If the bolts holding the mounting plate are tight, you might have a cracked EGR pipe. If the pipe is cracked, you'll need to replace it.

Also, please add your year and trim of your vehicle to your signature line. That way when you start asking questions in other areas of the forum, we all have a better idea on how to assist you. Click on "Quick Links" above, go down to "User Control Panel" and then click on "Edit your Signature" It'* pretty straight forward from there.

Sounds to me like a leak at the EGR. I had a cracked EGR tube once. Drove me nuts till I found it. Idled rough but ran OK when driving.

Ok, in looking at the throttle body I noticed a very slight gap along one side where the throttle body attaches to the UIM. I'm going to pull off the throttle body to take a look. I'm really hoping it'* not another impending UIM failure.

After further review it'* looking like a possible warped UIM. I've got some more work to do to verify it, but it looks like my 2nd time around on replacing the damn plastic UIM. Any suggestions for good quality replacements. I've seen some talk about the APN UIM'*, but thought I'd ask here.

The APN is supposed to be a good direct replacement. Another option is the L26 upper intake. Radomirthegreat did a swap as have a lot of grand prix and regal folks. It'* not necessarily the average swap, but it'* made of metal.

Just a quick update on where I'm at. Got called out of town for business for a few weeks and ordered the APN UIM. Just got back and took off the old UIM, while inspecting the LIM I found a piece of the LIM gasket that had cracked, so I'm ordering the aluminum frame LIM on Monday. Once I have it back together, I'll let you know the results, but from the symptoms and the issues I found I'm pretty sure this should get me back on the road. I don't see any coolant leaks, just vacuum so in the long run I probably got a little lucky that It wasn't a bigger issue.

Thanks again for everyone'* help with this!

Good luck.

After some major surgery the Bonneville is back on the road and running smooth as silk again! The long and short of it is that when replacing the UIM, I found the LIM gaskets were beyond shot and definitely had a vacuum leak from the gaskets. I replaced the LIM gaskets with the GM aluminum LIM gaskets and also had to replace a weak fuel pump.

While I was in there I did a massive clean up as 170,000+ miles caused some accumulation of the nasty black stuff, so I cleaned the throttle body, the LIM and did some spot cleaning of the valves and engine since everything was apart.

The end result is a Bonne that'* purring like a kitten and is ready for another 170,000

Thanks again for all your assistance. A couple of business trips got in the way of getting the job done, but the work is done and old reliable is reliable once again!