Hello!

I have a 1988' Volvo 245 DL Wagon that I've owned for about five years and has passed smog many times before. Today it failed nitrous oxide. It was close at 15mph, but failed 25mph test by about 70ppm (not much). Also, they said idle was high, at 1k rpms. Idle is supposed to be around 700-750 on this car according to my repair book.

Background-

Car has passed smog MANY times. I had a shop do my motor mounts recently, and they alerted me to a header pipe crack. I suspect it cracked from them jacking the engine up to do the mounts or from the header bending form the shot mounts, and sagging engine but c'est la vie, right? I understand header pipe leaks can greatly effect your NO reading, but I'd really rather try and make up the 70ppm from somewhere else because there may be other cheaper ways to reduce emissions. FYI, I replaced my radiator last year, and put it back together without the fan shroud, so it runs SLIGHTLY warmer than normal around town, lower rpms, idle and 0-50mph, which is where they check for smog.

1. I checked for vacuum leaks at basically every hose I could think of using starter fluid with the engine at idle. I didn't find any leaks, although I may have missed something. I don't really know the important spots to check, I was just spraying around the throttle housing/intake/fuel injectors/etc.

2. Tried to check ignition timing with my timing light. At the front pulley, where you're supposed to aim the light, there is this plastic piece with some markings that I don't understand. There are some lines that you're obviously supposed to correspond the pulley line when lit with the timing light, but they aren't marked in a way that I understand. I know the engine is supposed to fire at 10-14 degrees BTDC at 750rpms. At any rate I don't really understand where you adjust to change the timing, the repair book I have doesn't go into enough detail at all.

3. I pulled the dist cap, and examined the rotor, which is definitely a bit corroded. But I couldn't remove it. Seems to be stuck. Would replacing the rotor do anything anyways? My dist cap looks great, and plug wires look great. Plugs are probably kinda old, who knows. They always look fine; gray and healthy.

4. I twisted the idle screw all the way to the right to lower the idle as much as it would let me. Then I adjusted the top throttle cable in so the butterfly throttle mechanism touched it's stopping point, the engine seems to idle a bit lower but I have no tach so I can't tell.

5. Mass Air Meter or something was repaired right after I got the car five years ago. OS sensor couldn't be that bad with the rest of the emissions doing fine?

6. Cat convertor should be fine, because the other emissions were well below their max points. My guess.

Ya. So I'm a bit stumped. Any advice would be awesome. Thank you!