'96 3.8L Windstar Idles too fast.....

[Georgewin]

Hi every one; ever since I changed the PCM a while ago, the Idle speed has been an issue;
When I first start the car it revs very high (2500-3000rpm) and stays there for around 15-20 seconds, then settles down to around 500 to 900rpm (hunts a bit) until I stab the throttle, it then revs up again to 2500-3000rpm (this is with no throttle input) for around 15-20secs, then settles down again. This happens whether or not the engine is hot or cold.
Driving is even worse because it doesn’t throttle back when the car is moving,
only when I stop then after a few seconds it throttles back to normal. By now, which is some months, it should have relearned all the parameters!
Before I changed the PCM, the IAC, TPS, CCRM, were all changed because of an intermittent cut out issue. It doesn’t seem to do that anymore (I think!).
Unfortunately I don’t have the old PCM to troubleshoot.
Is this a PCM programming issue; is there a way of reprogramming these PCMs or at least checking to see if all parameters are OK? Thanks for any advice.

[wiswind]

The only way that I would know to check out your PCM would be to go to a dealership or a good shop that has the expensive equipment needed to do this.
If the CEL is lit, you could have the codes read (get the actual numeric code) and track down a problem from there.

[Georgewin]

Thanks Wiswind;
One other thing I forgot to mention; very occasionally it posts a 171 CEL (Bank 1 too lean). It may be a coincident but this has been happening since the PCM was changed;
Could an air or vacuum leak cause this whole issue?

[wiswind]

Yes, more air = higher idle speed.
The PCM will adjust the fuel mixture to match the incomming air.

Also, the most common cause of that code is a vaccum leak.
It is VERY seldom caused by a defective oxygen sensor, as auto part store clerks often recommend.
The sensor is just reporting the condition that it is seeing.
Finding that leak can be a challenge.
Check over all the vaccum lines, clean the Mass Air Flow (MAF) sensor with MAF cleaner or other non-residue electronic cleaner.
Verfy that you have the stiff line from the breather (front valve cover) to the back side of the big flex hose that goes from the air filter/MAF to the throttle body....in place at both ends.

Also inspect the vaccum line that goes from the top of the upper intake manifold to the PCV valve to make sure that it is connected, and not cracked.
Mine did develop a crack on the inside of the elbow....right on top of the intake manifold.

[Georgewin]

Found no vacuum leaks or split in the lines.
Here is what I have noticed on the IAC voltage from PCM: - when its idling correctly, input voltage is around 3volts. As soon as you touch the throttle valve it goes wild & it shoots up to 13 volts and stays there (whilst revving at 2500 rpms with no throttle input!!) for some time before it calms down again back to 750rpm and 3 volts.
So I'm suspecting the PCM is causing this issue & since the IAC is letting in too much air during these wild moments! it is seen as a leak hence getting 171 & 174 CEL.
Next is to get the PCM checked I guess which I was hoping not to!!

[wiswind]

I would also clean the IAC to make sure that it is not sticking.
If you punch the accelerator and let right OFF....the IAC WILL open up, then close back down......to prevent an over rich mixture.
It is "possible" that the IAC is opening up, as it should, and then sticking...which it should not.
I have used WD-40 to clean mine....although something like SeaFoam "Deep Creep" (which is the spray version) would likely be better.
You hold the IAC so that you can spray into it....and the liquid can drain out.
This will flush dirt out......and leave a lubricating coating behind.

It is worth a try, as the IAC is known to stick, and (my favorite) it is cheap and easy.

[Georgewin]

The IAC was replaced in March, but I did check it again and it looks OK; I even put on the original IAC back on the car incase there’s something odd happening with the new one, but it behaves exactly the same.
This voltage thing is odd, because I only have to move the throttle a fraction (very gently) & it jumps to 13 volts and stays there, meaning the IAC is then fully open & engine is revving like crazy. This really makes the car un drivable.

[wiswind]

OK, now I had this happen on another vehicle, but not on my windstar.
I had a throttle position sensor intermittently cause the idle to go high.
One time.....while sitting at a stop light, right next to a police car, my idle started to go up....and kept going up until I had to shut off the engine.
The PCM should sense if the throttle position sensor goes open or shorted, and light the CEL...and ignore the input, but I don't know about a reading that is slightly off.

Now, I did something with my windstar.....by accident....and it WILL light the CEL, but might be a test you could do.
I accidentally forgot to plug the cable from the Throttle Position Sensor (TPS) back in when working on something.
I took the vehicle for a test drive, I was able to drive it.
The CEL came on, but it drove fairly normal except for when I punched the accellerator, it did not downshift and take off.
You could try unplugging the TPS, and see if the high idle RPM issue happens.

It is a victory each time you eliminate a possibility, you can check it off the list of possible problems.
It is a better victory when it cost little to no money or effort to do so.

I personally know the frustration that comes from trying to track down a problem.

[Georgewin]

With the TPS disconnected there is even less control on the revs (remained at 2500-3000 rpms). It seems with no Throttle Position feed back the PCM commands the IAC fully open (very strange!). BTW the TPS was also an item I changed back in March.

[puumalad]

With regards to vacuum leaks, I've had two separate instances on my '98 where I've had the rubber elbows on vacuum lines get leaks in them. The rubber just started turning to mush and in one instance developed a huge hole on the bottom side (couldn't see it until you felt the bottom of it) and this last time (last month) there were no visible holes in it, but it was sucking air right through the rubber. This last time it was the rubber elbow that connects to the PCV valve. I just replaced it by purchasine the whole vacuum hose assembly that goes to the PCV valve.

If you are getting a Bank 1 lean error but not a Bank 2 lean error, I'd certainly look at something along those lines. Anything further upstream and you'd likely have both banks lean, not just one.

Dale