fuel pressure falls off...

[aboyandhisdog]

Jeep is running great but occasionally the fuel pressure will fall off from a steady 3.25psi to 0, and, of course the engine dies.  I have replaced the first AC pump with a known good Carter pump, but still have the issue.  It will cut out under hard load and also under no load like when coasting to a stop and then putting the clutch in.  95% of the time, no problem. 

I've checked all of the fittings in the lines both before and after the pump and they are all tight.  I have an in-line pressure gauge just before the filter just before the carb. which is how I know the pressure is at fault.

So my question is:  Is there anything besides a defective pump that could cause the pressure to fall off intermittently like this?  I wouldn't think I could have two defective pumps in a row.  Thinking now that neither pump is defective.

Thanks a lot!

[athawk11]

Because of the intermittent nature of the event, it could be a piece of debris in the fuel tank.  It occasionally floats close to the pick-up tube, gets sucked over the inlet, which cuts off the fuel flow and causes the engine to die.  The vacuum from the pump is released and the debris floats away, only to eventually find the pick-up tube once again.

[SteveKfl]

I've seen a similar situation described on a different site.  There, it ended up being an old rubber hose deteriorated on the inside, invisible to the eye, was collapsing inside blocking the flow.  They replaced the rubber hose and symptom was gone.  The debris in the tank and/or line is also another strong potential.  Good Luck!

[Rus Curtis]

Have you ruled out the cap creating a vacuum?

[aboyandhisdog]

Hawk and Steve, both very interesting leads!  I do keep a clean tank and lines (M38 tank and pickup) and if I had to guess (and that's pretty much all I do), I don't think debris is the issue - but would never rule it out.  As for the poor hose...would never have thought of that!  But, my oldest hoses are only about 6 years old, but still something I need to look into.

Rus, this cap issue is something that has perplexed me for a long time.  I have a vented M38 cap (vent is open), and I also have a vent on top of the tank.  I don't know how it works exactly, but it is there.  Now, as I understand tank venting, they are there to expel gasses in the tank as they expand and create outward pressure.  Do I have this right? 

But I have never really understood how a fuel pump can overcome the vacuum created as you use gas and the level in the tank drops.  Are the vents supposed to overcome this vacuum too?  If so, why do I get the typical rush of air when I open the cap?  Is it air going in, or gasses going out?  Does everybody get this when they open the cap?  I even get this on my modern vehicles.

Now then, to confuse the issue a bit more, the other day when I was having problems with fuel pressure, I actually reached down and loosened my cap to release the pressure.  Problem is, it made no difference!  I would have thought it would, but the engine died shortly after.  Additionally, the cap has worked like this for years without problems.

So, while we're here, could somebody please explain to me the physics behind vented caps?  Inquiring minds need to know!  Thanks a lot for your input!!!

[Rus Curtis]

... I actually reached down and loosened my cap to release the pressure.� Problem is, it made no difference!� I would have thought it would, but the engine died shortly after.� Additionally, the cap has worked like this for years without problems....

Can't rely on me as an expert either.  An observer at best.  I think you're understanding of the cap is about the same as mine.  The fact that you opened the tank and it made no difference and still died was what I was asking about. 

[aboyandhisdog]

I just came in from a drive and released the cap at home and air rushed OUT not IN.  Is this the way a vented cap is supposed to work?  I always thought the function of a vented cap was to equalize the pressure in the tank with the outside air pressure.  No?

[SteveKfl]

It would seem to me if you are getting that kind of vacuume release opening the cap, it probably is not venting, and it may be equalizing the vacuume from the pump and/or collasping the hose potentially.  I found this article on tank venting useful info....
http://www.tanksinc.com/index.cfm/page/ptype=results/category_id=160/mode=cat/cat160.htm

[athawk11]

Here is a specific write up on a 3A cap.  I'm not sure how the M38 cap works in comparison.

http://www.cj3a.info/tech/gascap.html

[aboyandhisdog]

Both links very interesting!  I don't think mine is working as well as it should be as I do get quite a bit of air pressure in the tank.  Of course, that would seem to actually aid the pull of the fuel pump, so maybe not the cause of my drop in fuel pressure.  In any case, thanks for the links - good reading!

[SteveKfl]

... but for your problem, it isn't "pressure" in the tank, it is IMO a "tank side vacuum fighting the pump" that may be your problem (hopefully solved that simply as a cap).  The "swoosh" you described I'm guessing was "sucking in air" not releasing pressure, or you would smell the gas fumes.  Key words in both articles... The other valve allows air into the tank as fuel is removed by the engine... thereby preventing a vacuum from forming inside the tank.  While looking for symptoms of a "bad vented cap article", I came across one chat box that the guy was saying his gas tank actually was caving in with a thump on long drives, so when he heard it, he pulled over and released the cap to pop it back out, and to release the vacuum before it stalled the car.  That is a strong vacuum (it was a 50s Chevy not a Willlys).  PS, if you're using Ethanol fuel, the rubber hose gets eaten very quickly on the inside, plus condensation can form in the tank in storage/non-use.  When replacing the hose, I'd suggest a "bio-fuel hose capable hose" with a PVC inner liner to fight the appetite of the alcohol in the fuel.  I use RV GAS a non-alcohol fuel for my low mileage equipment and vehicles.  Costs more at the station, but the "fuel related repairs" have stopped, including replacing my in-tank fuel pump in an Astro/GM product.  It is well worth the extra money IMO.

[aboyandhisdog]

Thanks, Steve.  We don't have RV gas here, at least I've not heard that term.  Occasionally an obscure station will have non-ethanol gas but they are few and far between and not an option for me.  I do use Sta-Bil Marine additive in every tank which is supposed to counteract ethanol and even prevent corrosion in a metal tank.  Been using that for years.  Right now I am playing with some other gas caps that I have as I do believe the one I was using was, in fact, defective (the valve seems frozen).   Just came in from a drive with a different cap and had no whoosh when I got home and opened the cap.  I sure appreciate your input!

[aboyandhisdog]

An update:  The fuel cap I used which is "open" (hole but no valve) makes no difference, as the jeep died again today - only to be revived by my turning on my elec. fuel pump when it began to stumble.  That was one of the best mods. I ever made to my fuel line.  Anyway, just installed my third fuel pump - a new Airtex.  Should know tomorrow if it is successful or not.  If not, about all I've got left is the possibility of bad rubber line or gunk in the tank.  Beyond that, I'm completely out of ideas.

Forgot to mention, installed a different carb. too and it still died. 

[SteveKfl]

Is that electric fuel pump inline with the regular pump?  I don't remember hearing of that before.  Could it be restricting the flow by having to pull the fuel thru the Electric pump mechanism starving it?

[aboyandhisdog]

Yes it is inline, and no it does not starve the mechanical pump.  It is a flow-through design and I've had it on board for about 9 years without an issue.  In fact it has saved me many times when there is vapor lock, boiling fuel, crud in the carb jet, or the issue I am having now. 

I just came in from another drive with the new fuel pump and had the same thing happen, so I'm certain I don't have a pump issue.  Tomorrow I'm tearing out the tank and flushing it and will get a couple of feet of new fuel line on order.  If this doesn't do the trick, I'm out of ideas!