sounds like a chain rattle

[ssjones]

I have a 72 c20 3/4 ton lwb that at times when accelerating I hear what sounds like a chain rattling from under the cab? Could this be bad u-joints?
any ideas?

Thanks

[matthufham]

tailgate support rods make alot of noise on my truck.

[Stocker]

Originally Posted by ssjones

Could this be bad u-joints?

Very possible -- the noise can be amplified by the driveshaft. Check 'em carefully for any sign of rust around the bearing cups.

You may be unable to notice any play if you try moving them by hand, but if you can put the truck on a lift and have someone start it up and put it in gear, you can often see the play in a bad joint.

[JDL]

sounds like the carrier bearing

[Sweet72]

Check the drive shafts first and , are you sure it's not detonation under acceleration? heh that kinda rhymes

[blazerparts]

I notice the same noise. I can rule out tailgate rods as I have a flatbed. U joints can also be ruled out as I replaced those when the trans was rebuilt last summer. That leaves carrier bearing or detonation. Color me ignorant but what would detonation be? I was kind of suspecting motor mounts, could they be a culprit?

[Sweet72]

Detonation is heard when the fuel is igniting too soon on the compression stroke and is more of a engine knocking sound like marbles rattling around on top of the pistons under hard acceleration. Sometimes caused by fuel with too low of an octane rating for the compression ratio of the engine, and may also be caused by ignition timing set too far advanced or carbon buildup in the chambers or pistons glowing and causing the fuel to ignite prematurely.

[jhwkns]

[ssjones]

I dont think it is detonation. it sounds like it is behind the cab and I thought it was a loose exhaust but I had that all re-done (needed to do it anyway). I have never replaced the u-joints or the carrier bearing. On the positive side I just finished up the front end with all new components and it drives and rides soooooo much better.

[powell72]

My wife's truck was doing the same thing, the metal shield that keeps dirt out of the bearing itself on the carrier bearing came loose and was rattling around on the drive shaft.

[Longhorn Man]

Originally Posted by Sweet72

Detonation is heard when the fuel is igniting too soon on the compression stroke and is more of a engine knocking sound like marbles rattling around on top of the pistons under hard acceleration. Sometimes caused by fuel with too low of an octane rating for the compression ratio of the engine, and may also be caused by ignition timing set too far advanced or carbon buildup in the chambers or pistons glowing and causing the fuel to ignite prematurely.

That would actually be pre-ignition... the act of detonation before the plug lights the mixture.

Detonation is actually the ignition of the air and fuel in a controlled environment IE, correct conditions.

U-joints wouldn't make that kind of a noise, although, the carrier bearing could.

[VetteVet]

If you care enough to read it.

Hastings Manufacturing
325 North Hanover Street.
Hastings, Michigan 49058-1598 USA
Phone (269) 945-2491 Fax (269) 945-4667


DETONATION AND PRE-IGNITION

When an internal combustion engine is torn down for rebuilding and badly burned and distorted pistons and valves are found, it is very likely caused by extremely high combustion chamber temperatures and pressures from detonation or pre-ignition. Generally speaking, most machine shop men know that such damage is caused from abnormal engine operation. Too often, the customer does not know what causes the costly damage, yet the machine shop man may be unjustly blamed for an engine "that didn't stand up".
Detonation and pre-ignition are forms of abnormal combustion in the combustion chamber. During normal operation of the engine, the burning of the fuel-air charge produces a steady, smooth push on the pistons of each cylinder. At the instant of ignition by the spark plug, the flame of combustion moves rapidly outward from the plug very much like the waves when a stone is dropped into a pool of water (see fig. 1).



Abnormal operation may allow combustion pressures to develop so fast that the heat and pressure will "explode" the remaining unburned fuel. This produces the knock, often called "ping", carbon knock, etc. Actually this is detonation. The knock results from the violent explosion when the normal flame front runs into the secondary flame front. Detonation will cause piston and ring damage, top ring groove wear, scoring, sticking rings, loose head gaskets and possible complete engine failure.



(Photo of detonation damaged piston). Fig. 2
Detonation can be caused by:
Lean fuel mixture
Fuel octane too low
Improper ignition timing
Lugging
Carbon deposits
Excessive milling of heads or block, which will increase compression ratio.
Pre-ignition, as the term suggests, is the ignition of the fuel-air mixture before the regular ignition spark from the spark plug. If the regular spark occurs shortly after the pre-ignition, the colliding of the two flame fronts will cause a pinging noise. Preignition causes loss of engine power and can cause severe damage to pistons, rings and valves.



(Photo of pre-ignition damaged piston). Fig 3

Detonation and pre-ignition are so closely related that it is en difficult to distinguish one from the other by sound. Each can lead to the other and either condition can cause extensive engine damage. Inspection of damaged pistons and rings quite often indicate which caused the damage.

Damaged pistons and rings usually mean replacement. The same damage can occur again unless the cause of the detonation or pre-ignition is corrected.

Pre-ignition can be caused by:

Carbon deposits that remain incandescent
Spark plugs too hot a heat range
Spark plugs not firmly seated against gasket
Detonation or the condition leading to it
Sharp edges in combustion chamber
Valves operating at higher than normal temperature because of excessive guide clearance or improper seal with valve seats.
Overheating
Ignition crossfiring. Induced voltage in spark plug wires that run parallel to each other for long distances


.

[wanted]

The rear brake on my truck fell apart and sounded like a chain dragging.

[ssjones]

Thanks Wanted. I did my brakes and that was not the problem. I think I will be checking my u-joints this weekend

[sneakysnake]

Originally Posted by powell72

My wife's truck was doing the same thing, the metal shield that keeps dirt out of the bearing itself on the carrier bearing came loose and was rattling around on the drive shaft.

Thats what I was gonna suggest, makes a nice jingly sound.

[dinnut]

coil spring or leaf rear end. friend had a bracket holding the leaves together kinda and it was loose and made a chain like noise

[dwaite72lnghrn]

Sounds like an exhaust hanger loose. Normally U-joints under load make a low grinding sound and turn into a knocking sound in coast mode(in between gas and brake) and a loud clang or bang when you rock between forward and reverse.
To check U-joints block the front wheels front and back then jack the rear off the gound use jack stands for safety. The grab the joint on each side and see if there is any movement up-down, sideways and twisting. They should not give any.