P1506 code and intermittent IAC valve failure

Tiny
GIG MARTIN
  • MEMBER
  • 1998 MAZDA 929
  • 2.0L
  • 4 CYL
  • FWD
  • MANUAL
  • 122,000 MILES
I have not seen this exact issue discussed anywhere online and need to get this cleared up to pass emissions.

Here is what happens:

The idle speed is okay and does everything it should when driving and turning accessories on or off, and when testing. There are no vacuum leaks or intake leaks of any kind, no throttle body damage or fouling, no ground faults, and no gasket leaks.

The problem comes intermittently so it is hard to diagnose. Setting at idle while not driving the car will only for a short time act exactly as if I unplugged the IAC valve electrical connection and the idle jumps down and runs very rough as if it were about to stall. Right or soon after the P1506 code presents itself.

My first question would be, is this a symptom of a bad IAC valve, or a precursor to it going totally bad?

Second question, because it is hard to capture a reading without knowing when it will happen, could this mean a bigger issue with my electronics or PCM specifically?

Nothing I have done so far has recreated the idle problem itself or the code except unplugging he IAC valve connection.

Please help.
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Sunday, October 30th, 2016 AT 11:55 AM

8 Replies

Tiny
HMAC300
  • EXPERT
Try cleaning throttle plate on both sides and IAC hole with choke cleaner then when done with that spray choke cleaner between number three and four cylinders intake gasket if rpm's change it is leaking and that is your problem.
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Sunday, October 30th, 2016 AT 3:35 PM
Tiny
GIG MARTIN
  • MEMBER
Thanks for your reply. I will clean out the throttle body locations you refereed to, and also the IAC valve parts that interface with the throttle body. I was wondering if using a mixture of gasoline and starting fluid would be better.

In either case I will take the added step of looking for vacuum leaks by spraying carburetor cleaner or starting fluid. I know starting fluid poses more of a risk, but it will give more immediate and definite result.

I have to say though, there never has been a steady (the symptoms are non stop) problem. Furthermore, a P0171 code or anything indicative of a lean mixture anywhere has been logged. The only code I get is for the engine speed being off, but it can take a week sometimes for the engine speed to be off enough to log that code. So I do not expect to find anything spraying either fluid near the intake manifold, but we will see.

When the code logs and I can sense the speed is off is as I said, intermittent. I do suspect it is something with the IAC valve, but do not know why it has not presented itself as a "constant" issue.

So for example, I can start the engine and let it run at idle for thirty minutes, an hour, or two hours even and all will be fine. Then out of the blue the rpm's drop, the engine starts running just as if I unplugged the IAC connection, and it feels like the engine is fighting itself. This will last for two to six seconds then it returns to normal. I can (hide) the rough running by using the accelerator during this rough running period, which makes me also start thinking "timing". It never runs rough out on the road, just intermittently at idle.
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Monday, October 31st, 2016 AT 4:27 PM
Tiny
GIG MARTIN
  • MEMBER
Correction to my last: the P0171 code has never been logged, (indicating a lean mixture). Only the P1506 code has, (suggesting the IAC valve action or inputs).
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Monday, October 31st, 2016 AT 4:33 PM
Tiny
HMAC300
  • EXPERT
Check what I have said in reply the intake gasket seems to maybe be a problem with a vacuum leak. Also never ever use gasoline as a cleaning agent. It only belongs in a fuel tank.
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Tuesday, November 1st, 2016 AT 5:26 AM
Tiny
GIG MARTIN
  • MEMBER
Thank you for another reply.

Okay yeah! I did read your first reply, but alas my reply back was not understood. The problem is "Intermittent", not 'fluctuating'. There is a difference. A leaking intake manifold gasket would cause fluctuating rpm's and a lean fuel mixture, and it would be continuous.

Perhaps a repeat explanation of my problem, and rephrasing the question would help being a more realistic possibility.

* When my RPM anomaly occurs, (maybe once or twice an hour. At idle), the rpm's go way down to barely running then return to normal after a few seconds. I get a P1506 code and no other (except the low rpm's did set of a number two cylinder misfire once).

* The at idle rpm's are between 700 and 750.

* when I turn on various power accessories, including the AC, the rpm's go down for a split second then go back to the normal idle range.

* when I turn accessories off the rpm's rises then takes a second or two to settle back to the 700 - 750 range.

* voltages and pressure/vacuum tests are good.

* At least visually looking at the plugs they at least indicate my fuel/air mixture is good (sometimes it takes and old racing mechanic who did a lot of carburetor jetting adjustments to tell these things. Sometimes old school works and just make sense).

* And to expand on the spark plugs again, none of the four plugs appear different from each other, meaning no hot (lean) cylinders, nor cold (rich) cylinders.

* Now let me rephrase the question and keep it simpler: Does an IAC valve always go bad all at once, or are there various degrees of it failing?

Hope this helps the "focus" issue.

Maybe I have the wrong forum. Without access to computer diagnostic tools, my salvation is going to lend itself to old school logic based diagnostics. Ask yourself, "is what I am saying "logical" and does it make good mechanical sense?". Would an intake manifold leak make rpm's go up or down? Would an intake manifold leak do nothing the the fuel/air ratio and cylinder temperatures?
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Tuesday, November 1st, 2016 AT 7:31 AM
Tiny
HMAC300
  • EXPERT
What I sent is a tip other mechanics have found and it can be intermittent like you say. you can also check your fuel pressure as that can cause it as well if low. It can also be things like poor electrical connections . Or maybe something is getting into a spark plug occasionally. your best bet if you have done the checks I have mentioned is to wait until it sets a code or something similar until it becomes a hard fault. see link for fp check

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator
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Tuesday, November 1st, 2016 AT 7:49 AM
Tiny
GIG MARTIN
  • MEMBER
Decided to use old school logic. Misfire turned out to be a corroded ground plate on the coil pack. Over speed issue turned out to be my leaking radiator which kept the coolant from the warmed up engine from circulating through my IAC valve.

Considering what each part does and what input they need to function was the way to go, especially when the symptoms are intermittent and everything seems to be working.

Because there is coolant flowing through the IAC valve body it was easy to warm up the engine and notice the coolant hoses to the valve never felt hot. Then it was easy to associate that with my lea!Kng radiator, considering the IAC valve is the high point in the flow of coolant through the engine.

The coil pack ground issue was made evident simply by removing the coil pack for inspection. I noticed all the metal salts crystalized on the base plate and wondered how the coil was working at all. There was that much corrosion.

Anyways, both codes have cleared by themselves.

Because the car has 122k on it though, I will be changing anyways. The intake manifold gasket, IAC valve, TB gasket (replacing the one I made with a factory one), and possibly the timing belt (if I can get a whole day set aside and standby transportation if I have to go buy parts. I only have the one car and use it to get to work every day).

IMHO, book smart only goes so far and ya gotta know when to put the book down. It is also my opinion and recommendation to consider what symptoms are NOT there. If something is not working but tests OK and should be working. It is "interface". Both my issues were because of " interface".
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Saturday, November 19th, 2016 AT 11:21 AM
Tiny
HMAC300
  • EXPERT
Well you have to remember the computer will automatically richen the mix to take care of minor vacuum leaks so it may have been doing that it can handle a small vacuum line off by richening the mix so that may have been your intermittent what you are referring to.
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Saturday, November 19th, 2016 AT 3:22 PM

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