Ford egine races at idle.

My 1989 Ford Taurus wagon with 3.8L engine started to idle at 2,500-3,000 Rpm soon after having the heads replaced last year. The idle speed control valve was replaced because the bellows was damaged. No
change! Replaced the ECA twice and still no change. Checked resistances to ground, +5V, +12V, on all 60 ECA pins and found no abnormalities-shorts/opens. The coil resistance is 10 Ohms which is in my opinion rather low but the same as the old units'. Voltage on the low side is 10V and at the high side is battery voltage. Obniously 2-4V across the coil is insufficient. The use of a current boosting circuit-an emitter follower transistor-reduced the idle speed to +/- 600 Rpm, but the valve never opens, causing the engine to stall. An alternative method using a plate with a small hole between the ISC and manifold works but this is not fixing the underlying problem. I've beeen climbing the walls for about a full year. Ford provided no assistance at all. "Just Answer" also provided no solution. I'm at the end of my rope and ready to give up. This is something I have never done in more than 60 years of servicing cars, TVs. audio, etc. Is someone able to help? Please help, Richard
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Are you sure you don't have a vacuum leak somewhere @ the intake?????

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So... if I have this straight:
1. The ISC actually WORKS if out of bounds of normalcy
2. You dont have a vac leak, because you can get the engine to idle lower than normal by 'overdriving' the ISC.
This started after replacing the heads, so I would suspect the engine to firewall ground strap.
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Thanks for the replies. Here is an update on the idle speed control failing to close. The signal to the coil is about 250-300 Hz and about 5V p-p. Either the ECA is not able to generate enough power, or the ISC is loading it down. The resistance is a suspicciously low 10 Ohms. It is a new unit-not rebuilt. Richard
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