My Maytag LDG8200AAW does not stop automatically and does not heat up to dry. I have cleaned the vents.

If your dryer does not heat at all, it will not advance the timer to off. Once the no heat issue is resolved, the timer should function normally.
To diagnose the No Heat issue, I have provided some technical assistance, courtesy of www.repairclinic.com.

Firstly, ascertain that you have made a positive heat selection on the control panel switch.

The links below show images of heat controlling components.

1. Igniter Image:
http://www.repairclinic.com/SSPartDetail.aspx?s=t-ldg8200aaw-%3d%3di3018&PartID=3018

2. Gas Valve Coils Image:
http://www.repairclinic.com/SSPartDetail.aspx?s=t-ldg8200aaw-%3d%3di3479&PartID=3479

3. Thermal Fuse Image:
http://www.repairclinic.com/SSPartDetail.aspx?s=t-ldg8200aaw-%3d%3di483890&PartID=483890


Parts breakdown/disassembly procedures.
http://www.repairclinic.com/Dryer-Gas-Maytag-Style-Appliance-Diagram


If your dryer doesn't heat, check these:

Igniter
Gas valve coils
Thermal fuse
Igniter Modern gas dryers use an electric igniter to ignite the gas from the gas valve. When it's working properly, the igniter glows bright orange. When it burns out, the dryer tumbles but there's no heat because the gas can't ignite. When the igniter burns out, you need to replace it. If the igniter is held by a tension bracket, you very well may need to replace the bracket too.

The igniter is inside the dryer housing, near the bottom front, usually in a cone-shaped metal tube (the force cone). It's about 2 inches long. It's mounted to the far end of the burner tube, and it has two wires attached to it--or to the tension bracket, if there is one.

Gas valve coils Watch the igniter. Does it glow bright orange, then shut off without igniting the gas? (When the gas ignites there's a large blue flame.) If so, there may be defective coils on the gas valve. Mounted on the top of modern gas valves, there are black electrical coils. The coils, when energized, open the gas valve. If one or more of the coils are defective, the valve doesn't open and the gas cannot ignite. Because it's often difficult to properly test the coils, it's usually best to replace both (all) of them at the same time.

Thermal fuse On many dryers, there's a thermal fuse (a heat-sensitive fuse that blows if the dryer overheats) mounted to the exhaust duct inside the back cover panel. The fuse is about an inch long. It's usually embedded in black resin and mounted in a white plastic housing.

If the fuse has blown, it has no continuity. When this happens, your dryer either just stops heating, or it doesn't work at all. Be sure to inspect the venting/heating system before replacing the fuse to put the dryer back into operation. (You can't re-set this type of fuse.)
The tips listed above are for the most common failure scenario If none of the above is helpful, you may continue your search through the repairclinic site to find less common parts that may fail, or check back with me more assistance.

HI, this will occur if the timer device has become unstable. Have this timer device replaced asap. it is damaged.