Whirring sound

[Manche757]

'89, 4.0, 5 speed.  A whirring sound is coming from under the bed that has not been there previously.  It is constant and not clicking and can be heard when parked and not in gear.  Is likely to be a fuel pump going bad?  There is no malfunction with engine performance.  If the fuel pump is going bad, is it something that should be replaced or is it like an old rear end roaring that could go on forever?  If not the fuel pump, is there any other moving part under the bed?

[AZJeff]

It is probably the fuel pump, as there are no other parts that move under the vehicle under the bed to make noise when it's not actually rolling.

 

I would be concerned about a pump that just started making noise out of the clear blue.   Perhaps others can offer more knowledge of fuel pump failure modes, but if it was MY fuel pump, I would be planning on replacing it.

[yxmj]

Without starting the truck turn the key all the way to on....is the sound there?

 

[Manche757]

53 minutes ago, yxmj said:

Without starting the truck turn the key all the way to on....is the sound there?

 

It does not.

[AZJeff]

5 hours ago, Manche757 said:

It does not.

Well then it's not the fuel pump.   So you are saying you hear a whirring sound with the key on, but the engine not running?

 

Dumb question---is your radio/audio system OFF?

 

There is nothing else in the back of a stock MJ that would make noise when the engine is not running but the key is "ON"

[Manche757]

1 hour ago, AZJeff said:

Well then it's not the fuel pump.   So you are saying you hear a whirring sound with the key on, but the engine not running?

 

Dumb question---is your radio/audio system OFF?

 

There is nothing else in the back of a stock MJ that would make noise when the engine is not running but the key is "ON"

You have the wrong end of the stick. When the key is in on positon with the engine NOT running, there is NO noise. The level of noise double today. I hope it is not a fine hazard.

Radio and Jimmy Hendrix off.

[yxmj]

The only thing spinning (aside from the motor) when the engine is running and the truck is not moving is your flywheel/clutch/pressure plate assembly.....

 

You would have to (SAFETY FIRST a friend might help here) block your wheels, engage your parking brake, start the truck in neutral and stick you head under and see if you can narrow it down. A Mechanic's Stethoscope is a handy tool to block out peripheral noises.

[AZJeff]

When the key is turned to "ON", the fuel pump will be engaged.  It will pump until the appropriate fuel line pressure is achieved, and then it will stop.   (It then starts up again and delivers more fuel as the fuel line pressure drops due to fuel consumption.  It repeats this pulsing process as needed as long as the engine is running.)

 

Does your whirring sound occur for the first few seconds after the key is turned "ON",but before the engine is actually running?   If that is the case, it's the fuel pump.

 

My wife's KJ makes some fuel pump noise before the engine starts, but then again, the vehicle is 15 years old.  Maybe yours is the same way.

[yxmj]

I think the above needs to be re-thunk

 

It is perfectly normal for the fuel pump to turn on when power is applied without the motor running......you will here it "whir" for the first few seconds after the key is turned on, then shut off....nothing would be unusual about that.

 

OP says the sound is not there when motor is off but is there when parked and in gear.......IMHO look at other causes than the fuel pump