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The recap and conclusion.... Idles smooth on startup, starts getting erratic "bumps" after a few minutes. A/C and power steering affect it, but unpredictably. When driving, it stumbles off idle but runs smooth above ~1500rpm. No check engine lights (except a rare "random engine misfire" when it...

You've been holding out on me! Just kidding. Just measured IAC resistance. 9.8, supposed to be 7.2 - 9.2, so just outside. Regarding my IAC functional tests, if it were giving no feedback to the ECU, the ECU might not tell it to operate. My tests wouldn't catch this "zero operation"...

Would you rule it out after those functional tests? No matter, I'm going to do the formal tests. It's brand new. With both the old and new I wiggled the connector like crazy (both old and new one) while it was running and it had no effect. It runs like total crap when unplugged, so seems to...

I've disconnected and reconnected everything at least once, done lots of wiggling with it running, no luck. Needles & haystacks, ugh.

I don't have the car for a couple days, but I'll start with the IAC tests. Previously, I disconnected the IAC and held it at idle with the gas pedal, still got the bumps, so doesn't seem it's giving bad info. I also put a blocking plate below the IAC, and with it hooked up still got the bumps...

My Actron reader displays live data. Guess I should look there, duh!

Finally had some more time to work on it. 2 mechanics suggested fuel filter or pump. Replaced screen & filter (fuel pump "body"), no change. New fuel pump, no change, grrr. [anybody wanna buy a fuel pump assy?] 008's comment on MAF jarred my memory of an erratic MAF on a Mitsu pickup I had...

I've never heard of Seafoam, but from what I'm reading it sounds like good stuff. Similar to Marvel Mystery Oil. I've seen MMV do a great job quieting lifters, but that's all I've ever seen it used for. I'll definitely give the Seafoam a shot.

The "instant" drops are when it sputters. When I turn on the a/c it goes down to 16 and stays there. Put it in gear and it goes down to 14 and stays there. Should it recover back to 21 while those are on? If it should, each of those systems must have a vacuum leak.

Thanks for that info. Plenty of places to go wrong it seems. So, the car has ~80 miles since the ECU reset, nothing has changed except I have a shiny new vacuum gauge. :-) Warm idle, no a/c, in Park, 21 inHg. Put it in gear or turn on a/c, 16. Add the other one (a/c or put in gear), 14. Is...

Yeah, it sure seems like the IAC is working. It kicks the idle up a bit when you turn the a/c on, put it in gear, or turn the wheel (idle always settles back down from the load in every case).

No kidding? 100 miles?

With the IAC unplugged it just dies on idle. If I hold it at idle with the gas pedal I still get the drops. I'm putting a blocking plate on it tomorrow in case it's an internal leak or something. Even if that doesn't change the behavior I'm about at the point of just replacing the IAC, and...

reset ECU, no tears in intake tube, can't find any split vacuum lines. I sprayed carb cleaner all over the base of the intake and every vac line I could find, didn't affect idle, though I'll probably get lung cancer from the fumes. I made a plate to block off the egr, no change, making another...

Pulled coolant lines off IAC/TB and plugged. Smooth idle cold, goes to crap before the temperature gauge starts to move. Grrrr.

Good info. I previously removed and cleaned IAC/EGR/Throttle body, they weren't that dirty for 150k. I checked EGR movement but not IAC movement, oops. Now knowing what that IAC does I ran a few more tests: Engine Cold (doesn't have the problem): Turn on a/c idle does go up a bit then...

Did you get this resolved? I've got an idle problem that's killing me: http://www.mazdas247.com/forum/showthread.php?123855161-Yet-ANOTHER-rough-idle-2002-P5

I've read about 1 bazillion posts on rough idle but haven't been able to resolve mine. It started about a month ago and has gotten progressively worse. Symptoms: Idles smooth at 700-1000rpm while in park, a/c off. Turn on the a/c, idle drops slightly (that's normal) then gets erratic. It...

Did you get this resolved? I'm in the same boat with a 2002 Protege. Have done everything you mentioned and no luck. Mine is a little different in that it idles smooth with a/c off, then sputters with a/c on. Also smooth in Park, same sputter in gear. I've also let it idle with the pcv valve...

I've read about 1 bazillion posts on rough idle but haven't been able to resolve mine. It started about a month ago and has gotten progressively worse. Symptoms: Idles smooth at ~600rpm in park, a/c off. Turn on the a/c, idle drops slightly (that's normal) then gets erratic. It never goes...