Hey guys,

Been trying this out recently while diag-ing 8's for stalling issues and/or jumpy idle.

Get in car, start, allow to fully reach operating temp (drive around, do whatever you do)

Once at oper. temp. park somewhere. Now with car in *neutral* (goes for a/t too) and clutch pressed to floor (obv. m/t only) rev the car with a quick jab of throttle to around 5-6k, let off immediately.

Now look at the tach, but rather then focus on the lines just stare at the middle so you can see the needle more then the lines. Monitor the speed it falls at, it *should* fall pretty fast. At approx 1375rpm the needle should almost bounce to a slower speed slow enough to be noticeable immediately.

I've had a few cars lately suffering IAC (idle air control) problems (IAC is within the TB, it's one unit essentially). None of these cars showed any codes, stored-pending-or otherwise. Only sign of failure was monitoring rpm vs time and comparing it to a known good rpm vs time, difference at times was noticeable. One car was easy to spot, could barely idle, very lumpy...would stall with a quick jab of the throttle, but if you eased on the gas it was fine.
Obviously car is taking in too much air at idle, with no vac leaks you assume car is expecting this extra air thats why you have no lean codes, computer is adjusting properly.
So first culprit is TB, due to internal IAC, this exact concern is typical of MPV owners, also multiple other ford models (they have external IACs however).

However, I've begun to see IAC's failing almost randomly, will allow car to idle only mildly off but will cause an intermittent stall in differing conditions.

One symptom they all showed was failure to hit the 1375rpm and slow down rate of rpm decrease, some would catch at 800 and get stumbly before leveling off, some would never level and just stall out or choke out and roughly catch itself, on some the idle was steady but low and car would catch the idle lower when revving. Which when looked at seems pretty obvious as an IAC problem.

Figured I'd get this out just almost as a poll to see how widespread this may be. This is all based on personal experience, not sure if the math and such behind it adds up, haven't looked that deeply.

All i know is cars with failing IACs have been showing up, if you're having problems why not give this quick test a shot, at the worst you waste a minute, youve wasted more then that reading this :P.

kevin.

edit: as an addin, this test isn't really conclusive of anything. Your IAC may be fine, there may be another problem being overlooked. I've simply found this symptom to correlate with bad TB's in a few recent situations, seems noticing this is also around engine replacement time not sure which comes first. I think this may cause some dead motors to seem even more dead which is why some are obvious and some aren't. I started noticing a trend, which nycgps' motor continued making it around 4/10. In all cases motors are confirmed dead, low compression. Removal TB when compression was found to be low and then re-tested showed near identical values so the TB was not hindering compression results. But then these brand new motors would be fighting to idle and no amount of memory clearing will get it to re-learn the idle. Replace with known good TB and new engine purrs. Again, personal experiences only, just throwing it out there as an *AID IN A LARGER DIAG SCOPE* ... to clarify.

 

i have this problem exactly. thanks for the info. i was just going to order a replacement (and polished) tb from BDC motorsports. i heel-toe tap the gas while coming to a stop to keep the engine from dying. my engine WILL die if i dont do this. it seemed to be worse when the weather was cold.

 

Dirt in fuel line causing Clogged fuel injectors?

Air filter too dirty?

 

yea like i said, one step in a diagnostic chain.

kevin.

 

I have noticed for a good 1-2 years now anytime the ECU is reset, my car completely struggles to catch idle. I could be cruising along, take it out of gear coming to a stop, and the RPMs would just fall way too far and the engine stalled.

I found when my ECU gets reset, I have to hold the throttle at like 5% open, the rpms will bounce around idle, then steady out and then I work my way down to not having to feather the gas. During the process no nasty fuel trims (short or long term) or misfires are ever really produced and the ignition system is up to snuff. After I go through the process, all is completely fine then...


could this be the issue? I've pretty much given up at finding the culprit.

 

Rotor that is normal when you re-set the ECU.

If ECU was loosing communication and causing the car to re-set you would have a cell code. The problem with a TB is a different issue that teknics wants to alert members about a paossible cause to their idling problems.

 

not air filter. i swapped between 3 intake setups. teamrx8s custom, k&n panel, and stock filter.

no difference.

uninstalled accessport for a few days.

no difference.

 

Mine started doing this after the Greddy install. Thought it was a vacuum leak but cant find one. It may be that I have no screens in the custom intake, however the previous FI owner didn't need screens with the intake. Engine feels strong just cant figure out the jump. Mine hits above 500 then jumps up to 1k. It will sometimes level off arounf 800 but it varies

 

Yea like Razz1 said, with a PCM reset the PCM has to relearn to how to make your specific car run properly. Basically when your car starts running good again youve most likely finished the necessary driving cycles.

kevin.

 

When you say IAC... do you mean that there is a separate passage/valve in the TB assembly that controls idle air? Do you have a picture or a diagram of it? I'm trying to find something in the 2004 service manual about this. I thought that idle air was controlled exclusively by the TB butterfly opening angle, controlled by the PCM and drive-by-wire system. The previous rotaries had an IAC valve mounted to the upper intake manifold, and this valve would eventually stick due to carbon buildup but could still be cleaned.

 

"IAC" vs "IAC Valve"

rx8 has IAC (idle air control), it uses the mechanics of the TB to perform it's function

mpv, escape, bla bla has IAC Valve, it uses it's own mechanical device to perform it's function.

I would say it could be "carbon buildup of the TB butterfly" but I've tried cleaning with no success (didn't do this on all of them, a select few, which when it did nothing i stopped wasting my time). So when i say "IAC failing" i mean the part of the TB dedicated to the calculations and operation of the IAC system (basically all electronics) is failing, simplified would be "your TB is bad" but that doesnt "solve" the problem, it simply fixes it.

kevin.

 

So, if the IAC problem occur, the best way is changing the whole TB?
Is that rightn?

 

Be careful when cleaning the TB as chemicals can get into the esensitive electronics from the pivot points of the butterfly.

Cars that have oil blowby can also have oil entering these sensitive areas. I am not certain if you can successfully dismantle the control panel it inspect the condition.

I think cleaning with a contact cleaner will be safer

 

Cleaning is by far the best first step. I know he is a Mazda tech (or was at his last login point 7 months ago), but I've never known an idle problem with an RX-8 to be solved with a throttle body replacement. It's always something else.

If you are worried about sensors when cleaning, MAF sensor is the safest you can get.

 

Usually i clean the TB by caburater cleaner, is that ok for that?

 

Contact cleaner of maf cleaner will be better

 

The previous owner of my 2004 RX-8 also installed the GReddy exhaust, and my car has the same idle issue. It may just be a coincidence... I don't know. I have an issue starting up my car on really cold days. On the tachometer, the needle constantly jumps back and forth between 3000rpm and 1000rpm until the car is almost warmed up to the first tick on the temperature gauge.

 

the issue u describe has nothing to do with TB/IAC, it's your neutral switch on the transmission. change it to the newest Version B and your issue will be resolved.