← Back to list

1996 Chevrolet Z28 Auto: P0507 Code - Idle RPM Too High, Solutions & Fixes

Model: 96Z28 Fault Code: P0507 Posted: 2006-11-22 15:00

I own a 1996 Chevrolet Z28 with an automatic transmission. The vehicle is currently displaying OBD-II diagnostic code P0507 (Idle Control System RPM Higher Than Expected). This issue has persisted for some time. The only way I've been able to temporarily resolve the code was by manually adjusting the idle screw to lower the engine RPM—though this is not recommended as a long-term fix. After doing so, the idle became unstable, fluctuating between 50 and 75 RPMs with a typical hangaround of around 750 RPM, which is elevated but still within acceptable range. I've already performed several repairs including replacing the Idle Air Control (IAC) valve, Throttle Position Sensor (TPS), rubber intake elbow (to fix potential vacuum leaks), and the throttle body (TB). I also replaced the engine control module (ECM), conducted a smoke test for vacuum leaks, installed new TB gaskets, and cleaned the fuel injectors. Despite these efforts, the P0507 code remains. I've considered having the PCM reprogrammed by Bryan at PCMforLess, who confirmed he can clear or modify the code. However, I'm seeking community input before proceeding with that step. The car runs smoothly and has good fuel economy (approximately 14 MPG in city driving), so the underlying performance isn't compromised—only the idle stability and diagnostic code persist. I've also tried a known IAC reset method: pressing the accelerator pedal halfway, holding it, cranking the engine, releasing after 5 seconds, turning off for 10 seconds, then restarting. This helped slightly but requires further validation. Any insights or suggestions on potential root causes—such as EGR valve status, exhaust leaks near O2 sensors, or ignition timing—are greatly appreciated.

Related fault codes

P0507

Comments (6)

Anonymous 2006-11-22 16:31

Could a stuck half-open EGR valve be causing the idle to spike? Even if it's disabled in software, physical blockage or partial operation can affect idle stability. Also, check for engine surging—this could point to an underlying sensor issue. I'd recommend rechecking basic components first before relying on PCM programming. A scan tool may be needed to relearn base idle settings. Ignition timing should also be ruled out as a contributing factor.

Anonymous 2006-11-22 17:15

The EGR valve is still present but has been programmed off; the exhaust pipe connection between the intake and headers was sealed before header installation, so that's not the source. The idle issue began prior to the header change. While the RPM is only slightly elevated now, manually turning down the idle screw helps—but the idle remains rough, fluctuating between 50–75 RPMs with a typical value around 750 RPM. I've tried the IAC reset method (pressing throttle halfway, holding it, cranking, releasing after 5 seconds, shutting off for 10 seconds, then restarting), which showed minor improvement. I'll need to drive it further to confirm if this resolves the issue long-term. Has anyone else experienced success with this IAC reset technique on a 96 Z28?

Anonymous 2006-11-22 20:48

It's possible there’s an exhaust leak near one of the oxygen sensors—this could cause incorrect fuel trim readings and lead to erratic idle behavior, even if the engine runs well.

Anonymous 2006-11-22 21:30

I’ve already replaced both O2 sensors. I’ll perform a thorough vacuum check to rule out any exhaust or intake leaks near the sensor locations. Thanks for the tip—appreciate it! Have a happy Thanksgiving, and I'll update with results later.

Anonymous 2006-12-04 10:57

I don’t know why it resolved after my last attempt—I drilled out the TPS adjustment holes to allow full movement, then turned the TPS back slightly clockwise. After restarting and driving briefly, the code remained but idle improved slightly. I then adjusted the idle screw upward to smooth out the idle—still had the code. Next, I removed the throttle body and took the IAC valve apart. Using a small piece of 300-grit wet/dry sandpaper on my finger, I manually smoothed the IAC seat inside the hole. After blowing it clean and reassembling, I performed another IAC reset (started with throttle partially depressed, released after 5 seconds, turned off for 10 seconds, then restarted). The idle became smoother. After driving a bit, the code persisted—then, after slightly loosening the TPS again (just a small turn clockwise) and tightening it back down, the P0507 code disappeared and has not returned. Idle is now stable and smooth. I have no explanation for this, but it worked—very surprising given how many components were already replaced.

Anonymous 2009-02-28 10:44

For others experiencing P0507 on a 1996 Z28: reference this resource for detailed code explanations — https://www.obd-codes.com/p0507