Failed smog

I took my 91 4.0 in to smog the other day and I passed everything except HC PPM at idle. At idle, it was reading 403 PPM well over the 120 limit. But at 2500RPM, it was reading 19 PPM out of the 180 limit. He said it was getting worse as it heated up too. I did a oil change, swapper cap, rotor, and plugs before the test. I used some smog-test fuel system cleaner too haha. I've heard it might be O2 sensor, knock sensor, maybe vacuum stuff...I haven't replaced wires either.

The main thing I'm curious about is why the hell one set would be so high, while the other is so low.

Could just be the o2 sensor. I would use a spray bottle with water set to "stream" and spray around the intake manifold at the gasket and the base of the injectors to check for intake leaks that would cause it to run rich, don't forget the underside.

May just be your o2 sensor, but the leaning out more as it got warmer could be an increasing intake leak as the intake manifold/head expanded from the heat.

You should run the engine up to full operating temperature before trying to pass the test.

Alright...I replaced the O2 sensor, installed headers (basically stainless factory stuff) and put on new plug wires. The old manifold was cracked, and I made sure to remove all gasket material when I reinstalled and of course a brand new gasket (including the one at the pipe). Took it to test...NO DIFFERENCE!! WTF?? This is whats going through my mind now...

Head valves--mechanic said they were going, Doc said it might do it.
Cracked head/block--yeah..don't like the thought.
MAP sensor--?? heard it a couple times, maybe?
Cat-converter--Doc said it might be the problem, and the mechanic said it was heating up faster than it should.

Any ideas anybody?? I love working on the thing, but I'm a student and have very little money, so I can't just shoot blindly. I need the car for work (hopefully firefighting this summer) and so I can get to school. I know a guy with some good diagnostic equipment, I've been thinking of going to him, see if he can help...

You could do a compression test on the cylinders and a leak-down test for the valves. (may have to have a shop do it) That way you'll know where you are with the motor.
If you're not in CA, you can get a flowmaster universal cat for about $60 through amazon.com. They are nice and small too. I think it is model #91005.

What do the plugs look like? Pull them and keep them in order, and let us know their condition.

After 20 minutes of operation, what does the temp gauge indicate?

HC = unburned fuel. Unburned fuel can be the result of a number of things--poor engine condition, poor combustion, running too rich.

If the cat is heating up faster than it should, you have a problem with too much fuel in the exhaust IMO - why is the question. Injectors could be leaking, valves could be leaky, or maybe you aren't entering closed loop mode. How does the exhaust smell? How does it idle? Does it heat up to op temp (210 or so) within 15 minutes and stay above at least 180-190 even on the highway?

If it's not the injectors or valves and you're reaching proper temps and thus entering closed loop mode, maybe the exhaust manifold isn't sealing properly against the block which would leave you with an air leak upstream of the O2 sensor, which makes the ECU run the engine rich trying to compensate.

I agree with the compression test. It's always good info to know.

EDIT: bah, beaten by Joe! :cheers:

Hey guys, thank you so much for your replies. I really appreciate the help. I found out #1 's dead, here's the new thread if you want to follow :

http://www.naxja.org/forum/showthread.php?p=244905587#post244905587

how's it run? Idle rough at all? You could just pull the plug off the injector for cyl #1 and likely pass smog but that will cause the exhaust to read lean, so the other cylinders will end up running richer than they should I think... not sure I'm reasoning this out right.

You can probably pick up an engine for a few hundred used, just make sure it's decent before you buy it. Spend a bit of time swapping it out and see what happens.