88gt with a 94-95 engine. Wiring issues, amongst other problems

The 302 block will only expand to 308 cubic inches, it is limited to a .040 overbore . To get 331 cu in, you need stroker crank and a .030 overbore.

Truck engines are problematic: there are two firing orders used in trucks. The HO firing order and the non HO firing order.The HO firing order is 1-3-7-2-6-5-4-8, the non HO firing order is 1-5-4-2-6-3-7-8

Determining what firing order you have:
Some trucks evidently use a HO firing order with a low lift cam.
Remove the #1 & #3 spark plugs. Put your finger in #1 spark plug hole. Crank the engine over until you feel compression on #1 cylinder. Slowly turn the engine until the TDC mark and the timing pointer line up. Mark TDC on the balancer with chalk or paint. Put your finger in #3 spark plug hole and crank the engine 90 degrees. You should feel pressure trying to blow past your finger. If you do not feel pressure, repeat the process again. If you feel pressure, it is a HO engine firing order.

No pressure the second time, remove spark plug #5. Put your finger in #1 spark plug hole. Crank the engine over until you feel compression on #1 cylinder. Put your finger in #5 spark plug hole and crank the engine 90 degrees. If you feel pressure now, the engine is not a HO model, no matter what it says on the engine.

Using a small carpenter or machinist square to mark the harmonic balancer off into 90 degree sections may be helpful here.

A 15/16 deep socket & breaker bar or ratchet may be used to turn the engine.

The HO firing order is 1-3-7-2-6-5-4-8.
Non HO firing order is 1-5-4-2-6-3-7-8



The wrong firing order will conflict with the wiring harness and computer. All 5.0 Mustangs used the HO firing order, and a computer programmed to match that firing order. If your 88 Mustang is a 5.0, use the harness from that engine if it is in good condition.

Computer list: Check the code on a tag opposite where the computer connector.
A9L - 89-93 5.0L Mustang 5-spd
A3M - 93 5.0L Mustang 5-spd
A3M1 - 93 5.0L Mustang 5-spd
X3Z - 93 5.0L Cobra 5-spd
A9P - 89-93 5.0L Mustang Auto
A9S - 88-93 5.0L Mustang California
C3W - 89-93 5.0L Mustang Auto
DA1 - 87 5.0L Mustang 5-spd
J4J1 - 94-95 5.0L SVT Cobra 5-spd
T4M0 - 94-95 5.0L GT Vert 5-spd
U4P0 - 94-95 5.0L GT Auto
W4H0 - 94-95 5.0L GT
ZA0 - 95 Cobra R
D9S - Lincoln LSC

Transmission wiring harness:
Originally Posted by 302EFI
The wires for the 02's and low oil did not change throughout the years, they are all in the same place.
The main ones you need to worry about are (on the harness end (ECU) that plugs into the 02 plug) is:
\- 1. Lightblue / yellow
- 2. White / Purple
- 3. Purple / Yellow
The White/Purple & Purple/Yellow gets looped for a automatic ECU
The Purple/Yellow & Lightblue/Yellow for a manual ECU

See Oxygen sensor Harnesses - Manual/Auto differences and year differences - Ford Mustang Forums : Corral.net Mustang Forum for more O2 sensor wiring harness info

Basic premise to use with transmission swaps:
Only run a auto trans O2 harness with an A9P
Only run a 5 speed trans O2 harness with an A9L
90 model year harness only works with 90 model cars without inspection/rework.
The 4 cylinder O2 harness uses 4 wire O2 sensors. It probably won’t work correctly without modifying it.