Electrical 1987 - IAC valve (Diode & Ohms)

Consider building a smoke machine. Not that complicated to make and comes in very handy.

View: https://youtu.be/crc6CJ_ni2w


I’m actually building one right now. Just waiting on a few parts from Amazon to show up.

7991LXnSHO said:

How do you fell of your homemade smoke machine is burning up?
The PVC case on the videos I have watched made that a serious question.

Click to expand...

Don’t temp me with over engineering a $40 DIY machine into a $400 machine. Because I can

Biggest culprit is the ports inside the heads themselves get clogged up. The path runs front to back through the head with perpendicular pathways to the exhaust ports.


And yeah...it's amazing how sensitive these cars are to vacuum leaks, especially 86-88 speed density cars as their fuel/timing curves are based primarily on manifold pressure. A small leak will change that

The answer to that question all depends on if you have cats. And if you do, do you have a stock 4-cat H-pipe or 2-cats?

If you don't have cats. None of that stuff matters at all. Plug it all up, disconnect the TAD/TAB and live with the codes. They don't set off the CEL and don't alter the way the ECU functions.


If you have a 2-cat H-pipe, it also doesn't matter. Here's why. The air pump system supplies o2 to the converters to make them more effective. These old cats relied on heat and O2 to be efficient. When you first start a cold Mustang, the cats simply aren't warm enough to do their job. SO ford added two small cats close to the headers so they would heat up quickly and do their job more efficiently. As a result, when you first start the car the TAD and TAB direct the airflow to the heads (specifically the exhaust ports) where the O2 flows into the headers and down into the first set of cats to help them be more efficient.

Once the vehicle is warmed up, the TAD switches the routing of the air from the air pump down to the lower set of main cats and this is where it will stay until the vehicle cools down again. This is why the heads and crossover tube tend to fill up with carbon. They only flow for a short period of time.

For the 2-cat pipes, you replumb the air pump if you'd like to just function for the main cats. This all depends on the setup of course, as my catted Bassani X-pipe has the air pump tube enter AFTER the cats rendering it completely useless. But if you wanted, you can just remove the diverter valve and the crossover tube plumbing and just cap off the TAD vacuum port. Leave the TAB and bypass valve in place and the car will just enter bypass the air to atmosphere or send it down to the H-pipe.

If you don't want to deal with the codes, a pair of 65-90ohm resistors should take care of the KOEO codes (81 & 81 i think) but you will still get the 41 & 91 when you do the Engine running codes.

Not a big deal.


Long winded i know, but helps to know how the air pump system work and what it's intent was.

Per the factory EVTM

TAB is towards the outside of the car and TAD is closest to the engine.

TAB is white/red wire to pin 51 on the ECU
TAD is light green/black wire to pin 11

Those are ECU controlled grounds. The other wire should have constant 12V

I dunno if you've seen this diagram, but this will visualize the air pump system. Flow is to the heads during warmup to feed the first cats (not shown here) and then it's directed to the main pair of cats when the vehicle is warm. I believe at WOT, the bypass dumps all the O2 to atmosphere.

The F0 part number is a 1990 engineering part number, signifying that part was made in 1990 or later. It will have a diode in it.

The diode and solenoid are in parallel, so you are still reading through the solenoid in both directions.

The reason it's in parallel is to basically short circuit the coil when it's deenergized and lets the internal resistance of the coil deplete the energy stored. Basically it's a constant loop until depleted. Without it, you get voltage spikes back to the ECU.