Hello everyone.

Towards the end of last fall my 92 Camaro with a 305 started having a rather odd issue where the idle surges when the engine is hot and not under load.

When it's idling the RPM will suddenly drop from normal then surge back up. It will then idle normally for a bit before surging again. This happens regardless of what gear the car is in. Any kind of load on the engine will eliminate the idle surge, such as turning on the AC/heater or raising the RPM above idle - even if only by a hundred. The hotter the engine is, the more often and harder it surges. Cooling system works just fine and temps stay within normal ranges. Car starts with no issues. Acceleration and driving seem to be unaffected.

Things I've looked at/tested/swapped since the issue started:

• Topped off the AC - thought it could be caused by the compressor clutch cycling on and off due to low pressure. It was a touch low so I topped it off but there was no change to the idle issue. Idle surge does not coincide with the AC compressor cycling when AC is not in use. Idle does not surge when AC is in use.
• Verified that the thermostat opens and fan cycles at appropriate times.
• Changed the spark plugs and wires with new ACDelco parts. All of the old plugs looked good if worn - no unburnt gas and no excessive carbon or melted electrodes.
• Changed the IAC with a new ACDelco part.
• Changed the intake air filter
• Changed the PCV with a new ACDelco (didn't think it was the cause but the old valve was clattering so I changed it out)
• Used a battery tender to ensure the battery is at the proper voltage

Things I've changed within the last 5 years, prior to the issue starting:

• New fuel pump
• New stainless steel fuel lines
• New fuel pressure regulator
• New injectors
• MAP sensor
• ICM on the distributor (can't remember if I've ever replaced the coil, though)

I'm at a little bit of a loss as to what the issue is and I don't want to just throw money at it. Any suggestions as to what I should look at next?

Thanks!

 

Sounds similar to my situation. Long story short, I replaced my ECM. Problem solved for me.

 

All those symptoms match up really well to how a vacuum leak behaves.

 

Check your alternator and grounds.

 

Wouldn't a vacuum leak cause the rpms to surge instead of drop?