Smog Failed :( change spark plugs?

[abcxyz]

I live in southern CA. Just barely failed smog check, Hydrocarbons (HC) were high at 15mph (max allowed = 55, my reading was 57).

I ran the car on the freeway for about 15 mins before the test.

Have a 1998bmw 328is.

I'm reading that high HC is caused by inefficient combustion and spark plugs could be the reason. The last time my plugs were changed were 5.5 years ago, 45000 miles ago. HOw long do plugs normally last?

Will I pass if i change them out?

Any assistance would be appreciated.

Also somebody said the bosch multi electrode plugs were no good and to get NGK. Not sure why he said bosch were no good, anyone have any experience with that?

[Manolito]

Depending on what kind of plugs you have in there, 45,000 miles is either getting close to their design life (platinum or iridium plated plugs) or well past their design life (conventional plugs). Either way, now would be a good time to invest in some spark plugs.

It looks like late in the 1998 production year was when BMW switched from 2 electrode to four electrode plugs. All of us have tried four electrode plugs in our earlier e36s and I think the general consensus is that they give little or no benefit. I use 2-electrode plugs in my '97s, and based on my experience, I would probably use 2-electrode plugs if I had a car built after 8/98.

Bosch parts are not necessarily bad parts. Most of them are just fine, but Bosch has lost a lot of its cache with Bimmer owners since the company was sold and much of the manufacturing moved to India and China. Most of us use NGK or Denso ignition parts now. I think BMW is currently using NGK in the new vehicles.

You can pay from $2.50 to $12 each for plugs that are appropriate for this car. They will pretty much all run the same for the first 10,000 miles. In view of the frequency with which you replace your plugs, you should spend $6 to $8 each and get plated plugs designed to last 50,000 miles. You should probably also use anti-seize on the threads, even with nickel-plated threads. If you use it, be careful not to over-torque the plugs, which is easy to do with that extra lubricant on the threads.

With regard to your smog check - does the car idle smoothly? If not, look for air and vacuum leaks. My '97s both got new rubber boots for their throttle bodies last year because they were cracking with age and leaking air. If you pull the boot, spray carb cleaner through the throttle body and clean it up. In fact, you may want to buy mass airflow cleaner rather than carb cleaner and while you are inspecting your air tubes, pull off the mass airflow sensor and spray the cleaner though that as well.

P.S. Make sure your air filter looks good.

[abcxyz]

Appreciate the detailed response Manolito. I don't know what kind of spark plugs were put into my car 5 years ago. I'm going to order the NGK Iridium one then.

Yes, my car idles smoothly. I get good gas mileage too, so not sure what the deal is. Maybe i dind't heat up the car long enough, only drove it about 15 miles before going to the smog check. I was reading one forum where someone was claiming to be a CA smog tech and he suggested over-inflating the tires slightly so the car doesn't have to use as much power to roll, and not going with a full tank of gas. I gather so the gas pedal doesn't have to be depressed as far to get to the 15 and 25mph marks. Seems it would only be marginal at best, but what do i know. Maybe i'll try it. What does having a clean air filter have to do with having high HC reading?

Thanks.

[Manolito]

I would expect that 15 minutes would be a long enough warm up.

Actually, you're right, a dirty air filter should not contribute to high HC readings at 15 mph, it just occurred to me that a good time to look at it would be while the air tubes are off for inspection.

I would not expect over-inflated tires to help with HC either. NOx maybe, but not HC.

Sounds like you have a good understanding of the physics at work here. High HC happens when a cylinder occasionally does not light off at all, or if the flame does not make it all the way through the charge due to lean spots in the charge. This can be due to poor injector patterns, but is more often due to vacuum leaks or a really bad plug or coil. I suspect it could also happen if the VANOS is not pulling the cam clear down to idle position due to solenoid problems or leaking seals. High CO happens when the mix lights off but does not completely burn due to insufficient air or cold cylinder walls. Both HC and CO will be high before the catalyst gets up to light-off temperature.

If it fails again, ask if the oxygen level is unusually high at 15 mph. If so, check the exhaust from the head to the catalyst looking for air leaks. At low load, a leak can actually leak inward, confusing the oxygen sensors and causing them to mess up your mixture. This would be more of a CO problem than an HC problem, but it is worth looking at.

[markdas]

I'd say change the air filter......if you are getting restriction in air flow then it can give you a rich mixture at the inlet manifold.....more fuel = more HC.

Plugs should be a yearly replacement if you do less than 12,000 miles or so.