5.0L HO Mass Air Conversion

For Converted or Transplanted 5.0L HO Speed Density Cougars/Thunderbirds ONLY

Page Revised: 10 May 2019


NOTE: This section is for converted or transplanted 1987-88 HO 5.0 motors ONLY. It is not intended for the stock Cougar/Thunderbird 5.0 motor. Any attempt to convert a stock non-HO motor to mass air can result in serious damage to the engine.

Article by
Eric Dess

The stock '86-'88 HO engine from a Mustang has a speed-density computer setup. It's good for inferring relative air intake, but it's inflexible as far as adding high performance components, such as a healthier camshaft. The best you can do with speed density is a bigger throttle body and a K&N air filter, some exhaust work...and that's about it. Ford began using the mass air system on 5.0 Mustangs beginning in 1988 (California only), and then it was added to all V8 Mustangs from 1989-1995, as well as HO Thunderbirds/Cougars from 1991-93.

Mass air is a more accurate way to measure airflow; the net result is better response and performance. In addition, the mass air computer is a "learning" computer, which means you can add a new component, and within a minute of starting up the car, the computer will automatically recalibrate itself around that item. As far as horsepower and mass air: the wallet's the limit! But first you need to get the mass air setup in your Cougar. You can usually find the parts needed at your local salvage yard, or swap meets, or online.

Parts Acquisition


You will need the following items from a 1989-1993 Mustang HO 5.0:

Mass Air Wiring Harness Adapter
Ford EEC-IV Mass Air Conversion Harness

Mass air requires a few extra wires running from the mass air meter to the EEC-IV computer. The easiest solution for your wiring is to reuse your existing, working 5.0 harness and add a mass air adapter kit to your harness. These kits are available in the aftermarket from companies such as Interactive Systems and Technologies, as well as others, and sometimes even on eBay. Basically it's a 4-wire hookup for the mass air sensor, and it runs from the sensor through the firewall, tapping into the main computer harness. It's very neat and efficient since you don't have to either purchase a new harness or take out your existing one. Plus, it's relatively inexpensive.

You may be asking, "Why not just change out the whole harness to one used in a mass-air equipped Mustang?" The reality is that the Mustang EEC harness is not fully plug-and-play compatible with your Cougar's existing internal wiring. For example, the 1990-93 Mustang 5.0 harness is its own beast, since those were the years that the previous-generation Mustang had a driver's side airbag, and it can create difficulties when installed (such as the fuel pump not getting power). Even a 1989 Mustang HO wiring harness, which should be a little more compatible, needs several wires repinned. It's not that these harnesses cannot be used at all; it's just that they're simply not easy to use. Several people have done it, mostly for the sake of aesthetics, but it required multiple shop manuals and some time to work out issues. In this respect the 4-wire mass air add-on kit is easier, cheaper, and much more efficient.

Mass Air Meter
Ford Mass Air Meter

The mass air meter itself is coupled inside the air inlet tube, and it measures the air coming into the engine. It's the heart of the mass air operation. The sensor on top has a two-fold job: one "hot" wire inside measures air volume, while the "cool" wire measures relative air temperature. In other words, it's a very sensitive piece and should never be touched with bare skin or any metallic objects. The sensor is calibrated to your corresponding fuel injectors. So if you're running stock HO orange 19 lb/hr injectors, the mass air sensor must match those. It is possible to remove the sensor and replace it with another sensor—for example, in the future if you're upgrading to 24 lb/hr injectors, you can keep the meter and just swap out a 24 lb/hr calibrated sensor.

Ford Mass Air Meter

The stock Mustang mass air meter is restrictive in diameter at 60mm and has the internal air vane which restricts airflow; a different mass air meter will allow for better breathing. Most people shoot for a 70-75mm unit. Ford did make some factory mass air meters larger—for example, the supercharged V6 cars (T-Bird SC, 1989-90 XR7) had 70mm mass air meters from the factory. Swapping out the top would be an easy task. But really, if you have the cash, an aftermarket mass air meter is the way to go. They are usually much lighter, stronger, and less restrictive than a stock Ford meter.

Ford Mass Air Meter Bracket

The mass air meter is attached to a bracket that mounts onto the passenger side strut tower; you may have to drill new holes to mount the bracket but 1986-88 cars should already have the holes. Do not attempt to use the sensor without the bracket, unless you are using a lightweight aftermarket plastic meter or a metal cold air intake system that can support the mass air meter properly (see below). By the way, the sensor on the top is removable in case you ever need to replace it. It's usually held on by a pair of safety Torx bolts. Be careful to never touch the electrode ends with your skin or any foreign material, as that can effectively kill the sensor completely. Always use mass air sensor spray cleaner to keep it clean; do not use carburetor cleaner.

Air Inlet Tubes
Ford Mass Air Intake Tubes

At first, these may not seem important; with speed density, you can run around all day without the air tube on with no side effects, save for all the dirt you let in. But mass air is a closed system; if you try running without these tubes, your car won't even idle right, much less run at all. They are now two short tubes instead of your usual one. Installation is exactly how you think it is—pretty simple. You will reuse your existing air cleaner box, or if you've installed a K&N Filtercharger kit, that will not be affected. If you wish to use aftermarket metal cold air intake tubes (such as those made by BBK, MAC and others), feel free to do so. They're going to give you more power due to the smooth inner walls and slight Venturi effect.

Mass Air Computer
Ford Mass Air EEC-IV Processor

You will need to replace your existing computer with the new mass air EEC-IV processor. There has been a lot of discussion on the Internet about which computer to use with which transmission. In the real world, it seems to be perfectly fine to use a manual transmission with an automatic; we've never seen any issues firsthand. However, some people say you can use either computer with a 5-speed, and not vice-versa. You will have to judge which computer will be best for you, but if you're uncertain then stick to the computer which matches your existing transmission type.

Ford Mass Air Calibration Code

Whichever computer you decide to use, there will be a calibration code on the sticker on the outside of the grey harness connector. You will need to keep this code handy for future reference, in case you need to purchase things like a BPS or new injector.

The following part numbers are for common Mustang mass-air EEC-IV computers with stock 19 lb/hr fuel injectors:

Automatic Cars (Calibration code A9P):

Manual Cars (Calibration code A9L):

Universal Rebuilt:


NOTE: Disconnect the battery before attempting any of the following!

The physical addition of the meter, bracket and air tubes is fairly easy and straightforward. The hardest part of the job is running the new wiring, and then pinning it into your stock harness. There are a few ways you can run the wiring. Inside the engine bay you can find a grommet on the firewall over on the passenger side and put the 4 wires through there. You could technically also drill a new hole and put a new grommet in, although you need to be careful when doing that. An alternative solution is to run the wiring inside the inner fender. With the car safely in the air, remove the wheel well liner and insulation and you will have clear access to run the wiring along the top of the fender support brace, and down into the area between the fender and the door. There is a second, empty hole beneath the antenna hole through which you can easily run the wires (note: if you have a power antenna you cannot see this hole and will need to remove the antenna housing to get to it). This puts the new wires exactly near the EEC-IV processor where you need them.

For wiring the mass air harness into the EEC harness, follow the instructions that come in the wiring kit. You will need to piggyback two wires onto existing wires, and place two other wires into empty slots in the EEC harness. In general:

Wire Color Function Position in EEC Harness
Red Main Power (12v+) Splice into wire at pin 37 or 57
Black Main Ground (12v-) Splice into wire at pin 40 or 60
Brown MAF Signal Ground Place in pin 9
Blue MAF Signal Place in pin 50

The brown and blue wires will need to be cut to fit, and new terminals crimped onto the ends. It's highly recommended that they also be soldered. For the red and black wires be sure to use either a high-quality splice connector, or to solder onto the skinned wire. Make sure all connections are taped up or covered securely and safely.

Now the mass air computer also requires that you reroute 2 wires in your existing main EEC wiring harness. These wires go to the TAB/TAD sensors, which control the opening and closing of the Thermactor air system, driven by the air pump. If the system does not operate correctly (in other words, if you do not change these wires around) you may experience hesitation, bogging and increased emissions.

Ford EEC-IV Processor Pin-Out

There are 60 pin locations in the stock EEC-IV computer harness. You need to switch #51 to position #38, and #11 to position #32.

You may also need to run a new wire to the fuel pump relay. In the Mustang, there is a secondary fuel pump monitor wire that runs from pin #19 on the computer to the fuel pump relay. It's basically a double-check to make sure there is sufficient power running to the fuel pump. If you do not run the wire you will get a code 95 (secondary fuel pump failure) when an EEC-IV computer test is performed. It doesn't mean your fuel pump isn't running or working properly, just that the back-up check doesn't acknowledge it. It's up to you whether or not to run this wire. Again, follow the instructions that came with your mass air harness kit.

Also, some pre-1988 vehicles may need two additional wires run to the vehicle speed sensor (VSS) on the transmission. Most people do not but if you find that your vehicle performs poorly or throws strange computer codes, you may have to run these wires. Positions: pin 6 to the O/Y VSS ground wire, and pin 3 to the DG/W VSS power wire.

Your MAP sensor can be reused; just disconnect the vacuum hose, cap off the hose and you're all set. Essentially that turns the MAP sensor into a BPS (barometric pressure sensor). If you experience idling or starting problems this could be due to the old MAP sensor being incompatible with the new mass air computer, which means you may have to get a new BPS from a mass air Mustang (1989-93). Some people need one, some don't, but most of the time you'll be able to reuse your existing one. Remember your calibration code when buying a new BPS.

Once the wiring situation is finished and all the other parts are installed, you can install the new mass-air A9L/A9P computer and reconnect the battery. If all goes well your car will start up as usual, and idle quality should be as normal. Now if the car doesn't start at all you'll need to double check your wiring, as mass air cars will not start without a correctly-wired mass air meter. Any rough idling may indicated a need for a new BPS sensor (see above), a bad mass air meter, or, again, possibly a wiring issue.