81 GP, 3.8L V6, rough at start, smooth(er) when warm

brrian

Greasemonkey
Jul 7, 2022
113
28
Pittsburgh, PA
My 81 GP has the 3.8L V6. I've owned it for almost two weeks, but covid has kept me from playing with it for much of that time. The day I bought it I drove it home, about 3 miles. It runs rough, almost like a cylinder is misfiring. I haven't had it running for any length of time, but the previous owner says it always started rough but once it warmed up it started and ran smooth. The day I looked at it, he drove it a good bit... we bounced around from bank to bank trying to work out payment. If it ran rough that day, I would've noticed it.

The smog pump is disconnected (belt removed, pump frozen). A hose from the vapor canister is disconnected. My initial feeling is that the roughness has something to do with this system being disconnected. I'll check other obvious stuff (plugs, wires, etc.) but I feel like the 'running smooth when warm' thing is a clue that it has something to do with the emission system.

Probably not related but worth mentioning - belts squeal on start, then quiet after a few seconds. I read about a missing belt causing that, but not on this engine (right)? The belts are shiny & might just need replaced.

& for what it's worth - my plan is to get the V6 running right so I can drive it when I want, and I'd rather get the emissions system working right than to bypass it. Eventually I'll swap something else in... possibly a Buick 350 because of how easy it is.

Thoughts appreciated. Thanks!

engine.jpg
 

CopperNick

Royal Smart Person
Supporting Member
Feb 20, 2018
2,407
113
Canada
That big old black canister is part of the smog-opoly mess that got inflicted on drivers back in the 80's. If you remove the two screws that hold it to the inner fender and flip it over you will discover that it has a filter pad that is removable and replaceable. The whole business is actually a evaporative canister that traps and filters fumes from places like the gas tank, yeah, if you check the names on the various ports on the top you should find one marked "gas tank" Track it back to the tank and it should be attached to the vent spigot on the fuel sending unit, or to a separate vent tube attached to the tank itself. it's all about scrubbing the fumes to make them clean and happy before they are discharged back out into the environment.

Tracing it back to the canister should tell you what that disconnected tube used to do for a living and give you a hint of why it was corked.

The A.I.R. Pump will either shake or. as you commented, be frozen. Pulled one off another motor once upon a time and it rattled like a maraca. All the little wiper blades inside it had turned to shrapnel. Finding another one that actually works might be problematic, but that is why this board exists. Someone hanging out on one or more of the threads around here could easily happen to have a crusty lump AIR unit that still works. (The big question here would be WHY???) or know of a yard or source that carries them.

I'm not so personally sure that having that pump disabled is going to be a factor in your rough idle. Being short two cylinders and dealing with a different firing order sequence may also be playing a part in it all.



Nick
 

brrian

Greasemonkey
Thread starter
Jul 7, 2022
113
28
Pittsburgh, PA
Dumb, dumb, dumb. I went out this morning to poke around a little & saw a spark plug with no wire :| Connected it and go figure, the problem went away. I wish I would've waited to post until I had a chance to poke around under there.

It's still idling a little rough but it's drivable. I'll work on that, and figure out what's disconnected & work on that. I ordered the old service manual today & that'll help.

Thanks for the reply!
 
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