I have a 1990 Buick Electra Ultra with an LN3 3.8. I've owned for nearly 4 years 30K miles. Currently has 114K miles. I have struggled with poor fuel economy for over 3 years. 10-13 mpg for over 3 years. Currently is only 9mpg. It smokes from fuel burning at idle. Manifold vacuum is always low. I'm wondering if the ignition is retarded. Starts easy and seems to run OK. Injectors don't leak down. I've replaced a lot of parts but nothing changes.
These have been changed. Rebuilt Harmonic balancer, O2 sensor, copper spark plugs, new plug wires, replaced the failing Magnavox ignition module with Delco (tried 2 different Delco module and coil sets) new knock sensor, new coolant temp sensor, new intake air temp sensor, cleaned the MAF. Tried a used MAF. catalytic converter removed and replaced with test pipe. vehicle driven without the muffler to confirm that the muffler is not plugged.

A scan tool is unable to communicate with the ECU via the diagnostic port. Key switch cycling does not activate a typical flashing light. This vehicle has the electroluminescent dash. When the key is turned on - the transmission gear indicator and the fuel level display alternately flash several times. This occurs any time the key is turned on or when the engine is started. This also occurs when the key is cycled to attempt to read any stored codes. It is always the same sequence of flashes. I have not found any posts describing this occurrence. I have confirmed that all dash lamps are functioning.

I have begun to suspect that there is an issue with the ECU or EPROM.
I can buy a used or rebuilt ECU. If I buy a rebuilt ECU I would need to use the EPROM from the original computer. What is the likelihood that the EPROM needs to be reprogrammed when using either a used ECU or a rebuilt unit?

Do you have any tools. Have you ever checked fuel pressure?
Do you know how to use a DVOM?

Was the catalytic converter clogged before you removed it?

The fuel pressure is fine. 32psi at idle with vacuum connected and 42-43 not connected. Pressure increases if the throttle is blipped. I actually replaced the FPR 2 weeks ago just to make sure. System holds over 20psi overnight. I know how to use a VOM. I do not like digitals. I like my Simpson 260.

The catalytic converter was starting to rattle at idle. Once removed it was found to be clear with no obstruction and very little movement of the honeycomb within the housing.

What did the old spark plugs look like?

Let'* see if we have any codes.
Ground the dataline (M) with koeo.

Operation CHARM: Car repair manuals for everyone.
Home >> Buick >> 1990 >> Electra V6-231 3.8L VIN C SFI >> Repair and Diagnosis >> Powertrain Management >> Computers and Control Systems >> Relays and Modules - Computers and Control Systems >> Engine Control Module >> Testing and Inspection >> Initial Inspection and Diagnostic Overview

Initial Inspection and Diagnostic Overview

ALDL Connector:

Use a "Scan" tool or ground the diagnostic test terminal with the engine not running and the ignition switch in the "ON" position in order to retrieve codes. The "SERVICE ENGINE SOON" light will flash a Code 12 three times and then flash each code stored in memory three times, if the "Scan" tool is not used. Once all Codes are retrieved, a Code 12 will follow. New codes cannot be stored in the ECM once the diagnostics mode is activated. See DIAGNOSTIC CIRCUIT CHECK for a complete system check.
pro multis · About Operation CHARM

Plugs have soot/carbon on the steel shell. Electrodes are darker than they should be. The engine runs better and with improved vacuum if I artificially lean the fuel by adding a few gallons of E-85 to the tank.

Well.....what service engine soon light? It must be over on the left side of the instrument cluster. I like the appearance of the electroluminescent dash but these sure are problematic - and costly to replace. I'll try to find what/where that indicator is and whether a bulb or if it is part of the display.

found it. Right side bulb. Of course my factory electrical systems manual says I need the chassis service manual for trouble shooting it...... I did find a note in the electrical manual that the lamp not functioning can be an issue with the ECU.