1996 cadillac sedan deville, when start up the idel is very high 2500 rpms and goes up to 3100 rpms, what could cause this?

Some of them has out side,most is inside.Big job,big money.

Unfortunately, it sounds like your HVAC control module is failing. You can try a reset by removing the negative battery lead (under the rear seat) for about 10 minutes, and then reconnecting it, but if that doesn't work, you will probably have to replace the module.

If when you push auto it goes to econ that normally means you are low on freon.

If the a/c does not engage as a result of the electronic control system in your 1996 Cadillac, the climate control system may need to be replaced.

However, if the problem is being caused by low charge of R134a in the system, have the system charged before changing that electronic part.

I have never seen a Cadillac Climate Control System do this, which leads me to believe that it may be more electronic than refrigerant pressure related.

If the refrigerant pressure was low in the system, the climate control system should still stay on 'auto' mode, I think, and simply blow warm air.

located underneath the intake manifold remove it and disconnect the knock sensor unscrew the knock sensor. installation is reverse

I would suggest checking your Oxygen sensor which will light up your service emission light. Check that and let me know Good luck

The lines should be differant sizes.. Did you check it? Ther should be a diagram on the cover over your radiator showing you the set up.. Hope this helps..

HAVE THE SYSTEM TESTED BY THE DEALER, THERE SHOULD BE A FAULT CODE. BUT THE MOST LIKELY CAUSE IS THE AMBIENT AIR TEMP SENSOR.

Check the following

1. Throttle position sensor
2. Mass air flow sensor
3. EGR valve
4. Replace the plugs and wires

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Firing order 1-2-7-3-4-5-6-8 ..............1-3-5-7 back of engine near fire wall...........2-4-6-8 front of near radiator