Another strange P300 error

[sneitzel]

This weekend I fixed the small coolant leak I had in my car. The flow restrictor had a hairline crack and was slowly leaking.

This morning when I left for work I got an SES light. It blinked a few times then went steady. Then it blinked again...and again...and again...

Instead of going to work I stopped off at AZ to have it scanned. P300 2X, 1 active, 1 stored.

About 2 years ago when I did my cap and rotor (MSD), I did not replace the opti body itself. Knowing that its days were numbered, I had ordered one, just in case. Figuring it had finally gone south after 117K miles, I decided to go ahead and replace it. While I was fixing the coolant leak before I had noticed there was coolant in the weep hole so, new water pump too.

Got it all back together, started first time, small coolant leak at the water neck, fixed. Looked inside and there it was, SES. I cleared the PCM and restarted it. Again, SES, acting just like it did before the opti change.

Here is the strange part. I cleared the code again and started the car without the AC running so the coolant would heat and I could bleed the system. I let it run for probably 30 min. and no SES. Turned on the AC and within 30 sec SES, flashing, steady, flashing, steady. Turned off the AC and it stays steady, ie. no new code. I successfully tested this behavior several times. With no AC, no code. With AC I get SES.

What could be causing this? I was thinking I bumped a sensor or something when I was working on the coolant leak, but which one? Why would it just be happening under increased load from the AC?

Any ideas?

Thanks.

[Injuneer]

The PCM reads the crank rotational velocity via the crank position sensor. Sensor is in the timing cover, near the bottom. The PCM compares the rotational velocity pattern to the know pattern resulting from the cylinders firing correctly. Might have damaged the sensor. Or, there's an outside chance you might have a problem with the A/C compressor that is causing irregularities in the crank pulley velocity, that the PCM is interpreting as a misfire.

[sneitzel]

Well, drove to work, back home, to the gym and back home yesterday and to work today, with the AC running, and so far no SES light.

Go figure....

I am now getting the infamous low coolant light however, how come when you so much as even look wrong at that sensor it goes out???