Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I'm working on a 95 Silverado, 5.7l automatic Z71. At stop in drive or neutral the engine idle rises and lowers until it "finds" idle. It is totally intermittent but seems to happen when the control system has to increase rpm. Turn the ac on and it may do it or put in drive it will hunt for 10 to 20 seconds. At idle once rpm's are stable it will run fine except pushing the throttle and dropping to idle . At speed no problem. At stop with the foot on the brake it may or may not surge.
Other symptoms I see. 1) I notice the RPM gauge seem to spike faster than the rpm of the motor change, so when the ECM starts to regulate the rpm it looks like your tapping a analog volt meter on a battery. Not smooth. 2) The TBI unit ticks loudly when it starts this RPM oscillation.
What I have done. I have put my scope on the map sensor and the TPS. The EGR has been disconnected for 70k miles but it did unplug the solenoid. IAC and harness replaced. Replaced ground connections at intake. Jumped ground wire to sensor ground. Checked for usual leaks and such at TBI unit, brake booster, and hoses.
What I found. Sweep on TPS was smooth with DMM. MAP responded correctly on DMM. When I checked them with the scope I noticed no dropout in the voltage but there is a fair amount of noise on the reference voltage. It looks like a rectified square wave with induced noise on top of it. The ripple is small but the noise seems to spike from the ripple. It's only about .2 volts and seems constant but a bit more prevalent when the hunting occurs. I put a capacitor on the 5v reference and cleaned the noise up some but it didn't correct the hunting. I felt like I was chasing unicorns at that point. I would expect the ECM to have some filtering on its input. I don't expect to see data here but would expect to see some induced noise. Should this Vref be a smooth DC or a biased square wave?
This acts like and under damped control system as the ECM attempts to regulate the idle if that makes sense. I'm looking toward experience. Any input is appreciated.
Solved. Replaced distributor. The truck started randomly skipping at driving speed after last session diagnosing. Th ignition module has attached Vref and provides signal to injectors and tach. The noise on the Vref , injector ticking, random misfire, and tach irregularities pointed toward a ignition module breaking down. All symptoms are gone.
Brian
I search repair sites on a regular basis and more often than not topics don't have solutions. I feel like if you seek help you should at least help complete the topic, even if you decided to junk the vehicle. It is what makes the forum helpful. I appreciate all the videos, I always pick up something.
I just read your original post and was thinking "distributor". Then I scrolled down and saw the fix. The magnets on the reluctor would deteriate, and the reluctors would crack. Fixed a lot of them by replacing the shaft when I worked at a Chevy dealer.
This truck came back after a few days and was loping a bit at startup until temperature came up. I set the timing at 0 degrees initially. I moved the timing up to about 6 degrees and it had similar behavior to the original issues. The correct timing was supposed to be 0 degrees but as it turned out 3 degrees worked best. It's been a couple weeks and truck behaves as it is supposed to. The point is these symptoms or very similar can occur if base timing is only a few degrees to high
