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Replaced the fuel pump on Dodge Intrepid 2001, 2.7L engine. Before dropping the fuel tank, drove it until about 1/8 full, so that it would be lighter. Installed the new fuel pump, strapped the tank back on, but when we went to fire it up, the engine wouldn't start.

After trying a number of things, we finally decided to pop off the vinyl quick-connect that connects to the steel fuel line to see if any fuel would pour out, but when we primped the pump we only saw a few drops, certainly not enough to start an engine. We added about 2.5 gallons to the tank, and it almost started. We added about 3.5 more gallons, and boom, it started right up.

Friend said the pump was vapor locked, but I thought that vapor locking had more to do with fuel boiling. Tried searching around the internet for an explanation, but didn't really find one. So, I ended up here. Anyone have any ideas why this happened? Thanks.

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Not vapor lock, but air lock. Different principle but the same effect. Air lock will only happen on a new install or if the tank is drained. The working parts of the pump are surrounded by air with maybe a little splash of gasoline, so the pump can not develop enough pressure to force the gasoline past the check valve at the pump.

The easy cure would have been to attach a hand-operated vacuum pump to the fuel line near the engine and pump until gasoline arrives. This would have eliminated the trapped air in the pump and would have given the pump enough liquid to create pressure.

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  • That makes a lot of sense. I'm still a little confused about the exact details though. As I understand it, the check valve of the fuel pump is located at the outlet. So does that mean that fuel below the check valve (e.g. fuel trapped inside the fuel pump) can flow back through the inlet and back into the tank? If so, I could see how the level of fuel in the pump would be at the whims of the level in the tank. But perhaps the turbine acts as a kinda check valve?
    – TLex
    Commented Jul 20, 2023 at 0:35
  • @TLex When the pump is air locked, the air and fuel in the pump before the check valve just churn around like a kitchen blender without developing enough pressure to pop the check valve open. Adding fuel to the tank raised the pressure under the check valve enough to pop it open and allow the air to escape into the fuel line.
    – MTA
    Commented Jul 20, 2023 at 1:57
  • I agree with the air lock on the pump, but I believe the action may be a bit different then you are describing. I believe if there isn't enough fuel in the tank and the new pump is only working on air, it is hard for it to get suction and therefore fuel up into the pump vanes. Once the fuel level was high enough and there was a constant fuel source, it's easy for the pump to push the fuel up past the check valve. After that, as long as the check valve is working correctly, the pump vanes are never without fuel and can create the suction to move the fuel through the lines. Commented Jul 20, 2023 at 10:45

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