Hi there my 1997 Monte Carlo Z34 is idling at 1400-1500 rpm as opposed to the normal 600-800. I am pretty sure it is the PCM but my question is if I go and get a used PCM from a junkyard or wrecker will I have to reprogram it or can I just pop it in?

Also as a side question how do I get the Change Oil Soon light to turn off as I just did an oil change today.

Thanks all.

 

Welcome to the forum.

What makes you believe it's the PCM? PCMs do go bad, but not often.

A high idle can be the result of things like vacuum leaks, failed IAC and a few other issues. Have you inspected for any of these? How does the car run elsewise (seem like it has additional problems)?

 

Sounds like a vacuum leak or a bad throttle positioning sensor. I have had both but its more likely its the sensor. Take it out and make sure it isnt bent. Unless the pcm has been messed with that shouldnt be the issue.

 

I have already tested for a vacuum leak and I have replaced the Idle Air Control valve. Honestly I hadn't considered the throttle position sensor, another idea I had toyed with was the engine coolant sensor but I felt it was the pcm due to the after market work that has been done on the car. I just got it used a few days ago and don't know the complete history on the car as the person I purchased it from was the 2nd owner and honestly didn't know anything about the astro start system that had been put in.

 

You may have answered your own problem BUT opened a new one. Since you bought a modded car and DON'T know it's history or what has been done, it's open to a LOT of possibilities. Such as, they may have had enough mods installed to require having the PCM tuned, in that process, someone may have said to increase the idle setting in the PCM. Well, sure, you might be able to put another factory PCM in it and depending on the mods, the car may not run again (such as, I know my car stalls itself out on the factory PCM now).
Perhaps this is a side effect of an aftermarket mod that was done.

Sadly, this is one of the reasons a lot of people don't buy modded cars, because of all the "unknowns" involved.

Do you know if the owner you bought it from had this high idle problem during their ownership? You may end up wanting to see if the DMV has record of the first owner and see if you have any way to get in touch with that person and learn more about what was done to the car.

 

You can NOT just put in another PCM, they have to be programmed to your Vin.

I also dont' think it is the PCM.

 

Sorry I didn't mean to suggest it was modded, I more meant the electrical around the PCM has been fiddled with during the installation of an Astro Start starter system the first owner had installed, but that doesnt even work now. First car and all so this all new to me heh, but if it isn't the pcm or the iac or a vacuum leak, it may be the throttle position sensor? What else could be causing this?

 

Does the car run fine? Or do you notice stumbling or mis firing?

Depending on the problem, the PCM tries to compensate for things and may have increased the idle as part of that compensation.

If you can get a hold of a good quality scan tool, you may be able to read additional info (such as, even though your check engine light is on, has the car thrown any error/diagnostic codes that may point to this.

I would agree that checking the TPS (throttle position sensor) is next in line. I have not seen those go bad, but they sell replacements for a reason. You can typically test a TPS sensor with an analog volt meter (not my strong suit, but it has to be analog as digital does not allow you to view the positions as clearly). A volt meter can tell you if the TPS has developed a dead spot.

Chibi may have more insight to this, but perhaps the MAF (Mass Air Flow) sensor needs cleaned (this is done with a specific cleaner for this task sold at the auto parts store). Not sure if this is a symptom of this issue.

I still have a goofy feeling there is something up with the IAC. I know the Snap-On scanner I typically borrow can allow you to change the IAC position.

Also, what engine is in the Z34? If it's a 3100/3400 I have another idea.

 

The 97 Z34 is the 3.4L DOHC. PITA to work on :P

When you bought it, was a complete tune-up done?

 

I have not wrenched on a 3.4 DOHC.