Cause of sudden very rough idle?
 

Just got back from a road trip: Denver--Flagstaff--LA--Denver in the 85 TD. Ran very well most of the trip. On the way back today got a very rough idle. I'd climbed eastbound to the Eisenhower tunnel in 3rd gear 62mph, about 4000rpms. Ran fine. Coasted downhill the other side for about 10 miles, rpm's at about 2000, no pressure on accelerator. Ran into a traffic jam and slowed down. The idle was horrible. Shook the whole car. Had to keep the rpm's at about 1100 to smooth it out. Stuck in the downhill traffic jam for about 10 miles, idle still bad. Climbed Floyd Hill under full power just fine. Coasted down to Mile High and drove at about 65mph about 10 miles home. Idle is now fine.
What caused that very bad idle? Some dirt in an injector(s) that blew out once I powered up again? Your thoughts.

A clogged injector sounds like a reasonable guess, I don't know what else would fix its self like that.

A piece of carbon broke away and got lodged in the valve seat? It might have eventually broke up and got blown out perhaps. If it never comes back in the next little while it was probably just a transient condition. Shaking that bad could have been a cylinder down for a time. Time will tell one way or another. Edit. I just found out it was very cold at the time in later posts. An additive in the fuel may be indicated if it was. Could have been a little water /ice in there or perhaps the fuel was starting to gel a little. Of course there are many possibilities but the cold conditions if extreme reduces them somewhat. You fellows live in the semi tropics I thought.:D Lately all we hear is it snows in colorado all the time. Must be the snow we are not getting this winter as weather travels west to east.

Not to change the subject Kerry, but I notice you have a 74' Custom Cruiser wagon. I may be wrong, but I think we talked about it before.... if so, disregard this post....

Do you have any photographs of it?

I have its "cousin". 73' Grand Ville - based on the same design, but mines not a wagon. :( I wish it was. :)

I don't have any pictures of it right now. I'm thinking of selling it because I don't use it much anymore, although it's my heavyweight towing vehicle.

It's pretty freaking cold around Eisenhower, I bet the engine just cooled down enough that it had to "reheat" the combustion chambers.

The temperature gauge was way down low on the scale, near 40 degrees but I hadn't considered that it was low enough to cause that problem.

Too bad you do not live closer, or I would have to buy it from you. :)

Heavyweight is right- tows heavy and is heavy.:D

You probably already know, the 1971-76 GM full-size wagons are one of the heaviest production cars ever made. In my research of these cars- I had discovered that the 1974 Oldsmobile Custom Cruiser was the heaviest of the lot- especially with the 3rd seat option. Does yours have that and the woodgrain option?

Yes, it has the third seat and the woodgrain option. Seats nine with ease. Even has the original fully functioning 8-Track. I live on the north side of the street so the woodgrain on the driver's side took a beating from Colorado's uv light. The other side is still like new. I replaced the original with new woodgrain from JC Whitney. What a mistake! It lasted about 6 months before shrinking, cracking, and peeling off.