Packard Forum Index » General

Can anyone tell me what the fuel pressure should be on my 51 327 at idle and also with the engine run up??

I'm having starting problems and I think they are caused by low fuel pressure. After the car sat for maybe 3 weeks it would not start at all. Found that the pump at cranking speed would not pick up any fuel. Had to fill the bowl in the carb to get it to run long enough to start pumping fuel.

I now have a fuel gauge connected to it. The pressure is around 4 pounds. After driving around for 20 minutes or so the fuel pressure read maybe 3 pounds max. Don't think this is right. Can anyone shed any light on my problem?

Specs are 4-5 PSI. Believe that is determined by the spring pressure pushing on diaphragm inside pump. I've had issues with the ceramic fuel filter being clogged to the point it was hard to pass much and pump didn't seem to be able to do much--even though it looked clean the varnish seemed to coat the pores. Does the filter bowl fill completely or is there always air present--even after running for a bit. The air was the symptom when I had that issue. Also possible the inlet or outlet valves in the pump are leaking. Either would cause low or no suction & lack of pressure. Today's gas does evaporate quicker than the old did so empty after 3 weeks is something I'm used to. Added an electric pump just to fill things up after a sit, then off as soon as engine starts.

Your fuel pump seems to be working normally. I believe there is another issue here, namely a slightly weak needle valve.

If the needle valve is not sealing quite tight it will not cause any problem in normal driving. But what if you stop the engine and there is pressure in the fuel pump? Fuel will slowly run into the carburetor until the float bowl overflows.

Because the jets and fuel passages start from the bottom of the carburetor, this starts a siphon action that siphons all the fuel out of the float bowl and down the carb throat.

Then, the float bowl is dry when you go to start the engine.

To me this accounts for the common problem of hard starting on old cars that have sat for a while.

If I am correct a new needle and seat will cure the problem. If no other changes are made to the carb, this can be done in half an hour.

Can any carburetor experts confirm this hypothesis?

Found that the pump at cranking speed would not pick up any fuel.

Whether or not as someone else has suggested you might have a leaking needle and seat, if your fuel pump really won't draw fuel from the tank when cranking, you've got another problem. Primary candidates are a leaking pump diaphram, leaking pump valves, or a small (could be a pinhole) air leak on the intake side of the pump, the flex line, or the steel line to the tank.

...jets and fuel passages start from the bottom of the carburetor, this starts a siphon action that siphons all the fuel out of the float bowl

If the float level is set correctly and the needle and seat is functioning properly, the main jet opening in the venturi is fractionally higher than the fuel level, hence there is no siphon effect. It's the negative pressure (vacuum) in the venturi caused by airflow that "lifts" fuel from the main jet into the air stream. Now if the N&S did leak, it would only do so until the residual pressure in the pump was relieved; during that time the fuel level could rise to or above the main jet and excess would be forced (but not siphoned) into the bores. But once the pump pressure ceased and the fuel level dropped back to a point below the tip of the main jet, there would be no siphon effect to empty the balance of the fuel.

I really don't know what the gentleman's problem is but I took his observation at face value when he said the pump will not draw fuel at cranking speed. Regardless of what issues the carburetor may have (or many not have), if the pump won't draw fuel while cranking, that's the reason why he can't fill the carburetor. As to why the carburetor is dry after 3 weeks, whose isn't? That's pretty typical with today's gasoline.

Some more information, I disconnected the fuel line from the carb and cranked the car. Pump would not pump any gas. Seemed it would not prime itself. Took the pump off the engine and removed the bottom half of the pump to look at the diaphram and the valves. Looked good to me. Didn't look like the diaphram was leaking. Put it together and back on the car. Filled the float bowl in the carb with gas hopefully so the engine would run for a time until it ran out of gas again. Car started right up and kept running. I now had fuel from the pump. I put a pressure gauge on the line between the pump and carb and initially got around 4 pounds of pressure. I have been told by you guys that that should be OK. Shut it off to check if the needle valve in the carb was holding pressure. It did hold but bled off over maybe 1 or 2 minutes. Drove the car around to warm everything up and checked the pressure at idle and it was low, Maybe 2 1/2 pounds, Started out being 4 to 4 1/2 pounds when I first got it started. Did not come up when the engine RPM was raised. BTW when I first started to check the pressure it didn't go up when the engine was run up. I figured that the pressure would rise a little.

What more can you guys tell me? Is it normal for the fuel pressure to drop off to 3 pounds when the engine is warm?

Thanks for the added info, it helps. 3 psi of pressure is still adequate but the big clue to my way of thinking is that the pump won't prime itself. 99+% of the time, if not the pump diaphram or valves, that means an air leak on the suction side somewhere between the pump and the tank, probably not one big enough to let much (or any) gas leak out. (You could try this, fill the tank, park on a slope with the nose of car down hill, and look for seepage or a leak). I'd suggest removing the rubber flex line and either replacing it with the right one (PAC and others sell the right one) or temporarily rigging some nice fresh rubber tubing, taking care to insure that it fits TIGHT. Then remove the fuel line from the carb, and try cranking the engine again. If that works, your flex line was bad.

If that fails, fit a longer rubber hose onto the pump inlet, put the other end in a gasoline car or temporary gas storage container at about a foot or so off the ground, and try again. If it then primes, you've got an air leak in the steel line between the pump and the tank. Remember, the pump would much rather pull air that fuel.

As to the pressure, it's controlled in part by the spring beneath the diaphram so was the spring gets hot, it wouldn't suprise me if the pressure dropped a bit, but can't say I've ever actually done that test and made that observation. It doesn't rise with increased engine rpm, just the volume of delivery increases, not the pressure. Still, 3 psi should be adequate except perhaps for blasting down the highway with the highway patrol in pursuit but if you get down to 2-1/2, I begin to think the pump is suspect (but it may not be your only problem). Why not just bite the bullet and send the pump to Gould or Terrill for a rebuild?

The lack of suction in the first place sure sounds more like a pump valve issue since the diaphragm is OK but the drop off after running could easily be a delivery/supply issue.

Any chance something is restricting the flow such as debris in tank or the screen inlet filter in pump? The flex hose liner has been known to collapse under suction when more fuel is called for & several have reported problems with the tank vent.

Thanks for all the input. Looks like I've got a lot more things to check.

Owen thanks for clearing up the float bowl business. Another fine theory shot to hell LOL.

Bob if you can put your car on a hoist and check the fuel line look carefully wherever it disappears inside the frame or behind a crossmember. Mud and dirt can accumulate in there and cause the fuel line to rust. I have seen fuel lines on old cars that looked perfect except for a 3 inch rusty piece which of course was in a hidden spot.