Terribly long post with a lot of boring info related to my problem. If you are an experienced mechanic and want a challenge, read on: Wife's Aerostar 3.0 (1992) has 89,000 miles and had original wires, cap, and rotor. Plugs had been changed at 51,000 miles (Platinum). Thing has a rough idle like an intermittent miss. So I look at the thing and think, "no way, this is going to be a bear to do." I call my mechanic and he says, "Aerostar. Plugs, wires, cap and rotor. About $350." So I decided to start with the easiest and do just cap and rotor, figuring maybe the wires are OK since they don't arc (no stray sparks when observed in dark." Well, that idea fell apart quickly when the distributor cap boots of #1 and #3 wires were pulled off and a lot of rust powder came out. OK, so next night its new wires. Still has intermittent miss at idle, but runs great. Ran pretty good before too. I tested the old wires resistance (spec is around 5000 ohms or so per foot) and they were infinity (#1), 20,000 ohms (#3), and the rest were 5500, 7000, 6000, and 7500 ohms, which would be fine (my idea to change only the cap and rotor would have worked but for the rust). Now the plugs are a real bear to change and I'd rather wait (they are supposed to be good for 60,000 miles per owner's manual), but I wonder if they may have gone bad, at least on #3 due to the infinite resistance (I understand the infinite resistance is to a 1.5 volt multitester battery and the 40,000 volts or so of my ignition coil must get through or it would run poorly). Regardless, the plug may be in bad shape from the high resistance which I believe would reduce current flow in the plug, but boost voltage (or so I think--and I am walking on thin ice here with my knowledge of electricity). Anyway, perhaps I should just pull that one plug (not the easiest to get at (#3 is easiest)) to see its condition. On the other hand, with the very high resistance, perhaps the electrode is less worn. And maybe the rough idle is due to something totally different. Mechanic had looked at it 5 months ago for the rough idle and said he cleand the plugs and they were in good shape (I find it hard to believe he did this and maybe he was mixing me up with someone else). None the less, he did not fix the rough idle and suggested the plug wires may be the culprit as they are old As I said, the car runs fine and did before I put the wires, cap, and rotor in. In fact, I was getting 19.5 mpg with it that way.

 

[updated:LAST EDITED ON 26-May-02 AT 11:26 PM (EST)]Tallpaul, you obviously had two bad wires, #1 and 3. You can also check the resistance on the plugs, although I have forgotten what the values should be. If it still idles rough, I would change the two plugs, or check the resistance on them.

 

Seems to run better since I did cap, rotor and wires. I plan to do plugs eventually. And will try #1 and/or #3 as soon as I get a chance. Odd thing is it seemed to idle smoother with the air con on than with it off.