Shaken Engine Syndrome

I have a 91 3.1L V6 Lumina Sedan (with over 260K miles on it) and I've been fighting this problem for a week now and am scratching my head like crazy trying to figure out why my car is acting the way it is. It all started with a loud clunk noise after a hard start on a really cold morning. The clunking and shaking killed the car but it started right back up without any problems, until I drove back to work the next day and I started getting a shaking engine issue.

In park and Neutral the car idles fine and has a slight vibration (not really worse than before the clunk) and if the gas peddle is pressed it will rev normally as well with only a slight increase in vibration. However, when I put the car in gear (reverse is worse) the engine struggles harder and shakes the whole car. While in gear, idling the lights on the dash, headlights and radio will flicker in time to the shaking. When driven the car is smoother around 5-10mph but shakes until it shifts into 3rd(?) or at about 35mph when it smooths out again (granted it still isn't as smooth as it was before the clunk). The lights are not affected at this time either. The transmission shifts fine from what I can tell. Was low on fluid but I've topped it off. I replaced the plugs thinking some of them were bad and causing misfires but that didn't fix the problem. I did check to make sure the plugs are getting a spark as well. I cleaned up the intake area and checked for vacuum leaks but didn't find any.

What I still need to do is check the fuel injectors but can't think of how to check them outside of the car without special tools and equipment. I do use injector cleaner. I'm also thinking that the shaking might be caused by worn out lower motor mount and transmission mount. Was also wondering if this could be a timing issue. I've never replaced the timing belt on the car and I have no idea if it was ever done before I bought the car.

Is there anything else I should be looking for or at. I've read a lot of threads but nothing else really describes the shaking that I'm getting with the engine. I just need to keep it running and driving until the end of January.

Because I'm thinking come tax refund time it'll be time to find another DD as I've well spent more on keeping this one running than what I've paid for it in the first place, and I don't want to sink even more money into it.

You have a timing chain, not a belt.

At 260k, it's probably shot. Most 3.1s seem to toss the chain around 175-200k, in my experience...

But that may not be your problem....I'd suggest spending $25 for a vacuum gauge, easily the best money you'll ever spend on a tool, and see what the reading tells you.

If something internal went away, you'll be able to determine that quickly. If the reading is solid, you can relax a bit, it's not critical, just gotta determine the cause from there...

BTW my lower mount on the motor is broken right now, and it still idles and runs smooth, maybe a very slight shake at highway speed, but almost undetectable ...I can just feel it moving around on it when it upshifts...gonna take care of that soon.

I don't feel like this is your issue.

Try taking the serp belt off and run it (no more than a couple of minutes though) to rule out a belt driven acc'y that went bad...

Broke down and checked the fuel injectors and this is what I found... Thee clogged injectors and a clogged fuel rail. Lovely.

Here's the carnage... flickr (http://www.flickr.com/photos/gregory_gdp/sets/72157625585181821/)

any good ideas on how to unclog the rail? I'm taking the injectors in to be professionally cleaned and check but don't think they are able to do the fuel rail.

machine shop. have them tank it (the aluminum safe one!)