P1506 code and intermittent IAC valve failure

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.

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).

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?

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".