I am looking for advice from the Crossfire experts out there as the whether the behavior from my ‘82 described below is normal and, if not, what I might pursue to correct it. I apologize in advance for this long thread. RPMs referenced below are estimates not precise.

When the car is started cold (like sitting overnight), the initial idle is about 1200. If left to idle, within 3 minutes-you could literally set your watch by it-the idle will have settled down to about 800 (which in my mind is “normal” or at least a number I am happy with). Put it in gear at that point, and the idle will be about 600-again “normal”. Once driving, if the car does not shift past second gear-like when youre leaving your neighborhood at low speed-it will drop to idle when I remove my foot from the gas and rest at 600 if you come to a stop.

HOWEVER…once the transmission shifts in to third, removing my foot from the gas leaves idle at about 1200, it will literally continue to propel the car down the road at a steady speed on a flat road with my foot off the pedal. If I come to a stop, the idle is about 800 but I am noticeably holding the car with the brakes against the partially open throttle. This will persist until I come to a complete stop for at least 20 seconds…after 10 seconds at a stop the rpm will start to relax, and by the time 20 seconds has passed it is back at 600 and not trying to push against the brakes. Once it settles back this second time, it will continue to have the proper idle behavior until the car is shut off again. If I never come to a stop for 20 seconds for it to settle, the idle will stay up for as long as the trip lasts until I get that 20 second stop-I have gotten straight on the highway after starting, driven significant distances, and when I finally come to a stop it will still be high until it has the 20 second rest period.

When I start the car warm, the idle will be at 800/600 in gear, but the same process starts over again once it gets to third gear…up to 1200, hangs there until it gets the 20-second rest period to settle back.

Once the car settles to normal idle, it runs beautifully. The throttle bodies have not been altered from factory settings. The temp sensor and O2 sensor are new. No codes of any kind. The car is totally stock including emissions equipment. Since all of this is a very methodic process, it does not seem likely that it driven by a vacuum leak or some other random fault-it happens the same way, every time, on a very consistent timeline.

Does anyone have any thoughts? The temporary high idle after hitting third isn’t a serious problem, more of an annoyance, but if I can make it go away I’d like to.

Thanks

 

I had almost the same issue a couple of years ago. I can't remember whether it was replacing the Engine Coolant Temp sensor or replacing the IAC motor on each TB. Neither repairs were very expensive and pretty easy, especially the IAC's.
That said, be sure to also run an ECM self-test to rule out any other low hanging fruit aa there are other potential culprits.
Good luck and check back to let us know what fixed it.

Dave

 

This almost sound like a thermo/ vacuum switch transducer. Heats up, opens vacuum switch and lets in AIR instead....high RPM is usually a vacuum leak. But you don't have the symptom until after warm-up.

 

I think that’s how they are programmed through the PROM Chip . A drive with a Scan Tool to verify the computer is commanding the IAC to open during de-accelerating would verify for sure, it’s been a while since I’ve had some Cross Fires here

 

Test the IACs, TPS, and coolant sensor...Then...Start looking for a ECM..........5550 (None out there that I could find), 6026 (Very few still out there) or upgrade to a 7747 (Best option).

 

Your ECM did not cause your idle irregularity. Useless to change it for this reason. But, on an entirely unrelated topic, I did install a Hypertech ECM chip that made a noticeable improvement to the transmission's shift performance which was originally calibrated with emphasis on achieving early/lugging lockup in higher gears entirely for CAFE- improvement reasons.

 

Based on what? Your gut feeling? Only one code for IAC that almost never sets. High resistance in IACs can damage the ECM and has the symptoms the OP describes. Testing the three sensors and their circuits will determine if he needs to replace the ECM.

 

How about a little history. Did this just begin, has the car sat for a while, recent engine maintenance, transmission alteration, fuel/TB adjustments, ECM changes, etc?

 

Ive only had the car a few months. Bought from a guy that seemingly knows very little about cars in general let alone the Crossfire challenges. Car had not been driven much in recent years but is bone stock. It needed some sorting when I got it but now it seems Ive straightened everything out except the issue I describe. I replaced EGR, temp sensor, O2sensor, both IACs (today), set timing per spec, and have verified no vacuum leaks. The car runs perfectly other than what I describe in my opening post. It is my daily driver.

Maybe this is just how they are programmed.

 

The absence of Engine Codes indicates that the ECM is happy with the engine operation. The ECM regulates the fuel delivery and hence the engine speed based on received information from engine sensors. The conditions causing this issue are predictable and constant. SO, there is an issue somewhere- either a failing component or an adjustment necessary. First, vacuum is critical to CF operation. When TESTING, block vacuum to all non critical components (ie headlights, etc), as well as checking all hoses for poor connections and replace hoses if doubtful. Also check manifold fitment and sealing. The Throttle Position Sensor should be checked- possibly the ECM is getting a faulty reading - thinking you have the throttle depressed under specific conditions. Fuel pressure is also critical to CF and can cause non-fault operational problems. Fuel filter is cheap and easily replaced (especially if car has not seem continual operation- and get fresh premium fuel). When diagnosing ECM issues, it is beneficial to Reset the ECM (remove 20 amp fuse for 10 seconds- always with the ignition key OFF) when making adjustments or replacing electrical components.
You may be concentrating on Third gear shift, when vehicle Speed is the real issue? What if you shift to Neutral without reducing speed? Disconnect the Cruise control. When you set the timing, did you disconnect the Brown wire behind the Distributor?
Well, just a few thoughts- which may get you thinking. Keep us informed. Just remember, with CF, any adjustment will affect the entire system.

 

it is not vacuum.

I have trued resetting the ECM.

Yes, I know how to set the timing properly.

I havent changed fuel filter but It seems quite unlikely a methodical, consistent, constantly repeatable issue like the one I describe is fuel pressure related. The car runs perfectly once the idle settles.