What does an e48 code mean on 1990 cadillac

EGR Valve issue (Exhaust Gas Recirculation)

https://www.google.com/search?q=1992+Cadillac+DeVille+Sedan.+error+code+E48

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Use Google & type--
1992 Cadillac OBD1 Codes
Lots of info their on your cars system & gm codes
OBD1 Codes E48,F11,F30 type that in search engine
google,bing etc

Better check the code with code scanner. Free from shcucks or auto zone.

Trouble Code #26 means "Idle Speed Control (ISC) motor/throttle switch circuit fault (short)

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Hello williamcole7: My name is Roger and I will help you. The code 888 is a test code. This tests the L e d's of the dash cluster. All segments must light so you can read all numbers. The 70 code means the system is ready and able to function. The ff code is a current BCM code and is happening at the moment. The f 40 code is a BCM history code and means it was a stored problem or something that has happened. The f 40 code is for a air mix door problem. THe BCM would set this if you had an A/C problem. The E 48 code deals with the EGR system fault. I would check for a vacuum leak at the EGR solenoid. The hose is prone to deterioration do to heat from the engine. Code E52 deal with loss of voltage. A dead battery or a disconnected battery cable will always set this code. None of these codes will keep the car from running.The repair schematics are to extensive to send in a reply. Should you need further help ao assistance I will be glad to help. Please rate the answer. Thank You for using Fix Ya. Roger

E20 means open fuel pump circuit could be failed fuel pump relay

What codes do you have??? Here is a link for you to retrieve your DTC codes that may indicate what your car's issue is for you to address.
Thank you for using fixya and good luck.
http://www.troublecodes.net/GM/

GOOGLE found THIS - EGR valve issue.
Good luck,
TD

timing should help the idle issue. and the vss probly needs to be replaced.check the egr diaphram its gold and is underneeth the air breather. push up on the bottom of diamphram it should move without alot of efort. if not replace it