update...I adjusted the valves today, and I have two new pieces of information.

1. The spark end of the cylinder 4 spark plug has carbon all over it; it's black, while the other 3 are white.

2. More urgently, my car will no longer idle. When I have the clutch engaged or when it's in neutral, instead of leveling off at about 1000 rpm, it never levels off and eventually dies. I tried the idle relearn procedure I read (turn on car, hold at 3000, rpm until radiator fan turns on, let idle for 5 minutes), but when I try to let the car idle, it does the same thing--it dies.

Any thoughts would be appreciated.

 

Do a compression test to see if there are mechanical problems with cylinder 4. These engines really don't work well on 3 cylinders.

 

Ok, thanks, will look into a compression test.

Also, did a different idle learn (link), and it worked, so the car will idle now.

 

Did the compression test. here are the results:
cyl 1: 125
cyl 2: 125
cyl 3: 110
cyl 4: 0
I did a wet test afterward, and cylinder 4 was still at 0.

On to the leak down test...

 

Check the valve adjustment, after you turn the engine by hand so cylinder 4 is at TDC, all of the cylinder 4 rocker arms should be slack (not pressing on the valve stem). If that's OK, the problem is clearly inside the engine.

 

Ok, thanks. I adjusted all the valves this past weekend. Would it have been really obvious (even to a noob such as myself) if one of the rocker arms had been pressing on the valve stem? Because I don't want to go through taking the valve cover off again if I don't have to.

If it is determined that the problem is inside the engine, is the leak down test the place to start?

 

Have you checked the mechanical timing? If so, do a cylinder leak down test to see where cyl 4 is losing compression.

 

No, I haven't checked the mechanical timing. I'm planning on doing a leak down test this weekend.

 

Finally did the leak down test. It seems to be an exhaust valve.

 

Timing belt probably skipped some teeth, causing the piston and valve to collide.