Talk:1991 Ford Mustang LX 5.0

smog pump

The Ford E8ZE-BA smog pump is an original equipment manufacturer (OEM) component designed for various Ford vehicles, primarily the 1982-1993 Ford Mustang 5.0L V8 (302ci) models, including the Fox Body generation. It is a belt-driven air injection pump used as part of the vehicle's emissions control system. This specific pump is also compatible with other Ford models such as the Bronco, Thunderbird, and certain Ford trucks from the same era.

Many owners have successfully bypassed or removed the smog pump, but there are important considerations regarding belt sizing and potential engine performance issues.

Owners of 1991 Mustang LX 5.0 V8s have reported successfully bypassing the smog pump by using a shorter serpentine belt to reroute around the pump, often without needing an idler pulley. This eliminates the parasitic drag caused by the smog pump, which is estimated to sap around 1-2 horsepower. Some owners also completely remove the pump and associated plumbing to reduce weight (approximately 10-20 lbs) and simplify the engine bay.

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.

Doesn't Always Show Faulty ECM

OBD-I self-tests (KOEO, KOER, Cylinder Balance) may not always catch a faulty ECM, especially if the issue is intermittent or doesn't trigger a diagnostic trouble code (DTC). These tests primarily detect issues in sensors, actuators, or circuits the ECM monitors, not internal ECM failures like logic errors or degraded components. However, you can still try affordable methods to narrow it down.

Enhanced Code Reading: Use an affordable OBD-I code reader (~$30-$50 on eBay or Amazon) instead of a paperclip for more reliable code retrieval. This ensures you’re not missing faint or complex codes. Check for "soft codes" (stored but not active) by clearing codes after the KOEO test (disconnect battery for 5 minutes) and retesting after driving.

Swap Test with a Known Good ECM:Source a used ECM for your Mustang’s specific engine (2.3L or 5.0L) and calibration code (check the ECM’s label, e.g., A9L for 5.0L manual) from a junkyard or online (~$20-$50). Swap it in (15-20 minutes, passenger-side kick panel) and test if symptoms (e.g., rough idle, no start, or misfires) resolve. Ensure the replacement matches your vehicle’s specs to avoid false results.

Multimeter Testing: Use a digital multimeter (~$15-$25) to check ECM pin voltages at the harness connector against a wiring diagram (available in a Haynes/Chilton manual or online forums like StangNet). Test power, ground, and reference voltage (5V) pins to confirm the ECM is receiving proper inputs. Abnormal readings may indicate internal ECM issues or wiring faults.

Visual and Environmental Checks: Inspect the ECM board for burnt traces, cracked solder joints, or capacitor leaks (common in early '90s Ford ECMs). A magnifying glass helps. Check for water intrusion or heat damage, as the ECM’s location is prone to environmental wear.

If these steps don’t pinpoint the issue and symptoms persist (e.g., random stalling, no spark, or fuel pump issues without codes), the ECM may have an internal fault not detectable without specialized equipment. At that point, you’re limited to:Borrowing/renting a breakout box (~$100 rental, rare for OBD-I) to test ECM signals. Replacing the ECM with a remanufactured unit (~$100-$150 from AutoZone or RockAuto) as a diagnostic step.