1991 Ford Mustang LX 5.0

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Engine

The 1991 Ford Mustang LX Convertible had two engine options:

  • 2.3L Inline-4: Displacement: 2.3 liters (140 cubic inches) Horsepower: ~105 hp (upgraded to ~105 hp due to a new cylinder head and dual spark plugs, not 88 hp like previous years)
  • 5.0L V8:Displacement: 5.0 liters (302 cubic inches) Horsepower: ~225 hp (High Output, or "HO" version only)

For the 1991 Mustang LX, only the "High Output" (HO) 5.0L V8 was offered. There was no non-HO 5.0L V8 option for this model year, unlike some earlier years where a lower-output 5.0L existed.

Sources:

  • Ford Mustang production data and specifications from "Mustang: Fifty Years of an Icon" by Donald Farr.
  • 1991 Ford Mustang brochure, accessible via auto-brochures.com.
  • Online reference: https://www.mustangspecs.com/1991-mustang/

1991 Ford Mustang LX Convertible 5.0L V8 Engine detail:

  • Engine Type: V8, naturally aspirated
  • Ford Windsor V-8 302 engine Displacement: 5.0 liters
  • Horsepower: 225 hp @ 4,200 RPM
  • Torque: 300 lb-ft @ 3,200 RPM
  • Configuration: Overhead Valve (OHV), pushrod
  • Fuel System: Sequential Electronic Fuel Injection (SEFI)
  • Compression Ratio: 9.0:1
  • Bore x Stroke: 4.00 in x 3.00 in (101.6 mm x 76.2 mm)

note: The 2011 Mustang GT generates significantly more power with 418 horsepower from its 5.0-liter V8 engine. This represents a substantial increase in performance offering nearly double the horsepower. Additionally, the 2011 model produces 389 lb-ft of torque compared to the 1991 model's 300 lb-ft.

Sources

ECM

KOEO Test

Key On Engine Off Test. Checking codes is one of the most important diagnostic procedures that you can do on your own for your Mustang.

Using The Check-Engine Light this Mustang will allow you to use a jump wire and count the check-engine light in the dash. The jump wire (I’d use spades on each end) would connect the STI (test input) with #2 pin on the connector.

The fox Mustang’s EEC IV computer has two connectors for self-testing and located on the driver-side firewall. The large connector contains the self test output (STO) and ground (SIG RTN). The small pigtail is the self test input (STI).

Engine Stalling or Misfiring Erratic engine behavior may also indicate a bad or failing ECM. A faulty computer may cause the vehicle to intermittently stall or misfire. The symptoms might come and go.

Mustangs VIP Connector Location
located behind the driver side strut tower

This image courtesy of dfwmustangs.net where there is a forum post that goes into far greater detail and covers more than one year of Fox Body Mustang.

  • 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.
  • Turn the ignition to the ON position (engine off) to run the Key On Engine Off (KOEO) test.
  • Observe the Check Engine Light (CEL) flashes or use an analog voltmeter to read the codes by counting needle sweeps.
  • 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.
  • 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.

Brakes

Drum Brakes

See: Drum Brakes on a Ford Mustang

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