Check the ground wires, that might be why it sometimes won't start also idles strange.

Make sure the ECU is getting backup power, check the backup fuse under the hood. Especially on 1996 or later models, the engine will run poorly for a while after every time the ECU loses backup power and needs to recalibrate itself.

To bleed the cooling system, park the car facing uphill so the radiator is the highest point. With the engine COLD, take off the radiator cap and fill the radiator if necessary. Start the engine and set the heater on hot. Keep watching the radiator and add more coolant as necessary to keep it full. When the engine warms up a little, the radiator should start to overflow. Put the cap back on then. Make sure there is coolant in the plastic tank.

 

Check every hose and connection that ultimately feeds into the intake manifold.

 

Where are the ground wires? What do you mean by checking?

 

There's one ground wire from the battery minus that goes to the frame and then on to the transmission, and there are also small wires that attach to the thermostat housing to ground the sensors and ECU. Make sure the connections are tight and not corroded.

 

Muchos Gracias. I will check it out.

 

I pretty much had no choice but to drive the car with the idling issues. I drove it yesterday and it here's what it did.

I got to work, it was still idling weird (up and down idling) when I put it in P at work. At about midday I went to the auto parts store and when I got back to work and put it in P it was just idling kind of high, but not doing the up and down intermittent thing it was doing before.

Then I drove home, CEL was on by this time, and when I put it in P at home it was idling just normal as I had remembered it before. I drove it this morning and it was again fine and seemed to idle normally.

So before this problem, it had been sitting for 4-6 weeks or so without being driven out of my driveway until I was able to get it started. I will see how it acts on the way home and then check the CEL code. Could the length of time undriven have anything to do with the strange idle?

 

If there is an AutoZone on your way home, have them pull the code with an OBDII scan tool (free service). Otherwise, use the jumper wire method described at the link in my signature. It'll be interesting to know whether the CEL code signifies a fault with the IACV or its circuit.

 

The car ran fine again on the way home and seemed to idle okay again, although it does seem to have some response issues.

My code reader gave two codes: P0505 - Idle Control System Malfunction and P0420 - Catalyst Efficiency Below Threshold, Bank 1...

...what the heck do these mean?

 

Troubleshooting those two CEL codes should identify the problems you are having:

 

Awesome! Thanks, I really appreciate the troubleshooting info. Hopefully my scan tool can help me do these tests. I don't have a tach though, so how do I know RPMs. Also, what is the ECM/PCM reset procedure?