Cab and Body Vibrations

[Ajax MD]

I did run through the search function and didn't find what I was looking for...not exactly, anyway.

Symptoms on a '71 M813:

I seem to have a fair bit of vibration at idle, just sitting. The vibration is everywhere. The tailgate rattles, body panels rattle, the tool box lid rattles, the cab rattles, the front bumper step REALLY rattles. If I mount my phone on the rear window, the video is blurry from the vibration. I cleaned up some of the rattles up by putting adhesive foam strips on the tool box lid and those removable air grates.

I don't have any drive line vibrations (wheels, bearings, transmission/transfer case). The engine vibration smooths out when I drive the truck.
When I climb up and observe the engine, it's not visibly shaking or rocking. The truck idles at a rock solid 600 RPM. The idle vibration reduces the most when the engine gets warmer, near operating temperature but it never fully goes away.

The truck is 47 years old. Is this just a function of age and out dated design or do I possibly have an issue?

The only search results I found that make any sense, were in a thread in the CUCV forum where it was pointed out that dirty or worn injectors that aren't quite opening on time can cause this, and also possibly a worn harmonic balancer.

https://www.steelsoldiers.com/showthread.php?175491-Idle-Vibration-Normal/page5

Thoughts?

 

[ross165123]

Sounds normal to me, it'll slowly vibrate stuff of the seat and windows at idle.

 

[Scar59]

Yep, nature of the beast. Sound just like my truck. Once started, use the hand throttle to raise the rpm just a skosh. My settles out around 725-750 rpm. These trucks do not like to sit around idling. Start, warm up, and drive.

 

[Ajax MD]

Ok, cool.

I do use the hand throttle to keep the RPM's between 700 and 1000 rpms during the 5 minute warm up period. Then, I release the hand throttle and it settles at 600.
The "wandering idle" that the truck did when I first bought it has completely stopped since I cleaned up the fuel system and started driving the truck.

I know some of my posts come across as hyperventilating over little things. I'm just trying to stay out in front of problems, especially big problems. As I gain experience and time with the truck, I'll mellow out.

Thanks!

 

[Scar59]

I have a installed a little self adhesive foam/weather seal here and there to quiet some of the more obnoxious rattles. Check your inner fender access panel, I had one not not secured and in sounded like the engine was coming apart. Place your hand on the panel and quiet. Removed it, gave it a calibrated flex and reinstalled, make sure all the fasteners align and secure. Engine bay side panels will drive you nuts too. Bent out the isolator flanges and fixed them.

 

[URSATDX]

I'm still working the rattles out of mine.
As stated above, walk around the whole truck and look/feel for the origination
of the noises. Cab insulation made a big difference in mine.
They all shake a lot.

 

[US6x4]

My truck also shakes enough to vibrate even heavy items off the seat, out the door, and all over my yard. This happens when the engine is cold and idling around 550. If I up the idle to 650-700 it smooths out a lot and I've been tempted to buy the idle adjustment tool to increase my idle speed, but the hot engine idle speed is smooth(er) between 600-650. For mine I think a 700 cold idle and 600 warm would be good but I don't know if we have the option of dual idle speeds other than the hand throttle which loves to gain rpm and run away on me.

My highboy has a balanced 428 Cobra Jet and when you shut off a balanced engine it just stops rotating without any shaking. Simply ceases to rotate and is very smooth. It would take a lot of weight but it would be cool to have a balanced 250 Cummins which would eliminate most of the vibrations

 

[Ajax MD]

I put foam on the inner fender panels. Those were the "air grates" that I referred to earlier. As you said, the engine bay side panels buzz a lot to I'll attack those next.
One reason I want to quiet everything down, is so that I can hear when "abnormal" sounds appear that indicate real problems.

 

[162tcat]

I did run through the search function and didn't find what I was looking for...not exactly, anyway.

Symptoms on a '71 M813:

I seem to have a fair bit of vibration at idle, just sitting. The vibration is everywhere. The tailgate rattles, body panels rattle, the tool box lid rattles, the cab rattles, the front bumper step REALLY rattles. If I mount my phone on the rear window, the video is blurry from the vibration. I cleaned up some of the rattles up by putting adhesive foam strips on the tool box lid and those removable air grates.

I don't have any drive line vibrations (wheels, bearings, transmission/transfer case). The engine vibration smooths out when I drive the truck.
When I climb up and observe the engine, it's not visibly shaking or rocking. The truck idles at a rock solid 600 RPM. The idle vibration reduces the most when the engine gets warmer, near operating temperature but it never fully goes away.

The truck is 47 years old. Is this just a function of age and out dated design or do I possibly have an issue?

The only search results I found that make any sense, were in a thread in the CUCV forum where it was pointed out that dirty or worn injectors that aren't quite opening on time can cause this, and also possibly a worn harmonic balancer.

https://www.steelsoldiers.com/showthread.php?175491-Idle-Vibration-Normal/page5

Thoughts?

Have you looked at how the truck is built? Of course it's going to rattle! The basic design was likely done in the 40's and lacking in creature comforts even in that era. Not many changes were made over the years other that drivetrain. If you hate noise and rattles, either get to work with felt, foam, linex and insulation or find a different hobby because they are loud and rattle. If it rains, you'll be wet. If it's hot, you'll sweat. The were build to do a job, not necessarily be comfortable.


Sent from my iPhone using Tapatalk

 

[Ajax MD]

Yeah, I get that it's crude and simple. There were/are perfectly valid reasons for that. I think you misunderstand the reason for my question.

I'm not looking for comfort, I'm looking for problems. As long as the rattles and hums are normal, I'm good with that. This wasn't a complaint, I was just asking if something might be wrong. I also said that the main purpose of taming all the rattles, is so that I can hear real problems if any arise.

One reason I was concerned, is because I've watched several in-cab videos of M809 trucks on Youtube and many of them seem smoother than my truck so I wondered if something might be wrong.
I'll put some foam on some panels but I probably won't bother with sound insulation and I'm certainly not going to hack the truck up to add any comforts other than what the military offered.

 

[US6x4]

I added about 12" of rubber edging to the rear flange of my hood on both sides where the hood curves down and eliminated most of my vibrations and rattles! What sounded like bad vibrations coming from behind the dash turned out to be the hood rattling against the cowl because of the slop in the hood hinge pin.

 

[Ajax MD]

Thanks for the tip, I'll try that!

 

[GopherHill]

Might check the harmonic balancer condition, just me being a worrier.

 

[Crf450x]

Mine idled at 550. I turned it up to about 600 and planning on going to 650-700. Everything outside rattled at 550 and some of it still does at 600. You don’t need a special tool. Just can’t do it while it’s running. Go for a drive between adjustments to get the air out.

 

[US6x4]

Mine idled at 550. I turned it up to about 600 and planning on going to 650-700. Everything outside rattled at 550 and some of it still does at 600. You don’t need a special tool. Just can’t do it while it’s running. Go for a drive between adjustments to get the air out.

I've noticed this as well with mine. I wish these engines were balanced. Imagine how much weight that would take!

 

[Crf450x]

I've noticed this as well with mine. I wish these engines were balanced. Imagine how much weight that would take!

Yeah that would be nice. I’m sure it’s been done. There are a lot of high dollar 855s out there. I’ve seen balanced 5.9s and it’s a night and day difference.

 

[US6x4]

I had my Cobra Jet balanced and is sweet. There's no shaking when it's shut off - it just immediately stops. Our 855s shake a lot when you shut them down. Almost feels violent.


FYI: in the 60s & 70s amongst the big 3 the mopars were the furthest from being balanced (require the most weight to get balanced)