93 Concorde Idle Problems

Here's the deal. This car is one of the first EVER Concordes. Production date was 11/16/92. Yikes. It's been used and abused and I'm just trying to keep it running a little longer until I buy me something better. I've already fixed myriad problems on this car and I'm sure I'll fix a few more before I'm done.

Anyway, paid $500 for the car, took it home, and it ran fine. Then it started to have really erratic idle problems. It would idle at a normal level (around 800 or so), then shoot up to 1400 or 2000 rpm's then go down to 1000 then back up to 2000, 3000...long story short, I changed the IAC valve...but I didn't have the money for a new one so I bought one from a junkyard. It looked brand new and it seemed to work fine when I tested it, so why not?

Now it consistently idles at between 1100 and 1400 RPM and will, occasionally, idle back down to 800 or so, but when it does, the engine runs like it's missing on more than one cylinder. But then it idles back up and there's no indication that it's missing.

I think the computer (PCM) was changed, but I haven't checked it against the numbers I recently dug up for the original part number. The code I keep getting is 56 (I think...OBD I) "short in iac valve".

So, the two problems are with the high idle and with the rough idle at "normal" idle speeds. (Oh, and my dad's been a mechanic for 30 years and even he is scratching his head.) Help?:banghead:

Vacuum leak? Can't exactly say. But don't doubt what the computer is telling you. Sometimes it can be valuable. Do resistance testing on your the IAC valve circuit. If the valve isn't busted, maybe it's the wires running to the valve?

I've considered the wires, but don't have the equipment or the time to test them properly. I've also considered that maybe it's idling too high to compensate for the lope in the "normal" idle speed. I've to remove the air plenum soon anyway to do some other work on the engine, so maybe I'll see then if there's a leak in that seal. Thanks for the reply!