Could a idle air bypass valve cause my cadilliac to die when I let off the gas

Those aren't fuel pump symptoms. Check the engine coolant temp sensor, clean the throttle body and idle air bypass valve.

Replace your Idle Air Control Valve (sometimes called Idle Air Bypass Valve).

Do the following

Here are the most common causes of idle surge, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake

This engine has automatic idle speed control, the computer controls a "air bypass" valve on the side of the throttle body where the air intake hose from the air cleaner is connected, the valve and the throttle butterfly get "sludged" up and this causes all kinds of idle problems and off idle stall, see full list below and fix for the problems

Here is the common cause of surges, cold stalls, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours.

To correct this problem Buy a can of throttle valve cleaner (it is made by CRC) from the local auto parts store, NEVER USE CARB CLEANING SPRAY, IT DESTROYS THE ANTI-STICK TEFLON COATING, spray it into the air intake through the throttle body of the engine until you have used about half the can, you will need to hold the idle up while you do this, it will try to stall, after this shut the engine down and disconnect the Negative (-) battery cable for 5 minutes. The intake is now clean, if you continue to have the idle speed control problems you may need to seek the help of a professional.

Check the idle air control valve its right on the front of the throttle body it may be stuck closed and not allowing air to bypass the throttle plate to allow the engine to idle, do you have a check engine light on at all, there could be a number of sensors that could have a problem not allowing the engine to idle when the throttle plate is closed like the idle air control valve, throttle position sensor......

There are couple of things I would check out that would cause this problem.

1. The Mass Air Flow Sensor. It measures how much air is going into the engine. Should be located on top of air filter box.

2. The air tube that goes from the Mass Air Flow Sensor to the throttle body. Check for leaks that would bypass air from going through the Mass Air Flow Sensor.

3. The Throttle Position Sensor. Located on the throttle body. This tells the computer what position the butterfly plate is on the throttle body. You control this plate with the gas pedal..

I would start by looking for a vacuum leak. You may even be able to hear the sucking sound if it is large enough, an assistant will be helpful in keeping it runnig while you look. An idle air bypass valve is also another likely culprit. But first verify no vacuum leak or intake air leak. Mass Air Flow sensor could cause it too, but is usually associated with a running issue off of idle as well.
Good luck

being a certified mechanic has its advantages. there are multiple causes for that problem: lack of air or lack of fuel.
make sure the air filter is mostly clean.
check throttle plate (where air filter tube goes into motor), most likely black with soot and carbon. clean with tooth brush and gas, wipe off with clean rag. repeat ultil CLEAN.
low air flow past plate when closed (idle) will cause stall condition.
dirty throttle body is a common problem with lots of cars. causes engine stall.

dirty fuel filter will also cause this problem.. low fuel pressure at idle.

I also believe that your car is equipped with an idle air control valve. (regulates a bypassed air flow to motor at idle) this valve may be stopped up or non-functional. have local shop take a look, but there are ways to check valve if you consider yourself mechanically inclined

Sounds like it's your IAC (idel air control sensor).  The IAC is a computer-controlled air bypass valve located in the throttle body. It helps control idle speeds by allowing air to bypass the throttle plates as to increase the idle speed.  If the IAC isn't working properly the car will stall when you are not pressing the gas pedal.  It is located on your throttle body and is relatively cheap and a simple change since it is normally held in my two screws.  Let me know if you have any other questions.  Thanks for an honest ranking and for using FixYa!

dirty intake throttle valve and air bypass idle speed control motor. ttry the following.

Here is the most common cause of surges and stalls at stops and low erratic idle speed and rough idle, it is the idle speed control air-bypass valve and throttle valve (IAC for short), they get full of gunk over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.