Engine miss fire doesn’t stay running.


10 Year Member
May 8, 2003
Stop trying to cheap out and go the speed density ecu route... Just verify the maf conversion is there like it should be and throw your purse at another a9l ecu
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StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
Dublin GA
Here's some basic information to get you headed in the right direction...

Computer system differences in 86-95 Mustangs.

Revised 15-Jan-2018 to add requirements for larger fuel system components to support large changes in airflow through the engine.

OBD1 comes in Speed Density and Mass Air Flow versions. It differs from OBDII in that diagnostic data cannot be streamed through the diagnostic port in a real time mode. The diagnostic data is stored in volatile memory and dumped on command by an external jumper or code reader connected to the computer’s diagnostic port. Watching the Check Engine Light, an external test light or voltmeter are all that is need to dump the codes on an OBDI system. An OBDI code reader can be used, but it isn’t an absolute necessity.

ODBII is capable of streaming data through the diagnostic port in real time mode. It requires a code reader that handles the OBDII data format. No code reader, no way of knowing what the codes are. The plus is you can watch changes in sensor data as they happen, and use the information to plan changes in the computer's program. OBDI requires a laptop & some specialized hardware to do the same thing.

Both OBDI & OBDII have adaptive learning to accommodate changes in sensor output, so as the sensors and airflow values change, the computer adjusts for them. Mass Air systems have a greater range of adaptive learning than Speed Density.

Speed Density uses Manifold vacuum (MAP), Throttle position (TPS) and RPM, & Air Temperature (ACT) to guess how much air the engine is pulling in. Then it uses all of them plus the O2 and ECT sensors to calculate the air/fuel mixture. It is dependent on steady manifold vacuum and minimal changes in airflow from the stock engine configuration to maintain the proper air/fuel ratio. Change the airflow or vacuum too much and the computer can't compensate for the changes, and does not run well. Forget about putting a supercharger, turbocharger or monster stroker crank in a Speed Density engine, because the stock computer tune won’t handle it. Every time you seriously change the airflow through the engine, you need a new custom burned chip to make the engine run at peak performance.

Mass Air uses a Mass Air Flow meter (MAF) to actually measure how much air is being pulled into the engine. The computer uses this information and inputs from the O2, TPS, ACT, ECT, RPM and Barometric Pressure (Baro) sensors to calculate the proper air/fuel ratio. It is very tolerant of changes in airflow and vacuum and tolerates wild cams, high flowing heads, and changes in displacement with minimal difficulties. Just remember that large changes in airflow require more fuel than the stock fuel system can deliver. At that point, you will need larger injectors and a larger fuel pump to make the engine run like it is supposed to. Larger injectors can be used with either an aftermarket calibrated MAF or a custom dyno tune. This makes it possible to use the stock computer with engine displacements from 302-408 cu in, and make many modifications without a custom dyno tune chip. Put a new intake manifold on your 331 stroker and the computer figures out how much more fuel to deliver without having to have a new chip burned to accommodate the extra airflow.

MASS air conversion instructions from http://www.stangnet.com/tech/maf/massairconversion.html FREE

Revised 2 Aug 2014 to add new links to conversion harness suppliers, diagram for repining computer wiring connector and YouTube video

A9L (5 Speed) computer from junkyard $100-$150
A3M (5 Speed) computer from junkyard $100-$150
A9P (Auto or in a pinch, it will work in a 5 Speed car) computer from junkyard $100-$150
70MM MAF from 94-95 Mustang GT - $40-$70
MASS Air wiring harness kit $30-$85

The whole thing is probably less than $300 using junkyard parts.

A9L computers are 5 speed only
A9P computers are automatic, but will work with a 5 speed.

The conversion harness seems to work well for most folks. It avoids the compatibility problems in using a harness from the junkyard. Simple and cheap, actually less work that swapping the wiring harness.
Conversion Harness kits & parts

Youtube video how to do a Mass Air conversion using a conversion harness kit (somewhat long but useful)
View: http://www.youtube.com/watch?v=mnnTgXkHU9Q

Wire side 60 pin computer wiring harness connector

Computer side 60 pin computer wiring harness connector

If the idea of moving & soldering wires scares you, here's a list of compatible Mass Air wiring harnesses.

Copied from bbunt302
Just for reference, here's a list of all the compatible years:

89 harness should work for 86-89 as long as you're using mass air.
90 harness will only work in a 90. (because of air bags and dual dash connectors)
91 through early 92 harnesses should be compatible (single dash connector, fuel pump relay under driver's seat)

Late 92 through 93 harnesses may be compatible (single dash connector, fuel pump relay under the hood). You may end up running some extra wire for the fuel pump relay. Comments and input from someone who has actually used the 92-93 EFI harness would be nice.

Larger MAF to go with Mass Air conversion:
94-95 Mustang GT MAF - $40-$100. It is 70 MM instead of the stock 55 MM on regular stangs built prior to 94. It uses a slip on duct on the side that goes to the throttle body and a 4 bolt flange on the other. You need a flange adapter to fit the stock slip on air ducting that goes to the air box. Wiring plugs right in with no changes. *1 *2

*1.) Metal flange adapter http://www.kustomz.com/cat3.html Buy the TR70 for $40-$45 depending on if it’s on sale or not. Or spend some time on eBay looking for one that may fit.
Try AutoZone and ask for 81413 - Spectre / 3 in. Aluminum Intake Mass Air Flow Sensor Adapter at $12.00. You may have to order it online.

*2.) MAF & sensor interchange
The 94-95 Mustang 5.0 MAF & sensor is also found on:
1995-94 Mustang 3.8L F2VF-12B579-A2A,
1994-92 Crown Victoria 4.6L F2VF-12B579-A2A,
1995-94 Mustang, Mustang Cobra 5.0L F2VF-12B579-A2A,
1994-92 Town Car 4.6L F2VF-12B579-A2A,
1994-92 Grand Marquis 4.6L F2VF-12B579-A2A,
Evidently the –A1A, -A2A, AA, etc. on the end of the part number is a minor variant that did not change the operating specs. You should be able to ignore it and have everything work good.[/color]