Failed smog for first time, not sure what to do.

[george graves]

Good post by e30devo.

Not so much on fuel injection BMW motors, but a super rich mixture can add so much fuel that it "washes" the oil out of the rings, causing galling on the cylinder walls. Too lean, and all the extra oxygen will eat the hot exhaust valve.

A worn out catalytic converter can be limped on for a smog test if you get it really hot. The hotter the better. Like 3'rd gear on the free way for 5-10 mins, and rev the hell out of it and they put it on the test as soon as you pull in.

[McGyver]

Ok, so a little off topic, but I've been having an intermittent misfire that is preventing me from being able to smog the new car. I have noticed that the car drives fine in the morning to work, but tends to intermittently stumble under load on the way home. It doesn't matter if it's light load of just maintaining speed, partial throttle of speeding up normally, or WOT. Sometimes it misses, other times it doesn't.

So far, I have done a timing belt, water pump, valve adjustment, thermostat, Bremi cap/rotor, Bosch WR8LC+ plugs, air filter, and Bosch CTS. I've tried a known-good ECU and smoke test looks good. No codes/warning lights. I've gone through 2 tanks for Chevron/Mobil premium.

I intend to give it an oil/filter change. Check the new plugs for fouling (some may be have been fucked up in shipping), plug wire resistance, TPS switch, and maybe toss in a different coil. I also have a spare AFM and O2 sensor I could try.

Any other thoughts on what I could be missing?

[e30devo]

Ok, so a little off topic, but I've been having an intermittent misfire that is preventing me from being able to smog the new car. I have noticed that the car drives fine in the morning to work, but tends to intermittently stumble under load on the way home. It doesn't matter if it's light load of just maintaining speed, partial throttle of speeding up normally, or WOT. Sometimes it misses, other times it doesn't.

So far, I have done a timing belt, water pump, valve adjustment, thermostat, Bremi cap/rotor, Bosch WR8LC+ plugs, air filter, and Bosch CTS. I've tried a known-good ECU and smoke test looks good. No codes/warning lights. I've gone through 2 tanks for Chevron/Mobil premium.

I intend to give it an oil/filter change. Check the new plugs for fouling (some may be have been fucked up in shipping), plug wire resistance, TPS switch, and maybe toss in a different coil. I also have a spare AFM and O2 sensor I could try.

Any other thoughts on what I could be missing?

When it misses, is it consistent.. like seems like every cylinder or multiple cylinders are missing?

Or does it seem like only 1 cylinder missing per RPM?

Is it pinging? like pre-ignition? knocking? I would attribute this to carbon build up, and heat later in the day... but dont jump straight to the sea foam thing until youve got it really pinned down as the cause.

Another test for spark plug wires is simply using a misty spray bottle with water and spraying the wires while running to see if they .. kind of arc.. outside the wire.. this is a pretty old school thing.. someone correct me if im wrong. There is a sound associated with this as well.. crackling or something.

I seem to remember checking and gapping spark plugs being important, although that wouldnt lead to anything intermittent.

You could T in a fuel pressure gauge and tape it to the windshield to be sure you are getting enough fuel pressure when it is missing, later in the day could mean more heat and hesitating fuel pump.

[McGyver]

When it misses, is it consistent.. like seems like every cylinder or multiple cylinders are missing?

Or does it seem like only 1 cylinder missing per RPM?

Is it pinging? like pre-ignition? knocking? I would attribute this to carbon build up, and heat later in the day... but dont jump straight to the sea foam thing until youve got it really pinned down as the cause.

Another test for spark plug wires is simply using a misty spray bottle with water and spraying the wires while running to see if they .. kind of arc.. outside the wire.. this is a pretty old school thing.. someone correct me if im wrong. There is a sound associated with this as well.. crackling or something.

I seem to remember checking and gapping spark plugs being important, although that wouldnt lead to anything intermittent.

You could T in a fuel pressure gauge and tape it to the windshield to be sure you are getting enough fuel pressure when it is missing, later in the day could mean more heat and hesitating fuel pump.

It's not a consistent misfire. So it will drive just fine, then start to miss. If I hold the throttle it will continue to miss, if I go WOT it will just miss "harder". However, when it starts to miss and I let off the throttle (TPS hits "idle"), then apply throttle, the miss sometimes goes away. Sometimes not.

When it misses, I feels like 1 cylinder (or maybe more) is missing it's combustion cycle. No pinging/knocking.

Good call on the fuel pump. It's not making any noise, but it might also be down on pressure once it warms up.

The exhaust smells a little stinky, which I attribute to it not running right. So for the car to run correctly, I need to check:
1) Fuel - pressure, filter, possible faulty pump, possible clogged injector
2) Spark - Fouled plugs, bad wires, bad coil
3) Air - already has a new filter, no vacuum leaks detected
4) ECU - Crank Sensor, Throttle Position Sensor, Coolant Temp Sensor, AFM, Ambient Ait Temp, and O2 sensor

Am I missing anything?

[e30devo]

It's not a consistent misfire. So it will drive just fine, then start to miss. If I hold the throttle it will continue to miss, if I go WOT it will just miss "harder". However, when it starts to miss and I let off the throttle (TPS hits "idle"), then apply throttle, the miss sometimes goes away. Sometimes not.

When it misses, I feels like 1 cylinder (or maybe more) is missing it's combustion cycle. No pinging/knocking.

Good call on the fuel pump. It's not making any noise, but it might also be down on pressure once it warms up.

The exhaust smells a little stinky, which I attribute to it not running right. So for the car to run correctly, I need to check:
1) Fuel - pressure, filter, possible faulty pump, possible clogged injector
2) Spark - Fouled plugs, bad wires, bad coil
3) Air - already has a new filter, no vacuum leaks detected
4) ECU - Crank Sensor, Throttle Position Sensor, Coolant Temp Sensor, AFM, Ambient Ait Temp, and O2 sensor

Am I missing anything?

Aside from compression.. but i dont think this sounds like anything compression related, compression issues are consistent because they are a physical mechanical issue. Even things like a slightly burnt valve, or bad valve guides i dont think would be intermittent.

That sounds like a solid plan, but this sounds textbook for poor spark under load. Jumping a spark plug gap is pretty hard, so if the electricity has anywhere else to arc to, it will do so.

Since you've replaced almost the entire ignition system, the wires are now the weakest link.

[McGyver]

That sounds like a solid plan, but this sounds textbook for poor spark under load. Jumping a spark plug gap is pretty hard, so if the electricity has anywhere else to arc to, it will do so.

Since you've replaced almost the entire ignition system, the wires are now the weakest link.

Yeah, and I have at least 2 sets of wires to play with. I know it's going to be something dumb and easy to fix. I just need to get it done so I can smog and drive without feeling nervous.

[McGyver]

Good news! I tested all the stuff, everything seemed to test within spec except my spark plug wires. I had difficulty getting a resistance reading from the #2 wire, which I figured was the cause of my intermittent misfire. I changed to a set that tested good and the car runs like a champ now! I also did an engine flush and oil change.

Went to smog yesterday and failed the pre-test. Plugging my numbers into a lambda calculator indicates the car is running with a Lambda value of 1.003 and CAT efficiency of 0.00%. I'm going to swap exhausts with one that has a good cat and will attempt again.