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I have a 1995 Nissan Sentra with a carburetor and a mechanical fuel pump. After being parked for a couple of days, the car doesn't start because no fuel reaches the carburetor. At most it, will start and then die after about 30 seconds, when the fuel that was already in the carburetor runs out.

I called a mechanic. After checking things a bit, he disconnected the fuel line from the carburetor, sucked from it (literally, with his mouth) until fuel came out, then plugged it back in. The car then started and ran fine for at least 15 minutes, including at high RPM. It restarted several times without a problem, too. It takes a couple of days of being parked for the problem to reappear. The mechanic said that there's probably some valve broken inside the fuel pump which causes it to be emptied of fuel after being stopped for a while, and then it can't pump fuel anymore unless it is "primed" like he did by sucking. Therefore, the fuel pump needs replacing.

I've been researching online, but I haven't been able to find descriptions of similar problems. From what I gather, the fuel pump either works or it doesn't, and if it's "sort of" failing, it's most noticeable at high RPM, not when starting. And it seems that fuel pumps don't need priming.

Does his diagnosis sound right? What else could it be?

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  • Replace the fuel pump , if that does not fix the problem then start looking for other problems.
    – blacksmith37
    Feb 28, 2019 at 16:37
  • The Mechanic is correct, fuel pumps have mechanical check valves, this is what makes them work, the check valve on the suction side of the pump is leaking.
    – Moab
    Feb 28, 2019 at 16:48
  • It is possible to get inline one-way valves as a temporary repair or just to test to prove the pump theory
    – Solar Mike
    Feb 28, 2019 at 17:00
  • @SolarMike - Which would probably cost more than a new mechanical fuel pump and be harder to install ;-)
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 28, 2019 at 17:01
  • @Paulster2 I used to have one in the toolbox just for that sort of testing - but did work on some old & weird stuff back then... gravity feed engines with magnetos etc etc
    – Solar Mike
    Feb 28, 2019 at 17:02

1 Answer 1

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When you stop your engine fuel should stay in the system all the way up to the carburetor. It may evaporate from the carb but the line should not be dry, which is what is happening to you. This can have a few causes:

  • A bad check valve: a check, or non-return, valve prevents fuel from flowing back into your fuel tank. If this stops working fuel will run back into the tank after the engine stops. It won't effect fuel pressure, so the engine will perform normally. I'm not sure whether Nissans have a check valve integrated with the fuel pump or not, if they do then replacing the fuel pump is one way to fix it, however installing an in-line check valve might be a cheaper option if there's space between the fuel pump and the carb. The part itself is less than $10, and it's a very simple installation. If the fuel pump is delivering the right pressure I'd see if that's an option and how much it costs compared to the fuel pump replacement. If the fuel pump replacement isn't expensive then I'd do that personally as who knows what will break on it next. If the valve is separate from the pump it's as simple as replacing it
  • Fuel leak: a leak in the fuel line or a bad connection could be dripping small amounts of fuel out, or a seal could be blown
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