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240 Humming noise at 1000 rpm

Maximum Turtles

New member
Joined
Feb 18, 2014
Location
Santa Rosa, CA
I've been trying to track down a noise for a while now in my 92 240 auto, to no avail. when the engine is running between ~950 and ~1050 RPM, there is a soft humming noise (sounds like someone blowing on a bottle, producing a C#4 note). The pitch never changes and it doesn't matter what gear I'm in (including park).

I really want to find out whats rubbing what and try to fix it. Any Ideas?
 
Have you checked for vacuum leks under the hood and dash?
 
engine is running between ~950 and ~1050 RPM, there is a soft humming noise...pitch never changes and it doesn't matter what gear I'm in (including park).

When vehicle is parked, put a decent battery charger on it, when engine is running. And older battery charger might be better...new ones quit when they sense enough voltage.

Constant pitch might suggest one of the fuel pumps...when engine revs higher, alternator is producing more current/voltage.
 
When vehicle is parked, put a decent battery charger on it, when engine is running. And older battery charger might be better...new ones quit when they sense enough voltage.

Constant pitch might suggest one of the fuel pumps...when engine revs higher, alternator is producing more current/voltage.

Fuel pump sound likely. I'm not sure I understand what I should be testing for with the battery charger.
 
I tried turning on the heater fan, but it was unfortunately much louder than the humming noise and I couldn't hear the noise anymore. I tried switching from no lights to brights, with no discernable change in pitch.
 
I actually just tried again and while I didn't notice any change in pitch, I was able to repeatably stop and start the noise by flashing the high beams.

So if I understand correctly, by changing the load on the system (high beams), I am changing the current to the pump, thus changing the speed of the motor.

Armed with this information, any ideas on how I could diagnose which of the fuel pumps, if either, is faltering?
 
92 240...by changing the load on the system (high beams),

Might use decent voltmeter....clean pumps grounds, btw

Both pumps can be energized at fuse panel...fuse 4

<big>1985-93 240, jump fuse circuit #4 to #6.</big>

With engine off, one might be able to hear which pump it is.
 
Listening, I could hear both pumps whirring when energized, so I got a fuel pressure gauge and checked the high pressure port on the fuel rail, which yielded 35 psi energized and held that pressure until i disconnected 20 minutes later. I then checked the port before the main fuel pump, which only had 1-2 psi energized and lost it immediately. I'm guessing that the valves that are supposed to hold the pressure in the in-tank fuel pump have gone bad somehow, and the in tank pump is otherwise not performing well. I'll continue my diagnostics (I won't have the time to replace for another 2 weeks) and replace when I'm sure.

Thanks a lot for the help, its great to finally have something to go on.
 
Yeah.

The 35 psi reading was engine off, key on, and the spec range is 32-38 for LH 2.4 according to Haynes, so that passed.

The spec range for the lower reading was 4-5 psi, so it didn't come anywhere near passing. It looks like the main pump is doing all of the work.

I'll update when I'm able to take the pump out.
 
spec range is 32-38 for LH 2.4 according to Haynes.

My main pump spec came from LH 2.4 bible....most likely with both pumps running, and engine off.

However, when two pumps are inline, the first pump (intank) will increase the second pump's output pressure. Iirc, intank should have around 6-8 psi....so 8 + 35 = 43 psi
 
I finally got some time to spare and popped the new pump in. Easier than I expected. Results:

Fuel rail pressure with engine off and key on is now 41.
No change on the low pressure port. still reads 1-2 psi with the pump running, then drops off instantly to 0.
More power, especially on hills.
Whining noise remains, no change. Damn.

I can hear the new in-tank pump running strong in there, and I tested the old pump when I got it out (could only get it to move if i smacked it while it was energized).

Not really sure what to make of all this. I'm going into the dash tomorrow to see if smokey was right about that vacuum leak.
 
Have you inspected you're exhaust system at all? Could possibly be a small hole, crack, or even a leak in the manifold gaskets. Also I would say check your belts and tensioners for noise by spraying the belts with water to see if the noise stops. if none of this works then it may be your transmission. hopefully not! good luck to ya.
 
I think I found the source. The adjusting screw for the AC belt has snapped in the bracket. The part of the bracket where the head of the screw goes has bent quite a bit, I think someone tried to tighten the belt without loosening the lower bolts, and the bracket gave. I think this is an easy replacement (correct me if I'm wrong), so I'll be heading down to the JY to salvage another.
 
That was it. I replaced the bolt/bracket I was talking about, which was in need of replacement:

IMAG0082.jpg

Guess which one is the old one.

Unfortunately, when I started up the car to hear how sweet it sounded, I was greeted by a banshee scream from the belts:

https://plus.google.com/u/0/1048900...6089858566304299314&oid=104890090381982058580

Turns out the pulleys were way out of alignment, but this bent bracket let the A/C compressor sit in such a way that the belts would not squeal. I replaced all of my accessory bushings, and started it up. I haven't heard any sounds from it since.
 
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