What type of motor mounts were used, solid or rubber?

 

Stock rubber replacement mounts

 

I take it this engine isn't equipped w/balancer shaft? Does it have a stock cam? No prob w/how it runs (no missing, etc.)?

You certainly can try loosening the exhaust system if it's solidly mounted to the chassis. Sometimes something as simple as increasing the idle speed will help a lot. The factory has used "counterweights" off the rear of the tranny to help w/vibration on other vehicles.

 

check the harmonic balancer

 

Apparently the V6 after '89(?) was a lot smoother because GM mounted a counter rotating weight (sort of like a cam shaft) below the intake manifold which took care of a lot of vibration. I rebuilt an '88 and it didn't have this counter weight and the engine runs rough. I've seen an '92 V6 run and it was smoother. Maybe you have an earlier engine. Hope this helps.

 

Because it is a 90 degree V-6, made by chopping two cylinders off a 5.7 V-8, it probably does not fire evenly.
Look at the distributor cap. Does it have two towers close to each other, opposite each other on the cap, and two single towers sitting farther away from the paired towers?

 

Are you sure you have the correct externaly balanced flywheel and harmonic balancer? I had a small block 400 years ago that had the wrong flywheel on it and it would rock the entire vehicle.

 

The flexplate register and bolt circle is different sizes on counter weighted and non-counter weighted flexplates. In other words, if the flexplate bolted to the crankshaft and the ring gear fit the starter gear, it is the correct flexplate.

 

Thanks for all the input guys. I have been snooping around the truck and here's what I found so far: When the engine is at idle, you can put your hand on it and it feels PERFECTLY smooth, yet a the same time, if you put your hand on the fender, you feel a vibration, at a much higher frequency than the idle speed. When you put the truck in gear, you feel the vibration on the engine and more pronounced when you hold your hand on the fender. Also, when you put the truck in gear, there's a "chirp" kind of noise. I also get this noise when the transmission shifts and over some bumps. I'm thinking there's something rubbing somewhere. There's also a vibration in the drivers door. I have traced that to the power lock motor assembly vibrating. As soon as you put your hand on the lock motor, the noise disappears, so that should be an easy one to isolate. I'm guessing that this is going to be a process of elimination type of thing.

And for you guys who asked: The balancer and the flywheel are the same ones that were on the engine when I got it. This is a pre-balance shaft, even fire, roller cam engine. I believe that the balancer and flywheel are correct. The brackets for the engine mounts and the transmission mount are home made and therefore suspect. I also going to take a good look at the exhaust system to see if it's hitting anywhere, there are a couple of tight spots.

 

I feel like a kid on Christmas morning, you know, when you get something you have been wanting for a long time? No, not that, get your minds out of the gutter, I already did that this year.

I found the source of my idle vibration problem!!! As I had mentioned, there was a chirp sound when the truck was placed in gear, every time it shifted and sometimes over bumps. Once in gear, the truck would vibrate...the hood, the fog lights, the steering column, everything. I have been poking around under the truck looking for the chirp, hoping that the chirp and the vibration would be one in the same.

The big problem is that chirps come in all shapes and sizes and it takes a skilled chirp sleuth (which I am not) to out the offending problem. I checked everything I could see and then finally started on things I couldn't see and had to assume were OK.

There is a spot where the passenger side exhaust pipe connects to the manifold that's directly over the right frame rail. You can't see a darn thing in there but I used a ty-rap as a feeler gauge and saw that I had maybe .040-.050" between the pipe and the rail. I had also developed an exhaust leak at that connection. I hauled up on the stud nuts and since, even with a mirror, I couldn't actually see the clearance, I decided to make more room by shimming the rear transmission mount 3/8 of an inch.

I started the truck and put it in gear and voila! It idles pretty much the same in gear as it does in park! No more vibration

I'm still going to try isolating by exhaust system with some new style rubber exhaust hangers to further isolate stuff from the body and frame but so far, so good. The truck is becoming more of a pleasure to drive every day. I still have a rattle in the glove box door to fix, a squeak that I think is associated with the hood latch and somewhat of a clunk on severe bumps that I have to chase down but this was the biggest bullseye I have hit yet!