to it? Mine is definitely getting soot, combustion gasses into the fuel bec the inside of my fuel filter and fittings etc is black… will changing the o’rings fix it or not?
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