Feeling Virbations cont.

[skokie]

Here is the original thread
http://67-72chevytrucks.com/vboard/s...0&page=1&pp=25

So far the balancer has been swapped along with the tranny, torque converter, and flexplate. What else could be causing this? Just a quick summary I am feeling a vibration when the truck is parked. Vibration is happenning from 1300-1500 rpm.

Thanks

[RON WOODGEARD]

What is the engine?

A vibration that is only apparent for 200 RPM is perplexing.. How bad is it? When and how did you notice it starting to vibrate? did you do something, or change something that made you notice it? Have you checked your motor mounts? Sometimes a motor mount will appear to be good until you take it out and then you find it's broken or worn badly.. all engines have SOME vibration, Albeit not much but some... The last time I had anything similar to this.. It turned out to be a loose torque convertor bolt... Just one !

[skokie]

It is a crate 350. Its close to moderate. On a scale of 1-5 I would give it 2-3 max. I noticed it was when I converted to short water pump. I dont remember if it was always there thats just when I remember noticing it. How do you check a motor mount? The motor mounts were new when we dropped the engine in last year. I put in a new balancer and today a new flexplate went in that seemed to make it a little better but its still there.

[Brainchild]

I noticed it was when I converted to short water pump. I dont remember if it was always there thats just when I remember noticing it.

I would drop the belt to make sure it isn't in the waterpump/fan/alt/ps area.

[rodnok1]

I would unhook the converter and see if it still does it, at least you could eliminate the tranny then.

[Sonny]

If you haven't already done so, take off all of your belts and strart it up and see if it is still there. My alternator and power steering pump didn't quite line up before I replaced the brackets and it used to do similar to what you are talking about. Mine would throw an alternator belt quite frequently until I put new brackets on.

Sonny

[Olden Days]

a dead cylinder will act simular. check that your firing on all 8

[skokie]

Sonny I will do all the belts tonight and see what that gets me. Olden Days I will double check the wires to. Rodnok1 the tranny has been isolated. Had a new tranny, torque converter and flexplate installed yesterday.

[Green Machine]

Try unhooking the vacumn advance for a test. Sometimes to much advance at light load can cause a shake.

[Chris H]

Is the engine a later model with a 1 piece rear seal?
I know that some of the newer engines (1987 and up) require a counterweighted flexplate.
Using an non-counterbalanced flexplate on an engine that needed on would definately cause a vibration at certain harmonic frequencies.

[RShaips]

I had a water pump that was causing a vibration, repalced it and everything was smooth. Although it got progessively worse as RPM increased.

[Mixup]

Did you change pulleys when you went to a short water pump? Maybe you have a bent one.

[skokie]

Yep I changed the pulleys when I converted it to a short pump. The pulleys dont seem to be bent. Rshaips the vibration is there at 1300-1500 only and it lasts 3-5 seconds, goes away and 2-3 seconds later comes back.

[pjmoreland]

My understanding of vibrations that come and go (pulsate) like the vibration you are having are caused typically by two otherwise insignificant vibrations that are at slightly different frequencies from each other. The vibration pulses will go through a repeating cycle of lining up with each other and not lining up with each other. When the pulses are lining up with each other, they combine to cause a big vibration. When they are not lined up with each other, the vibration goes away. You will feel a pulsating vibration as the two vibrations come in and out of phase with each other. Have you ever been sitting at a stoplight behind someone, and your blinker is almost exactly in rhythm with their blinker? The blinkers will blink at the same time for awhile, but then slowly they get out of sync and then slowly they get back in sync. That's the same concept, except sped up.

A common cause that I've experienced for this type of vibration is a drive shaft that has the u-joint angles set incorrectly on a vehicle with an automatic transmission. The torque converter allows the engine to spin at a slightly higher RPM than the driveshaft, causing the natural vibration of the engine to pulsate in and out of rhythm with the drive shaft vibrations due to the slippage of the torque converter. Of course this isn't your problem because your vibration occurs when you are not driving.

The father of a friend of mine worked at GM as an engineer in the vibration laboratory for many years, including the late '60s and '70s. I got a chance to speak to him a couple of years ago. He said that GM didn't do any vibration analysis on their trucks during these years. He said that typically what they try to do now is design the engine/transmission to have a natural frequency of about 7 hertz and the chassis to have a natural frequency of about 13 hertz. That way the engine is unlikely to set the chassis into a state of vibration since 7 and 13 are both prime numbers and don't divide into each other and are only common denominators of high numbers, like 91. If your chassis is damaged, or customized, this may have altered the natural frequency of the chassis. If the chassis natural frequency is now 14 instead of 13 due to damage, then it would be very easy for the engine to excite the chassis since 7 goes exactly into 14 two times.

If you have run your engine with all of the accessory belts on the front of the engine removed, and the three torque converter bolts removed, and you still feel the vibration, and all cylinders are firing as they should, then there may be some fundamental problem with your truck. If your truck was originally an inline 6 and was converted to a V8, and the original engine mount towers from the inline 6 were used, then the engine is mounted in a position that is a few inches farther forward than it should be on the chassis. This could possibly alter the natural frequency of the chassis. I suppose the exhaust system could be causing a problem like this as well. This is a difficult thing to track down.

[skokie]

pjmoreland thanks for the wealth of info. I apprecaite it. You mentioned U-joint angles. Are you talking in the rear of the truck? Here is why I ask. The truck came originally with a 3.73 rear end. I swapped it with a 3.07 rear end from a 70. If I remember correctly it is 1 1/2 inch wider from hub to hub than the original. I checked the exhaust and it looks ok. I dont see any no rubber braces on there. The funniest part about this is that I dont feel it when I am driving it. I tried putting it in first and going to 1300-1500 rpm and nothing. Its got me stumped. I have half a mind to replace the pulleys, water pump belts, alternator and the belts just to see if that doee fix it. How would I check the rear ujoints?

[pjmoreland]

If you feel the vibration when you aren't driving, then there is no way the vibration is coming from your drive shaft u-joints.

Since the vibration seems to have appeared when you installed the short water pump and pulleys, that is probably the cause of your vibration. I would highly suggest removing all of the belts and the crank shaft pulley and see if the vibration goes away. If it does, then reinstall the crank shaft pulley. If there is still no vibration, then add one belt at a time until you find the accessory/pulley that is causing the vibration. You may also want to try removing the fan (put the bolts back in to hold the pulley to the water pump.

As far as u-joint angles are concerned, the rear u-joint should be angled up the exact same amount as the front u-joint is angled down. A u-joint causes the driveshaft to speed up and slow down two times per revolution, and the more angle you have on the u-joint, the greater the speed difference is. The rear u-joint has to be angled the exact same amount as the front u-joint, but in the opposite direction in order to cancel out the speeding up and slowing down so that your rear axles spin smoothly. I've seen it done where trucks with extreme lifts have the rear axle rotated up so that the rear u-joint has zero angle. This is fine as long as a CV (constant velocity) joint is used on the front end of the drive shaft. Otherwise the rear u-joint will not cancel out the cyclic velocity induced by the front u-joint.

If a truck with coil springs is lowered an extreme amount, the rear axle will end up being rotated down in the front. This requires adding wedges between the axle and the trailing arms to correct the u-joint angle.

You can use an angle measurement level to measure the angle of the transmission output yoke and the differential input yoke. They should be the same, or at least darn close.

[skokie]

I am going with the belt and pulley removal plan tonight. Just talked to a machine shop. Was told to do what you said on this thread and if there is still a vibration to unhook the tranny from the engine and see what happens. I will post back tonight when I get the pulleys pulled off. Man I hope its something simple like the alternator or power steering pulleys HAHHAH

[pjmoreland]

Hope you get it figured out. Vibrations drive me crazy!!

[skokie]

Originally Posted by pjmoreland

Hope you get it figured out. Vibrations drive me crazy!!

Yep me to. Its one of the last major things I am doing in the engine compartment. Its keeping me from moving onto the much needed bosy work

[Brainchild]

Skokie,I have seen bent fans on the alternator cause a small vibration,and sometimes kind of a clicking sound,so you might have a look in that area while you're looking around.Hopefully its not something serious.

[Daubs]

Man, you guys are good! I'd have never thought of half these solutions to the vibration problem. I'm really crabby about rattles, vibrations and such in my trucks. Hope you find a solution...

[skokie]

I turned up the idle to 1300 so it would vibrate and got under the truck. I felt around on the oil pan, tranny pan and dust cover.Vibration is strongest at the torque converter and it matches with the vibration I am feeling on the steering wheel. Is it possible to separte the tranny and flywheel without having to take the tranny off?

[pjmoreland]

You could try removing the three bolts that attach the flex plate to the torque converter. Sometimes these bolts tread right into the torque converter, and sometimes they have nuts on the back. If you have a pair of heavy leather gloves, you should be able to rotate your engine by pushing on the flexplate (removing the spark plugs makes it easier, but it is a pain to remove the spark plugs) in order to gain access to the three bolts. Once the torque converter is separated from the flexplate, you should be able to slide the torque converter back toward the transmission a little bit (I think it should move about 1/2" or so). That will help reduce the chances of the flex plate rubbing on the torque converter. You should be able to start your engine up then, and the torque converter shouldn't spin. The torque converter may want to spin a little because the front center of the torque converter sticks into the back of the crankshaft. You can just hold the torque converter from rotating with your hand while you are down there feeling for vibrations.

If the vibration is still present, make sure you have the correct style of flex plate, as Chris H mentioned. What year is your engine? Are there any weights welded onto the face of the flex plate?

[skokie]

This is the engine.
10066036...350...........2 or 4...Target Master/Goodwrench crate motor, 2-piece rear
seal, "Hecho en Mexico"
I belive it is a 90 or 92.
The flex plate does have what looks like weights welded to it. They are about maybe 3/4 to 1 inch long and 1/4 to 1/3rd or a inch thick.

[pjmoreland]

According to the following link, your engine is a '94 with a 4-bolt main and a two-piece rear main seal:

http://www.classictruckshop.com/castgm_smblock.asp

It seems strange to me that the engine was made after 1986, and it has a two-piece rear main seal. I was under the impression that all 1986 and newer blocks used the one-piece rear main seal. I wonder if there is a chance that your motor has an internally balanced crank shaft, which would need a flexplate without weights on it. It might be good to give your local Chevrolet dealership a call.