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It cleared up a rear main leak on my '92 Z-28. Did great for revitalizing the seals.
There are so many things that can cause rough idle, can you give more information? Did the car recently overheat? Is there any fluids leaking on the engine? Are you getting any smoke or steam from the exhaust? When checking for steam, remember to wait until the engine has run for awhile as condensation can build up in your exhaust during the night. If you cannot tell if it is smoke or steam, hold a blank sheet of paper behind the tail pipe, if it is oil smoke, then oil dots will show up on the paper. Some things you can do and check; Check the spark plug wires, make sure they are tightly connected and not chaffed, broken or burnt. Check your PCV valve for excessive oil or clogging, (it will be in a rubber housing or hose leading from your valve cover to your throttle body/carburetor) I don't know the engine setup in that model, but if you have a distributor cap, check it for cracks and wear, also check the rotary cap. Check all rubber vacuum hoses on the engine for cracking. If you can afford it, change the spark plugs, plug wires, rotor and rotary cap, these parts all need to be changed over time and your car is getting older now.
First, go through the book and see if some option in one of the menus was set for this, it may be a security feature or something. If all your settings are as they should be, Go to the Chevy house in Marshall, in the service department, ask to speak with Roger . If there is a quick answer, he knows it (just don't tell him someone is sending him personal questioners from the net please), otherwise, turn it in and let them fix it, especially if it is still under warranty.
Do the checks that munkyrench said, but Texas mandated ethanol in the gas about 2 years ago, that could be part of it. Also, run some injector cleaner through it and some valve train lubricants. Check that the Air Filter is staying clean and check the pcv valve, if it is full of oil and gunk, it may need replacing also. Remove the air inlet hose and check your Throttle body area, use some carb cleaner to clean it up if it is dirty.
Knocking is normally due to low octane and experienced when accelerating or climbing hills or anytime the engine is under a load. Arizona being one of the "Greener" states uses oxygenation of gasoline and other "green" technologies which can reduce the real octane rating of your gas. Some stations now use nitrogenization as cleaning method, avoid it, as it is really only a filler giving you less gas for your money. As for cleaning, run a bottle of STP Full System Cleaner through every 6-10 thousand miles, not any more often as the naptha content can cause damage. After running the cleaner, run a bottle of Lucas Gas Treatment or Marvel Mystery Oil, this helps lube and condition seals. The Lucas and Marvel can be run more often. Run higher octane gas or put an octane boost in, using a booster is a little less accurate for measurements so skip every few tanks especially if you refill before empty. Lucas also claims, although I have no proof of it, to raise octane and to be safe every tank, but can be more expensive than a cheaper booster. So, Buy good gas, keep octane higher and clean regularly, I have done this for years and have never had a knock or had bad seals or dirty injectors.