Accepting any and all crazy ideas
 

I have a truck puzzle and I'm open to whatever suggestion flies up on the table. This might take a little bit so bear with me.

I noticed a knocking sound coming from the area of my gas tank the other day which is still behind the seat. I have installed bucket seats so it's wide open in the middle now.

I thought at first it was sound transfer from something like a loose torque converter so I checked it and it's tight. I looked the flywheel over and it doesn't look cracked. I checked the exhaust and it's all tight. It's also new.

So, I thought lifter maybe. However I can't really hear anything under the hood of the truck. It's loudest inside the cab with the doors closed and it really sounds like it's coming from the gas tank. I pulled the oil filler cap and pcv valve and neither valve cover is emitting any noise. By now I'm thinking it's a rod and the sound is transferring through the metal fuel line. It is definitely rpm specific. The trouble is, you can't hear anything under the hood or under the truck. I started it up today and reached down and I could actually FEEL it in the Big fuel line coming out of the tank.

I called my buddy Dr. Joe and he brought his stethascope over and we went to work trying to track it down and here's what we came up with. Nowhere on the engine could we locate a knock with the steth. Not from the valve covers, intake, block, or oil pan. I can't hear anything coming from the fuel pump that sounds like a knock. However, inside the cab, you can hear it faintly from the tank and faintly from the floor of the cab but it's very loud when you put the stethascope on the fuel line coming out of the tank. The "pull" line. I mean very loud.

Now here's the tricky part. That fuel line is insulated by a short rubber line from the tank and another rubber line under the cab and again at the fuel pump. So it's difficult to see how it could be sound transfer. But you can only faintly hear it in the tank and I think that's because the fuel line is laying against the tank.

I did recently change the fuel pump but as I stated before, the stethascope is not picking up a knock from the fuel pump itself or the surrounding block. So, I'm stumped.

Shoot the ideas at me. Keep in mind that it's rpm specific. Meaning it speeds up and down with the motor and when the stethascope is on that fuel line it sounds like a knock.

Re: Accepting any and all crazy ideas
 

Additionally, the motor runs strong. It's snappy and doesn't seem to have a miss so it doesn't act like a lifter issue. It also has high oil pressure so I'd be surprised if it were a rod.

Re: Accepting any and all crazy ideas
 

I'd check the fuel pump rod. Maybe the new fuel pump has a slightly different angle on the arm causing too much clearance. Just a thought

Re: Accepting any and all crazy ideas
 

They appeared to be the same when I switched them and it's been in there for a couple of months now. Also, the stethascope isn't picking up a knock from the fuel pump.

I may switch it out anyway just as an elimination process.

Re: Accepting any and all crazy ideas
 

i would isolate the tank sending unit, it could be loose in the tank

Re: Accepting any and all crazy ideas
 

I thought about that as well. I watched the fuel gauge and it doesn't even wiggle and the truck is sitting still in my driveway. And like I said, it's rpm specific. When the motor speeds up so does the knocking sound.

Re: Accepting any and all crazy ideas
 

i would pull it out block off the hole with a rag, then run it to see if the noise is still there

Re: Accepting any and all crazy ideas
 

This sucks. An old cab I had on the 4x4 had a sound coming from the gas tank area. Sounded like something in the gas tank hitting the side. Had that hollow sound. I had the tank out twice thinking something was in it or the float for the sending unit was hitting the side. When I had the tank out I noticed that there was some rust along the back edge of the cab where the floor is spot welded to the back of the cab. There was movement there when you lifted on the cab corner from the outside. Yep, the floor stayed put and the back of the cab was raising. There was enough flex that it was popping the gas tank. The tank is mounted to the back of the cab and the floor. It wasn't that noticable but I put 4 brackets across the back of the cab and bolted them together. I fixed it and the sound went away. It looked like that area had rusted just enough that the spot welds failed.

Re: Accepting any and all crazy ideas
 

are you getting reversion in the fuel line from a bad check valve in the fuel pump? similar to a burned intake valve causing a back fire in the intake but this is all in the fuel supply line. liquid transmits vibrations without regards to it's surrounding containment (metal or rubber has little or no difference.

or maybe you have a pi$$ed off gremlin hiding under the cab tapping on the line.

try this; have a buddy pinch off the hose by the pump while it's running and see if it goes away.

Re: Accepting any and all crazy ideas
 

is the fuel line vibrating against the frame rail or something? If you just replaced the fuel pump, perhaps you bent the line a little and its touching something?

Re: Accepting any and all crazy ideas
 

is the fulel pick up in the tank full of crap and the fuel pump makeing a vacum on it..might have been the cause of the first fuel pump failure

Re: Accepting any and all crazy ideas
 

motor mounts, flexplate, exhaust monuts tranny mounts tranny crossmember,body mounts sagging allowing tunnel to contact tranny, carrier bearing, universal joints, gas tank mounted without rubber isoolators

Re: Accepting any and all crazy ideas
 

Thanks for the ideas everyone. I'm gonig to pull the pickup/sending unit out of the tank to have a look at things and probably go ahead and swap the fuel pump and then check it out again. Once I rule those out them I'm going to move on to some of these other suggestions and eliminate as much as I can. I did hit the motor mounts with the stethascope and there wasn't any knock coming from them.

Re: Accepting any and all crazy ideas
 

Turn the radio up louder.

Problem solved. You're welcome. :D

Re: Accepting any and all crazy ideas
 

Is the tank vent clear? Just a thought, but maybe suction is causing the tank to oil can. As in, when the RPM goes up, so does the fuel suction. Then when you let the throttle off, the tank pressure releases in a way that causes the wall of the tank to pop? Don't know if this is even possible, just throwin out a crazy idea for ya.

Re: Accepting any and all crazy ideas
 

Mine started knocking like that... rpm specific and stuff. Turns out I was || <--this close to throwing a rod. Also, some thrust bearing or something in the crank was gone.
Have you towed anything lately? Certain conditions in towing can cause the torque converter to balloon which is what destroys the thrust bearing thingy...

Re: Accepting any and all crazy ideas
 

:lol: It would be like those geico eyes. "I always feel like somebody's watching me"

Yeah, we wondered about that. Those two fuel lines going up high into the passenger side of the cab................ is the freakin vent up there?

I hit the block and oil pan with the stethascope but no knock. However, in that fuel line it's really loud.

As for the tranny and hauling. The only thing I did was race an Impala SS last week and that wasn't anything crazy just a stop light run. The tranny is a TH400 so it should stand up to more than that. I did check the bolts and flywheel though. Looks ok.

Re: Accepting any and all crazy ideas
 

Snoring...It's got apnea!

Re: Accepting any and all crazy ideas
 

Have you tried pushing on the tank?
Try to hold, push ,twist the tank to isolate loose mounting issues.
Maybe a loose mounting bolt is allowing the tank to knock,knock against the cab.
Just throwing stuff out there for ya.

Re: Accepting any and all crazy ideas
 

Try taking off the gas cap, I thin VTBlazer may be onto something

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:lol: well then it's ok, I have sleep apnea too!

I'll try to move it but it's doing it sitting still in my driveway and it's rpm specific.

I'll try that tomorrow evening after I get the new fuel pump on.


Here's where I'm at right now. I pulled the new fuel pump off and the only thing I can see is that the rod wasn't making contact with the "center" of the pump lever. And the lever will move side to side and it makes a tin like knocking sound when it does. I'd be surprised if that was it though since I couldn't hear anything from the pump with the stethoscope.



http://i4.photobucket.com/albums/y14...C/fuelpump.jpg

Re: Accepting any and all crazy ideas
 

Just face it. You are never going to find the noise, so just sell me your truck cheap (after all, it does have a funny noise ;)). :lol:

I bought a car from my F-I-L because a noise it made at 55mph. When asked if I ever fixed the noise, I said "Yep, I only drive under or over 55mph. Problem solved."

Re: Accepting any and all crazy ideas
 

I had a similar noise a few months back, turns out some bark had gotten between the cab and the bed of the truck.

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A loose socket down in one of the cab corners ?....don't laugh, I once found a 30 year old screwdriver down in the rear quarter panel of an old Camaro...that rattle drove me nuts for years....LOL

Re: Accepting any and all crazy ideas
 

:lol:

Well it's not the fuel pump. I just installed a brand new one and it's still doing it. I checked the drive rod and it has the appropriate spin pattern on both ends and the bevels still match so no wear.

I don't think it's going to be a vent issue either since I had the lines disconnected from the pump and it made the noise instantly as the motor started. If anything it sounds worse. I'm almost going back to my original assesment of a rod bearing. I just can't understand why the stethoscope doesn't pick up any noise from the block.