'04 NB2 idle issues

[HarryB]

Now my idle issues got back, even worse this time. Approximately 2 months back, I had the car idling low and sometimes stalling with "external" load while idling (headlights on, turning the wheel). Back then I removed the intake tube, sprayed the ICV with brake cleaner and blasted with compressed air, disassembled the EGR actuator and lubed up the spring, and did a base idle set, which seemingly worked. However, the issues got back a week or so ago. I would normally believe it is the ICV again, but the interval seems too small for me to justify it. Symptoms:

Car idle is usually OK, but sometimes it is too low (650-700rpm), which causes engine stall. If I rev it up by throttle blipping, it goes back to normal idle (850ish). Strange thing is that sometimes, when stopping in traffic lights, it idles too high (1000ish) without any obvious reason. These two modes can happen back to back (from one traffic light to next). When idle sits high, if I blip the throttle, there's a slight hesitation in the RPM drop below 1500ish.

Also, when driving and decelerating, car in gear, sometimes it bogs when revs are below 2000 RPM. Seems like some sort of "clutch judder" or "hiccups". If I press the clutch and rev it up, it goes away.

The thing that drives me crazy, is that there's no consistency. All these things happen randomly, and sometimes not at all during my daily commute (approx. 8 km).

Ideas:
ICV
Loose ground point or connection
Bad spark plugs (will change them today)
Faulty RPM signal to ECU (cam sensor?)
Vacuum leak (PCV)? Not quite likely IMO because symptoms would be consistent.
COPS/Wiring issues?

Any other leads? This is literally driving me crazy...

[RustRat]

Only one solution: you undo your intake, maf etc. You lift the carpet at your passenger sde of the car, you remove the ECU. Are you following me so far? Here comes the good part, you slap in a Megasquirt, you stuff a supercharger in your engine bay, and boost the hell out of this damn thing.

[NobleRogue]

Try changing your fuel filters and then test your fuel pressure. That is where I would start. But then I'm not ASE citified.

[— kung fu jesus —]

Your list of items is exactly how I would approach it. Cleaning an IACV can be a temporary fix if is already worn. Some brake cleaners are pretty caustic, so be mindful that you can do more harm than good on the electronics.

[04msm]

I kind of had a similar issue but it turned out to be IAC Valve that was getting stuck I would clean it right were is at attached to the throttle body and the problem would always come back but ones I actually removed it from the throttle body and cleaned it really good problem solved.

[HarryB]

Thanks for your advice guys! I wanted to remove the IACV the last time, but I was too afraid of the Philips-head screws and my bad record with stripping them. I guess I will start with new spark plugs and checking all electrical connections/grounds/vacuum lines, them being the "easy" items. I also have a new fuel filter ready to go on my spares pile, so I might give it a go as well. Will report back with the results.

[—Martin—]

Thanks for your advice guys! I wanted to remove the IACV the last time, but I was too afraid of the Philips-head screws and my bad record with stripping them. I guess I will start with new spark plugs and checking all electrical connections/grounds/vacuum lines, them being the "easy" items. I also have a new fuel filter ready to go on my spares pile, so I might give it a go as well. Will report back with the results.

http://revlimiter.net/store/jis-screwdrivers.php

[— kung fu jesus —]

The NA has soft screws attaching the ICV to the throttle. I clamp vice grips around the head of the screw, use the closest size Phillips screw driver and turn both tools at the same time to break them free. I haven't ruined one yet doing it that way.

[— Agent☣Orange —]

Those screws are super tight for some reason. I had to eventually use Vicegrips on mine and then replace the screws with some Allen type. I cut out a new gasket as well.

[HarryB]

I will eventually replace both of them with allen-head ones, once I break them loose. Does anybody knows the size of these? Also, is there any sort of o-ring/seal I have to purchase beforehand?

[— kung fu jesus —]

Most likely. Search for a FSM online specific to your car, or look up part diagrams on Mazda OE part retailer online. An exploded view will show a gasket, if applicable.

[HarryB]

Confirmed. Looks like a paper gasket, so I can DIY cut a new one.

[HarryB]

Small update: Not a vacuum leak, not a loose connector as far as I could tell. Could not take the throttle body apart because a) the weather was terrible and I do not have a closed garage and b) I need new gaskets for the IAC and throttle body.

Locating a genuine throttle body gasket via e-bay was fairly easy (albeit LOTS harder than NA ones to come by). However, the IAC gasket costs anywhere from 28 to 35 euros, which for a gasket this small is ridiculous. Then I did what I do every time I find something awfully expensive for what it is; search the "part bin" to see which cars share parts. Found out that all NBs share the same IAC with Mazda 626, Protege and Kia Sephia. A Protege IAC gasket costs around 3 euros... ;)

[HarryB]

So, I have checked every single cable, changed spark plugs (they were a mess) and sprayed all connectors with contact spray, and thankfully symptoms mostly went away. I have a tiny suspicion of idle droop sometimes, but it cannot compare to what it was.

I got hold of all flanges, and ordered a second hand TB with IAC and TPS, as well as cam and crank sensors to check, they should be here soon(ish), so I will start experimenting. I am also about to change my gas filter and clean my injectors.

I also had a random moment of epiphany (or brainfart); for quite some time I hear a (what I presume as exhaust-related) rattle. It has been there since day 1, however it might got worse. It happens on load, between 1500-2000rpm and neither I nor the exhaust shop can find what it is. In addition, the car has developed a hesitation in accelerating some times, again in lowish rpm and partial load. Now, there's a little voice in my head telling me that the lambda sensor boss has cracked off the OEM manifold, rattling and causing all other problems as well... Thoughts?

[— kung fu jesus —]

Regarding the stumble, I would try an OBD2 scanner with datalogging capabilities to try to capture readings when the issue arises. Part-throttle makes me wonder if your suspicion on the lambda boss is correct. If air is getting in, it can make that sensor confuse the computer a little. If your lambda is in the same place the US versions are, it's not a terrible job to do. I seem to recall it being relatively accessible from under the car with a lambda socket tool.