Accepting any and all crazy ideas - The 1947 - Present Chevrolet & GMC Truck Message Board Network

*highperf4x4* · 10-06-2009, 12:34 AM

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.

*highperf4x4* · 10-06-2009, 12:38 AM

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.

cduster · 10-06-2009, 12:52 AM

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

*highperf4x4* · 10-06-2009, 01:00 AM

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.

72BLUZ · 10-06-2009, 01:13 AM

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

*highperf4x4* · 10-06-2009, 01:16 AM

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.

72BLUZ · 10-06-2009, 01:26 AM

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

Mudder · 10-06-2009, 05:40 AM

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.

68C15 · 10-06-2009, 06:53 AM

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.

BluTrukker · 10-06-2009, 07:14 AM

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?

Hottrucks · 10-06-2009, 07:45 AM

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

cdowns · 10-06-2009, 09:30 AM

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

*highperf4x4* · 10-06-2009, 10:42 AM

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.

chevy_mike · 10-06-2009, 03:30 PM

Turn the radio up louder.

Problem solved. You're welcome.

vectorit · 10-06-2009, 03:53 PM

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.

toddtheodd · 10-06-2009, 04:15 PM

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...

*highperf4x4* · 10-06-2009, 06:34 PM

Quote:

Originally Posted by chevy_mike

Turn the radio up louder.

Problem solved. You're welcome.

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

Quote:

Originally Posted by vectorit

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.

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?

Quote:

Originally Posted by toddtheodd

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...

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.

71tahoe · 10-06-2009, 07:09 PM

Quote:

Originally Posted by Hottrucks

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

Snoring...It's got apnea!

retodd7711 · 10-06-2009, 07:33 PM

Quote:

Originally Posted by cdowns

, gas tank mounted without rubber isoolators

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.

bigbadchev72 · 10-06-2009, 07:47 PM

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

*highperf4x4* · 10-06-2009, 09:13 PM

Quote:

Originally Posted by 71tahoe

Snoring...It's got apnea!

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

Quote:

Originally Posted by retodd7711

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.

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

Quote:

Originally Posted by bigbadchev72

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

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.

ERASER5 · 10-06-2009, 11:51 PM

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

).

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."

**kwmech** · 10-07-2009, 11:01 AM

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

Rack Man · 10-07-2009, 03:07 PM

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

*highperf4x4* · 10-07-2009, 09:01 PM

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.

10-06-2009, 12:34 AM	#1
highperf4x4 Account Suspended Join Date: Apr 2009 Location: Oklahoma City Posts: 4,709	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.

10-06-2009, 12:38 AM	#2
highperf4x4 Account Suspended Join Date: Apr 2009 Location: Oklahoma City Posts: 4,709	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.

10-06-2009, 12:52 AM	#3
cduster Registered User Join Date: Oct 2002 Location: S.W. Michigan Posts: 403	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 __________________ Good Judgment comes from experience. Most experience comes from bad judgment.

10-06-2009, 01:00 AM	#4
highperf4x4 Account Suspended Join Date: Apr 2009 Location: Oklahoma City Posts: 4,709	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.

10-06-2009, 01:13 AM	#5
72BLUZ Registered User Join Date: Nov 2006 Location: lafayette, ga. Posts: 832	Re: Accepting any and all crazy ideas i would isolate the tank sending unit, it could be loose in the tank __________________ Freedom is never more than one generation away from extinction. We didn't pass it to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same.... Ronald Reagan

10-06-2009, 01:16 AM	#6
highperf4x4 Account Suspended Join Date: Apr 2009 Location: Oklahoma City Posts: 4,709	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.

10-06-2009, 01:26 AM	#7
72BLUZ Registered User Join Date: Nov 2006 Location: lafayette, ga. Posts: 832	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 __________________ Freedom is never more than one generation away from extinction. We didn't pass it to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same.... Ronald Reagan

10-06-2009, 05:40 AM	#8
Mudder Registered User Join Date: Feb 2001 Location: East Central, MO Posts: 11,336	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. Last edited by Mudder; 10-06-2009 at 05:42 AM.

10-06-2009, 06:53 AM	#9
68C15 blood type; Retumbo Join Date: Jan 2004 Location: next to my reloading bench Posts: 10,269	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. Last edited by 68C15; 10-06-2009 at 06:55 AM.

10-06-2009, 07:14 AM	#10
BluTrukker BOHICA Join Date: Mar 2008 Location: Stewartville, MN Posts: 440	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? __________________ Trying to save someone from their own stupidity is like teaching a pig to dance. It is a waste of your time, and it annoys the pig!

10-06-2009, 07:45 AM	#11
Hottrucks Redefining LowBudget Join Date: Mar 2009 Location: lebanon Cow Hampshire Posts: 4,538	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 __________________ 1970 C10 CST fleetside 472 ....big dreams little cash... http://67-72chevytrucks.com/vboard/s...d.php?t=335721 SunShine Syndicate.. Mikes Sandwich Fair Run OCT 8th 2011 http://www.thesandwichfair.com/html/...auto_show.html

10-06-2009, 09:30 AM	#12
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	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 __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

10-06-2009, 10:42 AM	#13
highperf4x4 Account Suspended Join Date: Apr 2009 Location: Oklahoma City Posts: 4,709	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.

10-06-2009, 03:30 PM	#14
chevy_mike Never Ending Projects Join Date: Jul 2006 Location: San Diego, CA Posts: 3,836	Re: Accepting any and all crazy ideas Turn the radio up louder. Problem solved. You're welcome. __________________ . 1965 C10 Panel, Tiki Express http://www.67-72chevytrucks.com/vboa...d.php?t=506580 SOLD 1968 Chevy C10, Long, Fleetside, Hot Rod Hauler http://67-72chevytrucks.com/vboard/s...d.php?t=313233 SOLD 1965 Chevy C10, Long, Fleetside, Hot Rod C10 http://67-72chevytrucks.com/vboard/s...d.php?t=415702 SOLD We were given two ears and one mouth for a reason... listen twice as much and speak half as often...

10-06-2009, 03:53 PM	#15
vectorit What? Join Date: Jun 2007 Location: Southern California Posts: 7,617	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. __________________ Chris 1968 K20 Suburban 1972 K10 LWB PU



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10-06-2009, 04:15 PM	#16
toddtheodd The oddest Todd around Join Date: May 2004 Location: Miami, Florida Posts: 1,418	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... __________________ "The greatest pleasure is to vanquish your enemies and chase them before you, to rob them of their wealth and see those dear to them bathed in tears, to ride their horses and clasp to your bosom their wives and daughters." Ghengis Khan IIOY???

10-06-2009, 07:47 PM	#20
bigbadchev72 People know me Join Date: Jun 2007 Location: Hermiston, oregon Posts: 840	Re: Accepting any and all crazy ideas Try taking off the gas cap, I thin VTBlazer may be onto something __________________ 1972 k/5 blazer CST 4X4 Crate 350, th350/205, dana44with trac-loc/14 bolt with detroit locker, 4 wheel discs 4.56s, 8" suspension lift using 73-87springs, 1" zerorate custom front crossmember, HAD CV drivelines and driveshaft e-brake, full cage, PRP seats, Full autometer gauges, 38x14.5x16 Toyo M/Ts, 16x10 Polished aluminum rockcrusher wheels 1967 longbox 4x4, Strong 400/400/208, 44/12bolt 4.10s, 6" lift, 35" Toyo M/Ts on corvette rally wheels

10-06-2009, 11:51 PM	#22
ERASER5 Registered User Join Date: Jun 2006 Location: Indiana Posts: 4,859	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 ). 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." __________________ '70 GMC C1500 LWB Power disc brakes. WooHoo! Posi 6 Lug Dana 60

10-07-2009, 11:01 AM	#23
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,576	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.

10-07-2009, 03:07 PM	#24
Rack Man Registered User Join Date: Sep 2008 Location: Tempe, AZ Posts: 770	Re: Accepting any and all crazy ideas 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 __________________ My Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=309288

10-07-2009, 09:01 PM	#25
highperf4x4 Account Suspended Join Date: Apr 2009 Location: Oklahoma City Posts: 4,709	Re: Accepting any and all crazy ideas 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.

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