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Fuel pressure drops off and engine dies

guido666

guido666

1/2 ton status
Joined
Feb 16, 2005
Posts
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Location
Longmont, CO
I have a big block 454 in my K5, and the short fuel pump style out of a K20 with a 350. It ran fine and kept pressure in Ohio, but I just moved to Colorado. It worked fine here for a couple days, the started losing pressure. It finally gave out completely. I replaced it, and this pump seems to not be able to keep pressure either.

The fuel pressure sender in between the pump and the carb. I'm thinking that if it was a fuel filter problem (filter replaced in October with carb rebuild) it would increase pressure or read normal.

Could I maybe be sucking air into the line through a rust hole? Or is this new pump just crap?

Can someone tell me a big block that used the short style pump (the long style won't clear the crossmember)?
 
Does it run OK until the engine warms up? It sounds like a vapor lock problem, which is pretty common at these altitudes... :( Welcome to Colorado! ;)
 
Huh??...

A 454 (or any other BBC) uses a different fuel pump than a small block does,though a small block one WILL bolt up,and looks the same...the rocker arm is different where it touches the pushrod....

I put a brand new fuel pump I had for a small block on a 74 454 when its fuel pump died..I only drove it about a mile before it stalled again after putting it on..the pump snapped in two,the 2 bolt flange was still bolted to the block,but the rest of the pump was dangling by the fuel line hoses!...

I'd guess that ALL small block pumps would do the same thing,but who knows with todays china built crap?...

It is possible you could have the correct pump on it,and are sucking air through a rust hole in the metal fuel line,or porus rubber lines between the tank and pump...look especially close at the hose near the passenger side rear tire inside on the frame rail that joins the 2 metal sections together..I've seen many that are porus,or are collapsing inside,yet the outside looks perfect..


You need to do a "volume" test,as well as pressure...take off the fuel line to the carb,put it in a can or bottle,and run the motor for about 10-20 seconds..it should fill it right up,about a pint is normal..you may be losing pressure because the fuel supply is being restricted,like if the "sock" on the sending unit in the tank was clogging up....:crazy:
 
What is vapor lock? I think it could fit the description that it runs ok until the engine warms up, I would have to do more testing to know for sure. Is vapor lock the process where the fuel line gets heated enough to vaporize fuel, which then clogs the line with an air (well, vapor) bubble? How do you deal with this?

As for the use of a small block pump, I've been using it without incident since I installed the BBC 454 in September. That use was all in Ohio until this week though. That doesn't mean it's the "right" answer though. Are there any BBC pump that are the short style?
 
Your description of vapor lock is right on. The lower atmospheric pressure at altitude makes it even easier for the gas to vaporize in the fuel line. :( You need to make certain that the fuel line is well away from the exhaust pipes, muffler, cat, headers, etc.
 
yup...

There are short can fuel pumps for BBC's..mostly the long can ones are used on vehicles with A/C,and have an extra "nipple" for a return line..try looking one up for a 71 Chevelle or Camaro with a 402,no A/C ..

If I remember right I think thats the one I used on my 74 K20 when I swapped a 454 in it, it was from a 74 Chevelle WITH A/C,and had the long can pump on it..it didn't hit anything,but I cracked it while putting the motor in (or removing it from the car:doah: )..so I decided to use a short can version, to eliminate the return line..:crazy:
 
Ok, so I took aluminum sheet and mesh and built a series of heat shields for the fuel lines inside the frame rails. It seems to have helped some (but I only took it on a 20 mile test drive on a cool day, and not in the mountians). I got the fuel pressure to drop off to zero one time, while accelerating from a stop uphill.

I can only imagine this getting worse as the summer heat comes.

How do you "Coloradoins" deal with this problem? I'm hesitant to box in the frame rail with aluminum sheet for fear of turning it into an oven, and I'm not sure of any safer place for the fuel lines to live then in the frame rail. (I already have heat wrap on my headers, by the way.)
 
some ideas...

First off,I kind of doubt its vapor lock..but its possible I guess..

Most manufacturers used a fuel pump or gas filter with 3 lines..fuel IN,fuel to carb,and a RETURN line back to the gas tank..

You could look thru a fuel pump catolog ,I think there is a "short" pump for a BBC,that has 3 lines ..or install a fuel filter from a Mopar that looks like a universal inline one,that has the third "return" line back to the gas tank..keeping the fuel moving,and cooler by re-circulating it makes it much harder for vapor lock to occour..

You could also swap an electric pump in instead,and ditch the mechanical one..some are not designed for use with a return line,so make sure you get whatever one you need..


I think you have blockage or air getting into the fuel line upstream of the fuel pump...ever try blowing the fuel line from the pump to tank out with an air hose??...might have a kink or collapsed section of metal line too,or a rot hole in the upper part that wont leak,but lets the pump suck air..the "sock" filter in the gas tank can clog up easily too from rust powder or silt too..and a faulty gas cap might not let air IN the tank,and make it impossible for the pump to draw gas out!..I run into that on lawnmowers a lot..:crazy:
 
The fuel pump that was on the engine when I got it has TWO lines (in and out), not three. The replacement pump I'm using (from a '76 K20 w/ 350 SBC) also has only TWO lines.

There are two lines that come forward from the gas tank, one for the fuel (to pump) and one that went to the charcoal canister (when I had one, now it has a vent cap on it).

It makes sense that fuel would be harder to vaporlock while recirculating, but that's not really an option here (without lots of work).

Most of the aftermarket fuel pumps for BBC are of the short variety, they probably expect you to be trying to stuff it into something it doesn't belong in.

I have not tried blowing out the line, but I'm sure it can't hurt, and if it did have a small leak (allowing it to suck air) it might help find it.

What advantage would I have with an electric pump (to solve my problem anyways)? I think that an electric pump would perform poorly with all that heat, and I prefer the reliability of the mechanical pump (barring any pending discussions).

The reason I really think vapor lock may be the answer is that the only thing that has changed since two weeks ago (when it ran like a champ) is that I had it shipped to Colorado. And with coming to Colorado comes an altitude change, which probably lowers the boiling point 20-30 degrees. Another thing I didn't mention before that would support the vapor lock theory is that the worst failure was while traveling uphill from Boulder into the mountains to try to go wheeling. I was watching it, started at 7 PSI like normal, then it would hold at 6 PSI, then 5, ..., until it wouldn't start, run, or make pressure. This could be in part due to the buildup of heat from all that climbing, or the change in altitude. Luckily, I got it turned around by moving it with the starter, and coasted downhill and popped the clutch. I was able to travel several miles downhill letting gravity do the work.

What do you guys think about it?
 
high altitude..

An electric pump mounted near the tank will "push" the gas rather that suck it from the tank..they run cooler and keep the gas cool as a result..they also keep constant pressure on the fuel,not a pulsating pressure like a diaphram in a mechanical pump (some electrics DO use diaphrams too however)...in effect you have 6' of pressurized fuel line,rather than it being under suction or vacuum..

Adding a return line is no chore if your charcoal canister plumbing is intact..it has a return line to the tank you can use for the "3rd" line if you add a fuel filter that uses one..

Since this started when you moved to CO,maybe its the carb thats not liking the higher altitude??..but unless its flooding that does not explain the pressure drop...:confused: make sure the gas cap lets air in..try running it with the cap loose and see if anything changes...:crazy:
 
We should be going off the "winter blend" gas by now, so that should help. They add more "oxygenators" to the winter blend here, to try to reduce air pollution. However, it vapor locks easier than the summer blend, so early warm spells can cause problems for some folks. The problem may go away as the stations get filled with summer blend fuel.
 
Good point about the electric pump "pushing". Assuming that vapor lock, or fuel boiling, causes pressure which negates the vacuum imposed by the mechanical pump, wouldn't it just cause excess pressure with the electric pump?

And to answer the question, no, the engine is not flooding. I have a air/fuel ratio gauge (although it is only hooked to the driver's side exhaust) and I actually read slightly leaner here in Colorado then in Ohio, which surprised me. In Ohio I ran a little rich, here I'm closer to "optimal".
 
I'm pretty sure the vapor lock is occuring INSIDE the pump. I started it up, and drove it until I started losing pressure. Then I shut it off and let it cool. The exhaust, headers, and fuel lines were cool to the touch [after about 20 min.]. It was still vapor locked and wouldn't start. At the suggestion of a local wheeler, I poured cool water on the pump [which was hot to the touch] and after that, I was able to start it a few minutes later. Any input?

I found a short can BBC fuel pump (from a '70 Chevelle 454). It should be in tomorrow, and I can install it.

What is the actual differenc between a BBC and SBC pump? I've been running on the SBC 350 pump since I put in the BBC 454 and aside from this vapor lock issue, it seems to do the job well. Is there anything different about the BBC pump that may help this issue?
 
Welcome to vapor lock hell. I have been in that hell for some time, only recently resolving the problem. I have a carb'd '76. The mechanical pump was contributing to the problem, it wasn't just the fuel lines. Electric pumps help a lot, but my vapor lock problem did not resolve until I took the mechanical pump off and moved the fuel lines and wrapped any part of the fuel line close to any heat source with Thermotec wrap. Now, so far so good.

In the course of trying to diagnose and fix the vapor lock problem, I replaced and or had cleaned, every part of the fuel system from gas tank, sock, gas lines, fuel filters, mechancial pump, added electric pump, replaced carb. Vapor lock didn't resolve until moving fuel lines, wrapping them AND removing the mech pump. Oh, and BTW, I did have a return line and that still didn't help enough.

I poured cool water on the pump [which was hot to the touch] and after that, I was able to start it a few minutes later. Any input?
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HarryH3 gave me this same sage advice when I wwas trying to deal with the problem. That this worked is a pretty good indication that you have vapor lock. Right Harry?
 
Whatever is available at the pump, and due to my compression ratio I have to use premium (which here in Colorado is only 91 octane). I'm assuming it's straight gasoline.
 
last I knew all 'pump gas' was to be E10 by may of this year, atleast according to federal regulations. but E10, iirc, has a lower evap. temp than previous blends of unleaded gas.
 
hack500 said:
last I knew all 'pump gas' was to be E10 by may of this year, atleast according to federal regulations. but E10, iirc, has a lower evap. temp than previous blends of unleaded gas.
Click to expand...
In that case, just about every gas station around here is in big trouble, because almost all of them have at least one lever on each pump with plain old 87 regular unleaded and one with plain old 91 premium unleaded (in addition to ethanol versions of the same).
 
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