Engine Surging in park and neutral - Please help!

cgray · 07-13-2005, 06:14 PM

My 1999 Saturn engine surges when it starts or goes from a drive gear into park or neutral. After about 10 seconds it calms down. When i shift back into gear, the engine surges again. I've already replaced both O2 sensors, cleaned the EGR Valve, installed new plugs/wires. Could it be the TPS? If i UNPLUG the battery for 30 minutes and then reconnect it and start the motor, the engine runs fine for about 2 days. Then the surging happens again. It makes me think that it's something in the electronics relating to the PCM or something. Could it be the Coolant Temperature Sensor? Please help.

**Ssicarman** · 07-14-2005, 03:22 AM

Welcome to the board.

Have you cleaned the throttle body and the air bypass passage?
Also check the PCM engine coolant temp sensor. See if it has leaked coolant into the connector and corroded things there. If it has not damaged the connector then removed the sensor and check for any sign of the plastic tip (if it has not been updated to the brass tipped one) having cracked. If crackd or leaking replace it. If the connector has corroded terminals replace it.

cgray · 07-15-2005, 03:28 PM

I replaced the PCV Valve, the vaccuum hoses, and the Engine coolant temp sensor. The old coolant temp sensor DID have a slight crack in it, but the connector wasn't corroded. After starting the car, the surging has now gone away. How does the coolant temp sensor affect the surging?

However, now the car seems to idle low and miss when the air filter/air intake plastic hose is connected. If i disconnect it from the throttle body, the car idles fine. Should i clean out the throttle body? When i did my tune-up, i changed the plugs and wires, but left the coils alone. Should i check those and/or replace them? Any thoughts here?

PurdueGuy · 07-15-2005, 03:31 PM

Quote:

Originally Posted by cgray

Should i clean out the throttle body?

Can't hurt, might as well.

cgray · 07-18-2005, 09:22 PM

I checked the temp sensor and the probe DID have a small crack in it. I replaced it and the surging stopped. I then cleaned the throttle body and air bypass passage and the car runs smoother than ever.

**Ssicarman** · 07-19-2005, 02:49 AM

Thanks for the update.

The reason that the PCM coolant temp sensor affects the engine so much is that it is the way the PCM finds out what the engine temp is at and it uses that temp as a basis for the fuel delivery and and other control systems to kick in or not. If the PCM thinks it is one temp but the engine is really another temp strange things will happen. The list of things that it can effect/cause is long.