P0304 code, rough idle, better since code

[charlie3]

96 Ford Contour 2.0 105000 miles
About 6 months ago, engine locked up, would not crank when the plastic idle pulley for the timing belt wedged against the crank. Had a good garage replace timing belt, pulley, and water pump. Also, for about a year, I have to add 1 quart of antifreeze per month.

Now for the latest and have seen many post similar, except no engine codes mentioned in the other posts...

Has been idleing rough, shaking when in drive, almost stalls for 6 months. Engine seems to have good power and acceleration otherwise.

Last week, after driving about 20 miles, the engine just about died (stumble/shake) when pulling out from a stop. I thought I was ready for a tow. But then the check engine light came on and it started running normal. (never stalled)

In fact, now that the light is on, it idles great.

So, I thought a simple scan would tell me what sensor was failed, but the only scan code was P0304 which says cylinder 4 misfired.
I checked the plugs, they look great and re-gapped them. The wires look good, boots clean.
My confusion is....since the light is on, it runs better. This tells me the computer is using default values for some sensor that was detected as failed. But the scan code is not telling me that.
If it was bad wires or plugs, I don't believe the computer could change that.

Hope we can find what is going on with these rough idle problems so many contours have.

[beyondloadedSE]

if you disconnect the battery to reset the pcm and clear the CEL's, does it go back to idling rough?

[pre98zetec]

all zetecs idle rough

[charlie3]

I was able to borrow a snap-on scan tool, ran several tests and passed them all. The P0304 was the only thing in any memory I could find. I ran it at idle before and after clearing the codes with the scan tool. After I cleared the codes, it ran rough again, like before there was a code. Now, of course, I have a P1000 code until the emmisions re-learn.

With the scan tool, I can look at all the sensors, but who knows what are normal numbers for all that data??

Like the other reply, maybe it is just the way zetecs run when over 100,000miles.

[beyondloadedSE]

given the P1000 code, sounds like its time for a new MAF. That would definetely cause bad driveability if thats messed up. Try unplugging the MAF and see if it idles any better on 'limp' mode.

[charlie3]

Working better now for last 3 months.
Timing belt broke and needed to be replaced again. Was done just one year earlier. A year ago, the plastic idler on the timing belt broke forcing the replacement. This year, one of the plastic pulleys on the timing belt broke. My thought...replace all three plastic pulleys when servicing. Advance auto stocked a 3 pulley kit.
Anyway, when replacing, noticed that I had to pick the alignment of the crank, either between the timing marks or just after the second timing mark. I picked the between timing marks. (I didn't want to break loose the toothed gears on the cams.) Now it idles great and runs great. It still has some shake at idle and has some surging at certain light loads, but I am not worried about stall.
I can't be sure, but perhaps a year ago when the shop replaced the belt, perhaps they were one tooth off on the timing belt????