Toyota Repair: 92 Corolla Check Engine error 71, meg ohms, egr valve

Question
It is a 92 Corolla 1.6L vin code 6 california car with 248,000 Miles and runs perfect but with and intermitent error 71, slowly getting worse for over 2 years. I had one smog check that had high NOx reading 2 years ago. All other have been very low. I have the factory manual and have done some trouble shooting. At the connector for the temperature sensor with the sensor unplugged the ECM is supplying 5 volts. Also the ground is also good. With the sensor plugged in and the key on engine cold messure about 4.6 volts. At 75 degrees messured the resistence of the sensor and it was 160 meg ohms. Ran car in driveway for about 15 minutes and messured again and was down to 70 meg ohms.

I removed the EGR and cleaned out the the port acrossed to the other side of the engine with some stiff wire. Light vaccum applied to EGR it self and port opens. Port is clean with no plugs.
One thing my car has is a cracked exhaust manifold.

Could the crack affect back pressure enough to cause modulator not to work?

Can you tell me the resistence reading for different temperatures?

What is most common cause and can it be cleaned or does it need to be replaced?

Thanks

Answer
The most common EGR malfunctions are due to a clogged passage in the manifold intake chamber, which you have already cleaned, the other ones are a leaking egr modulator vlave, remove it and see if air can pass through from the bottom port, if so, the modulator is defective, another cause could be a clogged backpressure port on the EGR valve or a malfunctioning egr VSV.
Whether or not the backpressure could be affected by the cracked manifold depends on how bad it is leaking, so the answer would be yes, it is possible.
Check the system operation with a vacuum gauge, there should be vacuum at the R port of the modulator when the engine speed is increased. It is very uncommon for the temp sensor to go bad.