Have had a long line of electrical problems with this 87 302. It finally will idle but high idle. Extremely high idle. But when I unplug the iac it goes down to normal. And when I accelerate quickly the engine try's to die and backfires. But it's hazes black smoke out of the tail pipe. After testing everything but the ecu i use a friends out of a bronco with same engine and it didn't change anything but when he put the ecu back in his truck his has now has the same problems mine does. Any ideas?

 

It sounds like there's at least a partial short in your sensor wiring and it damaged the ecu. The outputs can only take so much.

Off the top of my head, I'd start by looking at the circuit that gets the reference voltage from the ecu. I think that it's 5 or 8 volts.

It should got to one side of the TPS, and to other sensors as well. You might try ohming out that line ( and others of course) and comparing the readings to what you get on your buddy's truck. You obviously have to do that with the ecu disconnected.

Good Luck,

hj

 

codes get them they can be a big help as far as the IAC and performance issues
could be timing to much fuel running rich not burning and a lot of other things the high idle could be very simple any idiot lights on

 

You might start by seeing what the 5 Volt DC supply is from the computer and see if is still 5VDC.

Turn the key on and check it at the TPS plug. it should be a orange wire with a white stripe. It also goes to the MAP and EVP sensors.