Rough Idle / Trans Modulator?
 

I have a 1985 500 SEC, has a little bit of a rough idle, but only when a/c is off, when a/c is on idle smooths out. I posted a question about this, and answers were cleaning the throttle body. Cleaned throttle body, still no change. I saw a thread about the Trans Modulator, my car does go into gear from park a little hard and jerky, not smooth, could the trans modulator be the answer for both the rough idle and the jerky gear change? All cylinders were checked for compression loss, all were perfect, which illiminates bad valves. The rough idle smooths out at around 1200-1300 rpms and then runs like a gem! It seems to be more of an air / vacuum problem, as with the a/c on, you cant even tell the car is running!! I would appreciate any help with this problem. Thanks!

Whether a bad trans modulator affects the idle quality can be easily diagnosed by plugging the vacuum line that feeds the modulator. Cap off the vacuum line....if idle improves, you've found your problem.

There are a number of ways to find vacuum leaks. One is to spray a solvent such as carb cleaner on suspect areas. If idle goes up when you hit a certain area, you've found your leak. Another is to flow propane gas over suspect areas. This method is recommended with the obvious disclaimer of its inherent danger, should you have a stray spark under the hood.

Rough Idle
 

Thanks Kestas!

Would the vacuum problem cause the rough idle with the a/c on as well? or is there some other sort of air regulator or restrictor that comes on when the a/c is turned on? Because, the idle is great with the a/c on!

When the a/c comes on, the computer tells the idle control motor to bleed more air to the intake, thus maintaining the idle speed. I can't comment much further on your questions.

I'm wrestling with the exact same problem on a Mercury product in our family. I have a feeling the problem may lie with dirty injectors in my case. This may also be the case with your car. Your problem may be everpresent, and only manuifests itself during marginal conditions.

Keep checking everything, including the egr system.

Most of the fuel system is brand new, im a little weird when it comes to buying used cars, bought the car and had everything fuel related replaced, injectors, pumps, relay, distributor, filter, had the lines blown, and the fuel delivery is excellent. Have to dive into the vacuum tubes, although took a look for the modulator tube, god help me trying to handle this thing. I'll check all the vacuum tubes and hoses i can for any leaks or brittle or hard hoses. Thanks for your help!

Check for vacuum leaks on all the lines you can find. It doesn't take much to upset the transmission.

The reason it is smoother with A/C on may be that the fuel system is feeding more air/fuel into the engine to compensate for the compressor load, however it is able to cut the air down, whereas at normal idle the airflow is so small it can't, hence the roughness.