Engine/Transmission (1998.5 - 2002) Is a pressurized fuel tank normal?- PLEAE HELP!

[Hohn]

Is a pressurized fuel tank normal?- PLEASE HELP!

Steve, hope this is in the right forum.



OK fellas, here's the poop: I have been having fuel system issues since Christmas. They have caused hard starting as I noted in this DTR thread



A week ago I noticed that I have a loud hissing coming from the fuel tank whenever i shut it down. It's there every time.



I also noticed that I have diesel fuel on the frame rail behind the rear wheel (driver's side). At first it was just a few spots.



Tonight I went to fill up, and when I popped off the fuel cap, it almost shot into my face!! There was a LOT of pressure in the tank. I have no idea why this is, as i have never had it happen before.



When I got home after filling it up, I went to investigate the tank area. It was hissing like it has been lately, and there was FUEL EVERYWHERE. Not enough to drip, but it was close! The vent line and filler neck were wet, as was the whole frame rail.



It was a lot more fuel than before.



So now I think I know why my fuel system has been losing prime every time I shut it down, and then it's hard to start.



What's up with this high pressure in the tank? Has anyone else experienced this?



The dichotomy here is the high pressure in the tank combined with an air leak that lets the system lose prime. How can the tank get pressurized if it has a leak sufficient to let air in? How does it get pressurized to begin with? What is that hissing sound?



At first I blamed my Max flow system, but then i remembered that the hard starting predates the max flow install. I am pretty confident that I don't have any leaks in the plumbing from the LP forward.



Please help!!



Justin

 

[HEMI®Dart]

You could check this. I don't think it's the problem though.



http://www.dodgeram.org/tech/dsl/ISB/low_power.htm

 

[Paulsdodge]

Your fuel tank should not have a positive pressure nor a vacuum when you remove the fuel cap. The fuel cap is vented to where the pressure can blow out of the cap or if the tank is under a vacuum the fuel cap will equal the pressure in the tank.

 

[BillGotthelf]

There is a small vent tube on the top of the tank in the 6" round plastic cap that has the fuel supply and return lines also. If you start with a tank that has fuel in it (any amount) and drive it for awhile the fuel heats up as the return fuel from the injector pump is hotter than tank fuel. If the lift pump is working good then alot of fuel is returned to the tank. Fuel can be 50 degrees warmer than tank fuel but as you drive you can heat tank fuel up. As fuel heats it expands but the air in the tank expands also. If the vent tube is plugged the expanded gas has no place to go so when you open cap out it comes. Check vent tube.

 

[Gary - K7GLD]

YUP check the vent - most likely plugged or kinked - if completely plugged, residual pressure MIGHT get high enough to rise above what the ECM allows for an engine start - that's why the LP only runs long enough to provide priming fuel flow then shuts down until the engine starts...

 

[Hohn]

OK, thanks fellas.



I crawled under there and couldn't figure out which one was the vent. i am pretty sure that this is the issue, as I will get pressure buildup in the tank after it's been running only a few minutes.



When I crack the fill cap, the pressure goes away immediately. There's also a fog-like vapor coming out the fill neck, and it's diesel. it looks like dry ice when you put it in water.



There's still some things that are baffling to me. First, how does the tank get pressurized to begin with?? Is it just the expansion of the warm fuel? If not, where is the air coming from?? I mean, the LP is sucking fuel out of the tank, right? So if the vent was plugged, wouldn't a VACUUM be the result?? I just don't see how the pressure could come from the warm fuel alone, because the tank will pressurize in as little as 30 seconds of run time just idling. Where is the pressure coming from?



Moreover, how come the truck starts hard when cold (no pressure in the tank) BUT starts INSTANTLY if restarted after it's been running (pressure in tank)?



In fact, the hissing sound I hear coming from the tank seems to play into this. If you restart while it's hissing, it will fire the first crank every time. But if you let it sit until the hissing stops, sometimes it will fire and sometimes it won't. if it's been sitting for 30 min or so, it will NEVER fire on the first try-- you have to bump the starter to get the FP to come up (18psi in my case) and THEN crank it until it starts.



So the pressure in the tank SHOULD help to push fuel to the LP and make starting easier, right?



It almost seems as if the truck won't start without the tank pressure that shouldn't even be there!!



FYI-- when hard to start, my initial FP is only 2-3psi. Once the key is bumped, I have a solid 17-18psi.

 

[Gary - K7GLD]

Wonder what would happen if you only LOOSELY installed the fuel cap - not enough to provide an airtight seal - might be a short term solution umtil you locate the actual vent tube...

 

[Hohn]

It's worth a try.....

 

[A Johnson]

A bad copper on an injector could possibly be allowing compression into the fuel return galley in the head. I'm trying to think how a bad injector itself could cause this without having a miss fire, and it doesn't seem likely.



I would try running my return line into a bucket and shorting out injectors, and see if there is alot of air in the return and if a particular cylinder is causing it.



I've been trying to think up a way to use turbo pressure to pressurize the tank, kind of an "active fuel pressure setup", but decided it wasn't worth the risk of blowing up a fuel tank if things went wrong.



The ECM has no way of measuring fuel pressure at start, it is just an educated guess, therefore it cannot tell the engine to not start due to high fuel pressure.



It is the #&*^^@ VP-44 that is on its way out, causing hard starts. The reduced lift pump duty cycle reflashes to cure hard starts were just a band aid fix in my opinion, to keep you out of the dealer for a new VP-44. Once again it is only my opinion, and I can't at this time back it up with proof.



A Johnson

 

[Gary - K7GLD]

"The ECM has no way of measuring fuel pressure at start, it is just an educated guess, therefore it cannot tell the engine to not start due to high fuel pressure. "



UMMMmm - well SOMETHING in MY truck was smart enough to not allow it to start after I First installed my Carter pusher - strarting PSI was above 10 because I first wired the pusher as always on with the ignition - had to wire a relay keyed to the stock pusher to shut OFF the extra pump and reduce PSI at startup or I'd be walking today...

 

[juicedcummins]

My last 3 diesel pick-ups have all done this. The only solution even after tying new caps was to drill a small hole through the cap. aprox 5/32. I'm not saying to do this but it is an option and NO you do not get dirt in the tank with this small hole.

 

[A Johnson]

That something was the extra fuel pressure was pushing on the backside of the three pumping plungers holding them away from the internal cam ring. I'm not sure if it is caused soley by a weak internal lift pump which cannot overcome the 10 PSI in your VP at cranking speeds or if it can also be compounded by a cracked pulsation dampner inside the VP.



On a few VPs' that I have tore apart, where the complaint was hard starting, one had a damaged internal vane pump I believe due to water intrusion, and two of them had cracked dampners. I just do not have the resources to do the proper testing to make these matters clear in my mind. The people who do are not talking.



If I remember correctly the specs for lift pressure were less than 7PSI during cranking when the ISB was first released, it was then lowered to and is currently less than 5 PSI during cranking. So it is not to say with 10 PSI on a new VP there may still be starting issues.



A Johnson

 

[Hohn]

IF the VP is on its way out, then how come I have no problems once it starts?

I mean NONE- - zero zip nada. It runs like a dream.



Since the issue is STARTING ONLY and I have only 25K on my truck, I think the pump is the LAST things to check. No codes either.



Justin

 

[A Johnson]

Originally posted by Hohn

IF the VP is on its way out, then how come I have no problems once it starts?

I mean NONE- - zero zip nada. It runs like a dream.



Since the issue is STARTING ONLY and I have only 25K on my truck, I think the pump is the LAST things to check. No codes either.



Justin

Because engine rpm is high enough at idle speed and above to mask the problems.



I didn't mean to imply YOUR pump was on the way out, it could be, by all means fix the other issues before condeming the pump.



A Johnson

 

[BillGotthelf]

When the tank has warm fuel in it and is pressurized you say it starts easy but when cold it takes longer to start. You have a vacum in the tank when its cold. As the fuel and vapor in the tank cools it shrinks and puts the tank under a vacum if the little vent hose is plugged. This also puts the lift pump under more pressure as it has to pull against a vacum. I changed out my vent to a 3/8" line when I redid my fuel supply to 5/8" from the tank to FASS to injector pump.

 

[Hohn]

Update:



I have confirmed that the "hissing" is from the tank vent. When I shutdown, the vent bleeds off the tank pressure. Note: I have pressure, not vacuum, in the tank.



Besides, isn't the purpose of the vent to relieve BOTH pressure and vacuum situtations?



The problem is that the tank is being pressurized faster than the vent can relieve it. Where is the air coming from???



Someone suggested to me that this sounds like a bad injector internal seal, which is allowing compression pressure into the fuel return, and thus pressurizing the tank. This is so far the ONLY explanation which seems to make sense.



But how would a leaky injector not affect running performance? The engine runs great, and there's no hint of a miss. Smoke level MIGHT be a hair more, but I can't tell because it's been like this so long that I don't remember what the smoke level was like before.



I don't know if it's my imagination, but It sounds like you can HEAR air in the injection lines when you listen to the running engine with your ear close to the line bundle. But there's nothing conclusive.



I am presently trying to acquire some stock injectors from a guy who's upgrading and then I can see if the DD2s are the fault.



By the way, Bill: the hard to start has almost nothing to do with fuel temp that I can tell. It will be hard to start if it's been sitting for 10 minutes, no matter if it was run for 10 minutes or 4 hours.



This is just really strange to me!

 

[15w40]

A while ago someone posted that they had a problem with pressure in the fuel tank. After trying everything to find out why it was happening, it was discovered that one of the injectors was leaking from the connector tube. The head has a channel that returns leaking fuel back to the tank through the return line. It was the high pressure from the injection pump leaking back to the tank and causing the “huff” when the fuel cap was removed.



I would think that a leak in the high-pressure fuel system would cause a miss at idle. How does your idle sound?

 

[Hohn]

The idle is as smooth as it's ever been since the DD2 install. The injectors gave it a little attitude, but nothing major.



the other thing that's interesting is that there's a diesel "fog" in the filler neck if you look into it while the engine is running. I am pretty sure this was NOT there before.



This "fog" COULD be a possible result of the return line getting air injected into it at high temp and pressure?? Like from a combustion leak?



It's got me wondering now...



Justin

 

[BillGotthelf]

We had a CTD that had a plugged vent line and was a bear to start. As the air and fuel in tank cooled it created a vacum in tank which the POS lift pump could not pull until it had cranked for about 10 seconds. It would do this only after it had been shut off for 15m to 4 hours. If it sat overnight the vacum went away. The truck also would blow out diesel vapor and air when you opened filler cap to refuel after it had run awhile. Having dissected the fuel system I cannot see how air could be mixed with return fuel as it is under pressure and would leak out not let air in. The unused fuel from the injector area is still under pressure. If your vent tube is not blocked it sure is a mystery.

 

[Hohn]

The other thing I was thinking was this:



Those injector holes are REALLY small. Under compression pressure, it would take a LONG time to move enough air to pressurize the tank, especially assuming only one injector is leaking or bad.



But MY tank will pressurize in as little as 2-3 minutes. After running just briefly, I can hear the hissing of the vent tube trying to bleed off pressure as the engine runs. It's just hard to think that an injector could possibly leak that badly, let alone leak that badly and still have the engine run smoothly.