Fuel pressure

[Donovan]

I have a 69 383 with a Carter carburetor. Recently, I've been getting surging, and now almost stalling problems. My fuel pressure gauge says that my pressure is dropping to 0, and fluctuating from 0-4. I figured my electric fuel pump was going bad, so I replaced it with a mechanical pump. Same problem. Pulled the gas tank out figuring it was clogged up, and it is pristine in there. Re-plumbed every thing in case there was a leak or a kink. Same problem. I seem to have fuel coming from and to the pump, but by the time it gets to the carb fitting, (where my pressure gauge is integrated) it comes and goes. As far as I can tell, I either have two different types and fuel pumps with identical problems, or two brand new filters that are clogged, or a problem with the carb. I'm stumped on a carb issue that would cause the fuel pressure to drop at the gauge though. I've used two different gauges in two different locations, btw, same result. Can someone help me figure the right direction?

[Wildergarten]

My truck has these exact problems, and you probably won't like my explanation of what I've found so far. The first point is: why did anybody install an electric pump? The reason is likely that the lobe on the cam driving the mechanical pump or the push rod are worn because of a lack of ZDDP (zinc phosphate) in the oil. Hence, at low RPMs, the pump having little throw made little pressure. Solution: Install the electric.

So the scenario you won't like goes like this:
1 Owner makes installs new cam.
2 Owner balks at the price of the ZDDP additives or doesn't know any better.
3 The cam and push rod start to wear.
4 Metal in the oil results, eating bearings. Oil pressure drops. So does low RPM fuel pressure.
5 Owner installs electric pump.

A responsible installler of an electric pump will include an oil pressure switch such that if the motor isn't running neither is the pump. If the pressure drops below 15psi when running it will cut out the fuel pump.

Make up a jumper and put 12V at the pump. Does the fuel pressure come up? If so, your electric pump works. Put a volt meter on the pump. If there is nothing only when RPMs are low, the problem is possibly low oil pressure, with the switch cutting out when the pressure drops below 15psi.

Do you have an oil pressure gauge? What is the pressure at low RPMs? Is it enough to hold the switch closed? Is the charging circuit delivering 12V at low RPMs? Mine had resistive connnections at the switch making that a problem, but the source of power when running as opposed to starting is different.

The oil pressure swhich is single pole, douhle throw, with the "common" supplying votage from the switch to the pump. When starting, the power comes from the "start" lug on the starter relay and goes to the swtiched side of the oil pressure switch that closes when the pressure is low. Turn the key to "start" and the pump starts to run. When the ignition switch is in the running position, the power to the oil pressure switch usually comes off the "run" side of the ignition switch to the ballast resistor and goes to the other side of the oil pressure switch that closes when the oil pressure is above 15psi.

[Donovan]

Thanks Mark. The electric fuel pump was wired simply to keyed 12v and ground. It was also a simple pump for carbs, it doesn't produce much more than 7psi. I kept a regulator just it case, and never needed it. I had considered, and still consider a cam/push rod issue, but I appear to be getting flow from the mechanical pump. And, the problem existed when I had an electrical pump and the mechanical pump.

[Wildergarten]

Thanks Mark. The electric fuel pump was wired simply to keyed 12v and ground.

That is unsafe, especially in an accident. One can still get those pressure switches.

It was also a simple pump for carbs, it doesn't produce much more than 7psi.

Sounds like the Purolator. Is it a small cube-like affair?

I kept a regulator just it case, and never needed it. I had considered, and still consider a cam/push rod issue, but I appear to be getting flow from the mechanical pump. And, the problem existed when I had an electrical pump and the mechanical pump.

You say you replumbed it, but you say "two filters." Where are the filters in this system, before or after the pump or both? Do you know for a fact that the truck is bogging at the same time the pressure is low? Or could it be that the float is stuck at the top of its travel, the truck runs out of gas and bogs, shakes it loose, and while the gas is refilling the bowl the pressure reads lower?

[Donovan]

Since the last time I was on here, I’ve changed the carb to an Edelbrock 750, and discovered that it was a 440 with an aftermarket cam. I don’t know the cam specs, as I haven’t taken apart the front of the engine, but saw the cmc ep-45 marks on it. It also had a vacuum leak and I have addressed that too. Today I fired it up, tuned it. It’s idling high, but had 16 on the vacuum gauge, so I took it for a drive. The problem was even worse. I watched the fuel pressure gauge finally rise up to 5 psi, and drop down to zero as I gave it gas. I’m stumped.

I know what you mean by the electric pump. It was the PO and I have burned down a Charger that way.

[Wildergarten]

I’ve changed the carb to an Edelbrock 750, and discovered that it was a 440 with an aftermarket cam. The problem was even worse. I watched the fuel pressure gauge finally rise up to 5 psi, and drop down to zero as I gave it gas. I’m stumped.

You said you pulled the tank and cleaned it. I assume you removed the gauge/pickup tube unit. When you did that, was there a strainer/screen on the end of the pickup tube? I assume you cleaned that too. What type of filter does this truck have? Is it toally replaceable or is it an element type (which may have a screen)? What size is the tube? Is it at least 3/8 back to the tank? Are all the hoses new? Have you blown compressed air backward through the tube (after the filter)? Might there be a clear plastic bag inside the tank (it's an old and nasty prank)? Have you tried running it with the gas cap off?

[johnah]

Could be vapor lock.

[Donovan]

This has been frustrating and escalating. When I pulled the tank, it was very clean inside, I didn't clean it. The sock was missing, and I have not been able to find another. I did use compressed air to clear the lines, and I have replaced the rubber hoses, and added hard lines on any large sections of of the line. I thought some of the bends might be a little tight. I bought a brand new Edlebrock 1407 750cfm carb. I realized that the pressure dropped radically after the float bowls opened and I have since learned that the NAPA pump is very low flow, so I have put the electric pump back on in series. I am now getting enough fuel pressure now, but it runs horribly. I found a vacuum leak around the intake, and the base of the carb. I replaced the intake valley pan, and put new gaskets on the carb. Now my vacuum is even lower, it runs horribly, pops out of the exhaust, stumbles and runs very rough. I started with just a little surging, and it seems like each repair points out a whole new problem. The timing has always been pretty high to keep it running, (around 25 initial), so I'm thinking it still has a vacuum leak somewhere? Maybe a cracked intake? Frustrated.

[Wildergarten]

Frustrated.

Once those bowls are filled you shouldn't need that much flow unless you're hammering it, which one doesn't expect if it's running like a dog. Have you put a vacuum gauge on it? Try puting some gas in a squirt bottle and spritz it in spots onto the manifold to see if something changes. Are you using ported or manifold vacuum for the distributor? What do you know about the distributor itself? Have you checked the shaft for wear (seems to be a common problem around here)?

[PwrWgnDrvr]

Popping out the exhaust is un-burned fuel in the exhaust system. There are better ways to find a vacuum leak than squirting gas onto the engine. Ive seen too many fuel fires to do that one, and Im a pyro! Sound like u have a hellva ignition problem. I'd absolutely be looking at the distributor.

[drbugs]

i had a similar problem with a 69 imperial 440, it was wear on the rocker arm from the cam to the fuel pump

[Wildergarten]

i had a similar problem with a 69 imperial 440, it was wear on the rocker arm from the cam to the fuel pump

Don't you mean the push rod? Yes, they do wear, and so does the camshaft lobe.