Talk:1991 Ford Mustang LX 5.0

statistics

98,737 total Mustangs were built in 1991, with 19,447 notchbacks and 57,777 fastbacks (hatchbacks) sold with all engine types.

Below is a breakdown of the 1991 Ford Mustang LX Convertible and LX Notchback (sedan/coupe) production numbers, categorized by engine type (2.3L Inline-4 and 5.0L V8). The data is sourced from reliable references specific to Fox-body Mustang production.

1991 Ford Mustang LX Production Numbers by Body Style and Engine Type

LX Convertible (P44):2.3L Inline-4: 6,391 units

• 5.0L V8: 7,850 units
• Total Convertibles: 14,241 units

LX Notchback (P41, Sedan/Coupe):2.3L Inline-4: 12,218 units

• 5.0L V8: 7,229 units (including 385 Special Service vehicles)
• Total Notchbacks: 19,447 units

Notes

The 2.3L Inline-4 was rated at 105 hp in 1991 due to a revised cylinder head with dual spark plugs. The 5.0L V8 was rated at 225 hp (High Output version only). Production numbers for the LX Convertible include the “Summer Special” Limited Edition (2,019 Vibrant Red units with white wheels and tops), all equipped with the 5.0L V8. Notchback figures include Special Service vehicles (used by law enforcement), which were exclusively 5.0L V8 models. Total 1991 Mustang production (all models, including GT and hatchbacks) was 98,737 units, with notchbacks being the least common body style.

ECM KOEO Test

You can test the ECM (EEC-IV) in your 1991 Ford Mustang LX affordably at home using the following method:Manual Code Reading with a Paperclip:Locate the self-test connectors under the hood, near the driver’s side fender or firewall.

1. Use a paperclip or 3-inch wire with spade terminals to jumper the STI (Self-Test Input) to the SIG RTN (Signal Return) pin on the diagnostic connector.
2. Turn the ignition to the ON position (engine off) to run the Key On Engine Off (KOEO) test.
3. Observe the Check Engine Light (CEL) flashes or use an analog voltmeter to read the codes by counting needle sweeps.
4. Record the trouble codes and compare them to a Ford OBD-I code list (available online or in a service manual) to identify ECM-related issues.
5. Optionally, perform a Key On Engine Running (KOER) test after warming the engine to check dynamic performance and a Cylinder Balance Test to detect misfires.

• Tools Needed:Paperclip or short wire (free or ~$1).
• Pen and paper to note codes.
• Optional: Analog voltmeter (~$10-$20 at hardware stores).

Visual Inspection:Check the ECM (located under the passenger-side kick panel) for physical damage, burnt components, or leaking capacitors.

Inspect wiring harness and connectors for corrosion or loose connections.

Common ECM Issues:Look for codes like 31 (EGR issues), 67 (neutral safety switch), or 95/96 (fuel pump circuit issues), which may indicate ECM problems or related system faults.

A continuously running fuel pump may suggest a faulty transistor in the ECM.

This method is nearly free, requiring minimal tools, and avoids expensive dealer testing. If no codes or obvious damage are found but issues persist, the ECM may need professional testing, but this is a solid first step.