1991 Ford Mustang LX 5.0

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. See Affordable Horsepower Upgrades below to bring your 91 5.0 closer to the hp of the 2011 GT.

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

Affordable Horsepower Upgrades

Tuning older 5.0L engines is more involved than modern ones. You have to ensure proper ignition timing and fuel delivery through ECU adjustment to maximize the benefits of additional bolt-on modifications. Performing the work yourself will significantly eliminate most labor costs and increase personal satisfaction with the build.

Changing the rear axle gears to 3.55 or 3.73 is a modification that improves acceleration and throttle response. Remove the front sway bar or unnecessary interior components to reduce weight. Replace stock accessory pulleys like the alternator and water pump with lighter, smaller pulleys will reduce parasitic drag.

Cold Air Intake (CAI): Replaces the restrictive stock airbox with a high-flow intake (e.g., K&N, BBK, or Steeda) to improve air intake efficiency.

• Horsepower Gain: 10–15 hp
• Increases air volume to the engine, improving combustion efficiency. Pair with a high-flow air filter for best results.

Performance Exhaust System (Cat-Back and Headers): Install a cat-back exhaust (e.g., Flowmaster, Magnaflow) to reduce backpressure and long-tube headers to improve exhaust flow.

• Horsepower Gain: 15–25 hp (cat-back: 5–10 hp; headers: 10–15 hp)
• Enhances exhaust scavenging, boosting power and adding a deeper exhaust note. Long-tube headers provide better gains than short-tube headers.

Throttle Body and Intake Manifold Upgrade: Upgrade to a 65mm or 70mm throttle body and an aftermarket intake manifold (e.g., Ford Explorer GT40 or Edelbrock Performer). The Explorer manifold is a budget-friendly option often sourced from junkyards.

• Horsepower Gain: 15–25 hp
• Improves air delivery to the cylinders. The GT40 manifold is a popular, cost-effective swap for 5.0L engines.

Ignition Timing Adjustment: Advance ignition timing by 2–4 degrees (e.g., from stock 10° to 14° BTDC) using 91–93 octane fuel to optimize combustion.

• Horsepower Gain: 5–10 hp
• Optimizes spark timing for better power output. Must be done carefully to avoid engine knock.

Aftermarket Tune (Handheld Tuner or Chip): Use a handheld tuner (e.g., SCT, DiabloSport) or performance chip to recalibrate the ECU for increased air/fuel flow and timing adjustments.

• Horsepower Gain: 15–30 hp (when paired with above mods)
• Maximizes gains from bolt-ons by optimizing air/fuel ratios and ignition timing. Essential for CAI, exhaust, and manifold upgrades