Poor Idling

I'm building a factory five roadster using a 1996 Mustang GT donor with 4.6 L SOHC motor. I started up the rolling chassis for the first time this past weekend. The engine idles a little high for the first minute or so, then it drops to such a low idle that it stalls. Is this a problem that is caused by the idle air control valve. My chiltons manual at 4-19 says that idle speed is not adjustable on this motor, that I need a special SCAN tool to extract working parameters (voltage signals) from the EEC-V system while it is running, which chilton says must be done by the dealer or shop with this equipment. What is the easiest way to check the function of the IAC.
Thanks
Joel

Herb Butler said:

More than likely you will be changing the IAC. Be sure you buy the Motorcraft IAC. The copycat units are not reliable.

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Thanks Herb. Now I'm thinking that the Throttle position sensor and IAC both have to be reset. I'm thinking that because the battery has been disconnected so long - well over 9 months - the sensors have to be running for while, then the engine shut off with the battery still connected in order the computer/PCM to be reset. However, I'll be the first to admit that when it comes to electronic fuel injection, I'm well over my head.

Herb Butler said:

Are all the stock sensors connected? alldatadiy.com will have the diagrams and you would have to purchase the use (for a given vehicle and engine).

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I believe all the sensors are connected. However, my intake manifold has only one coolant temperature sensor, not two like most mustangs from that era. But I don't think that is something which will effect idle. I have an original Ford wiring diagram manual for the mustang - it's a vintage book! - which should help trace the wires.

I don't know if you're familiar with factory five roadster kits. In addition to buying the kit, you also need a Mustang GT donor car from which you remove dozens of parts for use in the kit. My problem is I started with a donor that had been modified, drag raced and abused. The engine had a blown head gasket, some parts missing, wires cut, and emission controls disconnected. While the engine started, it did not run well when I purchased it. I shouldn't have purchased it, and I overpaid on top of that; $2000 for a car that didn't work. I had the engine bored and ground before I reassembled it. I have had to remove the EGR valve and replace it with a block off plate because the new headers are not compatible with EGR. The previous owner had glued a homemade sheet metal block off plate behind the IAC valve, effectively blocking it off, but still keeping the computer and sensor connected.

I'm starting with a mess, and expect I will have to trace every wire, and test every sensor, to figure out what is wrong. And to make matters worse, I'm sure there will be other problems to address after the idle is tackled.

Joel

Herb Butler said:

One more thing Joel,,, Get the codes after running. The PCM will tattle on the problem inputs.

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Thanks. I spent much/all of today working on the car. I put a hose clamp on the IAC hose, which may have smoothed out the idle a bit. But then I wen to work on the radiator fan, which hasn't been turning on. However, when I unplugged the coolant temp sensor, it started working. The sensor is working; I tested it in boiling water. When I reinstalled it, not only didn't the fan come on, but the idle problem came back. The motor has the old plastic intake manifold which uses just one temp sensor, on the passenger side, not two. But the previous owner has it set up using the wires from the driver side. He spliced an extra 12" of wire into the harness so the two wires would reach across the motor. Now I'm thinking I have to rewire.

And to add to my wiring problem, the directionals aren't working. I'm not too worried about it because the headlights weren't working either but I figured that problem out.

Thanks for all your input.
Joel

ARBOC39 said:

Mustan5L5 helped me fix mine, was quick and easy, worth a try...he wrote

Oh, to set idle. Unplug the idle air controller, twist in the throttle stop screws a few turns and start the engine. Now back down the idle stop screw to as low as you can go and have the engine idle smoothly.

When you get it to where you want, check TPS and make sure between 0.6 and 1.1 volt. Adjust only of outside that range.

Once set, shut car off, plug IAC back in, and disconnect battery for 30 mins.

Reconnect battery and start engine. Let idle 2 mins, then turn on every power accessory and ac (if it works) and let run 2 more mins. Shoul start to idle better.

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thanks Arboc, but I actually tried this fix, but it didn't work. But I think I'm getting closer to a fix. thanks

If you decide to build a FF car, my suggestion is start with a good, complete and working donor. Don't do what I did, which was start with car that had been butchered and didn't work. There are just too many surprises, and none of them good.