Hi guys, Since I've owned the 1980 Buick LeSabre it has always had a rocking, almost uneven engine idle when it is up to operating temperature. I did get this a lot better as the vaccum hose from the distributor was split, so it is not as apparent, but there is still an underlying rock. I've done the usual service things and it is timed correctly. The cap and rotor look good as do the leads and all spark plugs fire straight away, so I don't think it's electrical. The car does not have a cat either but does have aftermarket pipes from the headers. I've been looking at the various mess of vaccum pipes of which this is a completely standard unmolested example vehicle that was not used from 1990 to 2016, and i've replaced all the various tubes I see fit. But I had a simple idea and that was to remove them all completely, what is the reality that my problem would be solved by this apparent (although no proof) vaccum leak somewhere in the system? Also, i'd be intruiged by what degree on mileage this would have, I'm expecting little if anything- but clarification would be nice
Try putting a vacuum gauge on it. It may be carb . Carb throttle shafts wear and can leak vacuum, or it may be an adjustment. . You want a steady vacuum. If it's fluctuating a lot or is down around 16 you may have a problem. is it the stock camshaft?
After the carb, check the EGR valve, if it is still so equipped. A little carbon in the valve can cause exhaust gases to enter the intake manifold at idle. Also, if enough stuff is causing trouble, and the idle is raised too high, the EGR can open. That's rare, but given that this car has sat a long time, it's possible. You will have to remove the EGR valve to check if it is stuck, so be prepared with a new gasket. If it is bad, I'd recommend replacement Vs a block-off plate, since the rest of the controls are calibrated for it. Also, the dilution of the incoming charge to the cylinders cools it, (the purpose of the EGR valve is to lower combustion temperatures, thus lowering emission of Oxides of Nitrogen) and your engine may start pinging if you do leave it off.
All thoughts so far are possibilities. Headers can contribute to lean conditions as well, if you are already ...leaning that way (sorry). An issue with complete removal is that the distributor's mechanical and vacuum advance both can be a part of the emission device strategy. The mechanical and vacuum advance curves both need to be looked at and rethought if need be. There is a basic and easy to read primer on ignition curves here on the forum.
That's a good point the headers leaning out an already lean carb. And add ethanol diluted fuels . And a possible carb that has sat for years.
As far as mileage increases... If you have a good condition engine and remove said items, recurve the distributor as needed with proper vacuum advance, there's usually a noticeable mileage increase.
