Falling fuel pressure

[Torrance00]

My friend has a 68 dart slant 6.
He said the car was running rough, not idleing as before. So he did a pressure check
on the fuel line between the pump and carb. He said it was barely 1 psi.. So he replaced
the fuel pump and (I was there) the car was started and the pressure was a little over
6psi. but after about 5 minutes at idle it went down to 3psi.. I had noticed all this time
that there was hardly any fuel in the filter (clear plastic) I thought he was picking up
dirt in the tank. Yesterday he tell me he checked the tank and the filter was fine..
he says it's a worn cam ... I don't get how a worn cam can be "good" one minute and
then bad.. Mechanical parts don't usually show that type of symtom, good to bad
and back to good..
Just would like another opinion...
On another subject. I've never heard of or done it myself, but soaking a new timing
chain in oil before installing it. I see no harm, but would not doing this cause the
chain to wear faster.. So this on the "garage squad" the other day...
Best

 

[diymirage]

i would be looking at a vacuum issue
if the tank is clean, check the vent lines, and the soft fuel lines (make sure they arent collapsing)

 

[fjr]

If I understand your post correctly, you have a pressure gauge measuring fuel pressure at the carburetor. When you start the car, the pressure starts at 5 psi and then drops to 3 psi.

How long does it take for the fuel pressure to drop and does the fuel filter also show less liquid fuel as the fuel pressure drops?

Does the car continue to run OK with the lower fuel pressure?

What happens to the fuel pressure if you run cold water from a garden hose over the fuel pump after the pressure drop to 3 psi?

Whereabouts are you located?

 

[A56]

My friend has a 68 dart slant 6.
He said the car was running rough, not idleing as before. So he did a pressure check
on the fuel line between the pump and carb. He said it was barely 1 psi.. So he replaced
the fuel pump and (I was there) the car was started and the pressure was a little over
6psi. but after about 5 minutes at idle it went down to 3psi.. I had noticed all this time
that there was hardly any fuel in the filter (clear plastic) I thought he was picking up
dirt in the tank. Yesterday he tell me he checked the tank and the filter was fine..
he says it's a worn cam ... I don't get how a worn cam can be "good" one minute and
then bad.. Mechanical parts don't usually show that type of symtom, good to bad
and back to good..
Just would like another opinion...
On another subject. I've never heard of or done it myself, but soaking a new timing
chain in oil before installing it. I see no harm, but would not doing this cause the
chain to wear faster.. So this on the "garage squad" the other day...
Best

Not sure how oiling the chain would cause increased wear. As far as dropped fuel pressure : how many filters are in the system? one at the tank and one by the carb? Replace them no matter how clean they appear.

 

[Dana67Dart]

Try presure tests without the gas cap on.

 

[72bluNblu]

What type of fuel pressure gauge are we talking about? Liquid filled fuel gauges will read lower pressures as the engine compartment heats up.

Straight from the "notes" section on a Summit fuel gauge...

"Liquid filled pressure gauge readings will change as ambient (under hood) temperature changes. A liquid filled gauge compares line pressure to the pressure inside the gauge case. A change in ambient temperature can cause case pressure to change as much as 7-12 psi. As a result, gauge readings can fluctuate from 40 psi cold to as little as 28 psi hot."

Summit Racing® Fuel Pressure Gauges SUM-800115

Now, for some reason they're talking about EFI type fuel pressures in their example, but it doesn't really matter. If the gauge shows 5psi cold, and drops to 3 psi when everything is heated up, it could be completely due to the way the gauge reads the pressure, and not an actual drop in pressure.