Engine "shuddering" when stopped at lights? '90 Celica

In the 1970s I had a car (GM/Vauxhall "Chevette" -- European car) which did much as you describe. Fault proved to be a perforation in the diaphragm of the carburettor float chamber. How modern is your car?

Or how about dirty spark plugs? Plugs once were such a source of fun. (Modern ignition systems seem to have spoiled all of that.)

That happened with my Nissan because a bad spark plug wire occasionally arced to the sharp edge of a piece of metal. There were no visible cracks in the insulation, and I found the defect through the old method of running the engine in dim light and spraying water over the ignition system and looking for sparkles.

Another possibility is the idle air control valve, which routes air around the throttle plate to regulate the idle speed, or its wiring, but sometimes it's not the valve itself but a dirty throttle plate and throttle body. Cleaning it with throttle body spray, rags, and a toothbrush (not wire brush!) can help, and I was told that the difference between throttle body and carburetor choke spray is that the former is made to not damage any anti-stick coating.

There is a good change that whatever is causing the shudders is the culprit.

If you are wondering what to check, try the following:

- Turn on the AC and see if engine idle speed increases by 100 ~ 200 RPM. If the AC is already on, try turning it off and see if it clears up.

- Check to make sure the air filter is not clogged and the air filter housing is tight.

- Check the plastic snorkel between the air filter and throttle body for cracks and holes.

- Check for a vacuum leak with a vacuum gauge. There should be at least 14 to 16 in Hg at idle.

change the plugs every 30 or 60k. either NGK or ND, only.