Chris Gower, via CarCraft.com: I have a very strange fuel-pressure problem. The motor is a 350 Chevy with a 284/296-duration, 0.507/0.510-inch-lift cam. It's topped off with a single-plane intake and Dart Iron Eagle heads. I am running a 650 AVS Edelbrock carb. I installed a fuel-pressure gauge this spring and discovered 2 to 3 pounds of pressure. I then replaced the fuel pump.

There are as many opinions about fuel pressure as there are Internet gossip gasps about Britney Spears. For a typical small-block street engine, 4 psi is more than enough to feed the beast, while 6 to 7 psi is more a common pressure for carbureted performance engines.

 

This time the pressure started at 7 pounds. I thought the problem was fixed, but within 10-15 minutes the fuel pressure dropped to 0 pounds. I installed three different pumps, a new ARP fuel-pump pushrod, and new fuel filters. I still had the same problem. I then replaced the gas tank, the sending unit, and the tank-to-pump fuel line. I still had the same exact problem. I am at my wits' end! Here in Montana, winter comes early, and I want to at least take a few drives before then. Thank you for your help!

Did you know that the reason the rotor turns as you plunk the distributor in the block is because the cam and distributor gear are both helical-cut?

 

Jeff Smith: One of the things I've learned from 30-odd years of working on and writing about cars is that you should never lose sight of the common denominator. At first, your situation seemed very odd, but then it occurred to me that the common denominator in this situation is the one component in the entire system that you did not change-the fuel-pressure gauge. It's entirely possible that the gauge is not accurate. You did not mention the type of gauge, where it's mounted, or the manufacturer. For example, those small 111/42-inch-diameter gauges that mount to chrome fuel lines that you can buy at the swap meet for $2 should be immediately tossed. I doubt they are accurate.

 

Another reason we suspect the gauge is that you said you have been fighting this situation all summer, yet you did not mention anything about the car running poorly. If the fuel pump was actually making zero fuel pressure, chances are the engine would quit running or at least would be incapable of supplying sufficient fuel to allow the engine to run at higher rpm. However, if you just failed to mention that the engine is running poorly and that it's difficult to get the car to accelerate, there is another possible explanation. It may be that for some reason there is either no vent for the tank, or there is an insufficient vent situation creating low pressure in the tank as the pump attempts to do its thing and get fuel to the engine. A restricted vent will not be able to make up for the change in volume that occurs as the pump moves fuel. This creates a vacuum in the tank. Pumps work off of pressure differentials, meaning that they need atmospheric pressure to help push on the fuel level in the tank to move fuel to and through the pump. All you have to do to test this principle is put liquid in a soda straw and plug the end of the straw with your finger. Now try sucking the liquid out of the straw. The straw quickly collapses due to greater pressure on the outside compared to the inside. But if you release the seal on the end of the straw, pulling liquid through it is easy because it is properly vented. Simple, no?

 

If the tank is not vented, a vacuum in the tank would lead to the fuel pressure dropping off as you described. The test for this is to drive the car for a short distance and then open the gas cap after the gauge begins to show low fuel pressure. Listen for a quick rush of air entering the tank. If this happens, there is a vent problem. If there is no sound of moving air, then my guess is that the problem is the fuel-pressure gauge. Bolt in a high-quality gauge and you might be surprised to see you've had pressure all along.