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Adin3, Nissan Mechanic
Category: Nissan
Satisfied Customers: 3423
Experience:  Autozone Manager 3 years, mechanic 10 years
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Rough idle..tank..gas dry and 93 octane gas its still idling rough

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Hello, I recently had a problem of rough idle at a 1/4 tank. I figured I had water in the gas. After fill up it went away till 1/2 tank and now after putting in gas dry and 93 octane gas its still idling rough. I replaced the spark plugs cause I figured they were fouled and replaced some bad vacume lines. The truck idles at 750 rpm where it always did but still runs rough.

Hello and thank you for choosing Just Answer.

Just to clarify here for me, you had a rough idle at 1/4 tank but when you put in more gas the problem would go away?

Now you are having that problem no matter how much gas is in there?

Customer: replied 8 years ago.
it did go away and then when it reached 1/2 a tank it started back. No matter how much gas is in it now it does it all the time

Thank you.

This indicates that the fuel pump is going bad. The fact that the car runs fine when there is 1/2 tank, but not 1/4 tank indicates that the fuel pump is weak. The fuel pump sucks in fuel, if it's weak it need additional pressure to be able to suck the fuel and delivery it to the rails. With more gas in the tank there is more pressure exerted on the pump, allowing it to pump the gas with less efforts. You may consider doing a fuel pressure test on this car when the fuel is low, this is verify if the fuel pump is infact bad. You also may consider checking and replacing the fuel filter.

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Customer: replied 8 years ago.
I wasnt sure because trhis truck is carburated injected. So even though its idling at normal idle the fuel pump could be failing.. That always happens with a full tank of gas. I was a mechanic about 10 years ago and went into machining. So everything I checked just dead ended.

Yes even it the car is idling normal the fuel pump still may be going bad. The car requires the least amount of gas during idle operation. The fact that the problem was there while the tank was at 1/4 and not there at 1/2 indicates that the fuel pressure is having an issue. This is most likely due to a bad pump and is having trouble pumping gas, more gas in the tanks makes it easier for the pump to work.

Customer: replied 8 years ago.
Ok I will replace the fuel filter and test the pump. Thank you reply to this and I will accept
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Customer: replied 8 years ago.
Now it has cleared up the started back again! Whe I run at 1500rpm it seem to do ok. still the fuel pump?
It may not be the whole pump, there is a strainer attached to the pump that serves as a filter for the pump. You may also want to check for more vacuum leaks.
Customer: replied 8 years ago.
I have replaced the fuel pump and replaced strainer, fuel filter , Plugs, checked vacuum pressure, fuel regulator, No check engine light. I am truly stumped. I have talked to local mechanics and there not sure with out running up a huge bill for an old truck. someone said intake gasket could be bad, I check distributor for flash and its good, ran it in the dark to see if the wires were arching and there clean. what stumps me is it comes and goes but more goes than anything.

Well you can run a quick test on the intake gasket. Here is how you would perform that test.

  • Start the car
  • Spray carb. cleaner around the intake gasket
  • If the RPM's rise as you spray , the gasket is leaking. (The rpm's will spike)
Customer: replied 8 years ago.
nothing happened. any other suggestions. I have trie dthe idle air controle valve as well.

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