High idle 92 Dodge Shadow

  • 4 CYL
  • FWD
  • 152,000 MILES
My 92 dodge shadow is idling at about 2200 rpm. We have fixed vacuum leaks and replaced the idle air control and tps. We recently had it timed by a mechanic after we found our distributor was put in wrong, who could only get it down to 1200 rpms then we found 2 different vacuum hose leaks. We fixed those, timed it again and it ran great drove it from south dakota to iowa and back. Its been about a week now and now its idling at 2200 rpm but when put in gear its at 1500 rpms. There is no check engine light and it only does it sometimes. What could cause this?
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Friday, December 3rd, 2010 AT 3:47 PM

1 Reply

Find a mechanic with a scanner, or you might be able to borrow or rent one from an auto parts store. View "live data" while the engine is running. In particular, you want to look at the "AIS steps". That is how many steps the automatic idle speed motor is opened to out of 256. A typical value at idle is 32. If it is higher, that suggests something is causing the engine to slow down and the computer is responding by turning the AIS motor to increase speed. That could be caused by simply putting it in gear.

You can expect to find one of two things. If the number of steps is very high, say 50 - 100, the Engine Computer is trying to raise idle speed in response to some incorrect condition, typically a sensor value. If you find the steps closer to zero, the computer sees that the engine is running too fast and it is trying to bring the speed down. That would be typical of a vacuum leak.

Try spraying water when the engine is still cold, and watch for places it gets sucked in and the engine slows down at the same time. Spray around the base of the throttle body and the bushings for the throttle shaft. Pinch off the large vacuum hose going to the brake power booster. To be running that fast, you will be looking for a pretty good-sized vacuum leak, not just one of the little black hard plastic hoses. If you have cruise control, see if it works. If it can't hold a steady speed, that too suggests a vacuum leak. If it doesn't work at all, check the vacuum hose going to the servo. It should be near the battery. You can also follow the throttle cable to the servo to find it.

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Friday, December 3rd, 2010 AT 4:27 PM

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