Fuel Tank Pressurization Diagnostics Question, Vent?

[74M35A2]

I don't really understand how people are pressurizing their fuel tanks to prime and find leaks, such as the infamous flexible line permeation.

I would assume our fuel tanks are vented, and if so, how is one pressurizing the tank to find such leaks? Must either block the vent, supply enough air continuously to back-up and overcome the discharge through the vent, or disconnect the vent and use that port/line/fitting to inject the compressed air? But I see stories of folks just placing rags over the 4" cap and going to town with their air gun. I don't get it. Is their vent plugged up?

OK, who is going to be the first to tell me to read the manual....but, fuel tank pressurization is not in there, so I play my trump card before you!

 

[doghead]

You guessed it, blow more air in than leaks out the vent.

The infamous leaking flex line is more an issue on 5 tons with the NHC-250.

What kind of issues are you having?

 

[doghead]

One more simple thought you may have overlooked.

Simply blow air into the vent line!

 

[74M35A2]

Thanks. I think you posted and I edited at nearly the same time about blowing into the vent line. It seems to be a good diagnostic method, but I didn't see/understand how people were actually doing it besides saying they pressurized their tank(s), so the vent thing made me curious.

To answer your question, it belongs in a separate posting really. I have some homework to do on it first, but my A2 starts on only a few cylinders and runs at about 400rpm for the first several seconds during the first cold start of the day, winter only. It is fine the rest of the day. Also blows blue smoke when doing so. Runs clean with good power and zero smoke after several minutes of idle warm up.

I bought a turbo rebuild kit and 12 new Cummins valve seals to try and cure the blue smoke issue, neither yet installed. Will post procedure when doing so. For the fueling, I have a replacement mechanical lift pump/primer yet to install because I did notice my primer button leaks fuel when used. Finally I'd install a pressure gauge after the lift pump to monitor the situation before asking further. I believe there is a port to do such atop of the lift pump outlet fitting which is normally capped off with a plug. I think a fuel pressure sensor goes here in other applications.

Thanks for the info about the NHC fuel lines vs the 6CTA 8.3, I didn't know that. I do have the large frame mounted fuel filter canister such as most of the NHC's do (and the smaller spin on fuel filter also), yet only few of the 8.3's seem to be equipped with. I'm going to refresh the lines on the larger filter anyway just in case, as I would think those to be nearly the same as the NHC setup.

I don't believe the blue smoke is air in the fuel line. Oil can only get into the combustion chamber one of a few ways, so I am hoping it is valve seals or turbo seals. Have not inspected any turbo ports yet, just going to stuff a kit in it anyway since it is likely decades old.

2 separate issues maybe. We'll see. Neither is a show stopper. I have seen where people installed a Schrader valve directly into their fuel fill cap, and I like that idea and may follow it if enough can pass through there to overcome the vent opening, or valve the vent off.

Thanks again.

 

[doghead]

I did not mean to exclude the 6cta engines. I have not worked on any of those so I'm not familiar with their lines. Do they use the PT pump?

I have seen several trucks with a shrader valve in the cap.

 

[doghead]

I've had a miss at startup on my nhc250 motors when they have an air leak in the fuel system.

 

[74M35A2]

6CTA8.3 does not use a PT pump. They use a block mounted mechanical lift pump (with built in push button primer), that pushes fuel to a 6 piston inline injection pump (Bosch MW) similar but not same as what is commonly used on Cummins 5.9L engines.

I'm not familiar with a PT pump other than to know the lower # of "button" that is installed, the higher the fueling rate. I don't think a PT pump is a 6 piston inline design...? I'm on a limb of talking about what I don't know here.

I think my cold start multi-cylinder misfire could be due air in the fuel system, or to a cylinder loading up with oil overnight, hence the blue smoke at cold start. I hope to chase them both and be confident on an air tight fuel delivery system at the same time.

 

[Wildchild467]

If any fuel line looks questionable, replace it. I know a good hose shop you can get top quality replacement hose at.

Your cylinder is not loading up with oil. If your valve seals did leak, it would hardly be any oil to drip down the valve stem. 2 drops, if that. Your engine does not look to be that old, so I bet your seals are fine. Seeing that your engine has been only running at 120-140 degrees quite regularly because of the defective thermostat, i wouldn't be surprised if your injectors are upset and not that clean. When your engine was running as cold as it was, I think that can effect combustion. If your injectors are not spraying a good stream, it could cause a misfire and cause the fuel to burn slower thus making it smoke more. That is my theory anyway. I would take it for a drive and give that thermostat a chance to help the engine run better by getting the engine temp up to 180 consistently. Who knows, maybe your rings are carboned up and you now have a loss of compression and that is causing your misfire (joking). I think your engine runs good, but that's just me. If your turbo seals are leaking it could be from not letting the turbo cool down prior to shut down... that is hard on turbo seals. I think turbo seals need to warm up to properly seal too. So your smoke at warm up might be acceptable. Talk to a Cummins rep and ask.

 

[M35A2-AZ]

If you have a M939 or M939A1 it may have the vent tube running up the motor intake, that is a good place to pressurize the the tank from.
3-5 psi.
On the M939A2's I am not sure where a good place is.

 

[74M35A2]

I have an M925A2. Apology for not specifying and thanks for the tip.