safulop

OK, here's a story for you. Got my 85 944 smogged this morning. HC parts per million measured 68 and 23 at the two test speeds, which is very good, while NO PPM measured 655 and 750, which is a big fat F.

Car has been used little in the past months, and I forgot to do an Italian tune-up. So, off I go down the highway for a 40 mile round trip. Take the car to a different smog shop this afternoon, and now I have the NO down in the 300s which passes, but the HC is up to 135, which did pass but it is the absolute maximum allowed.

???????????????

Somebody school me in smog, please.

I thought that NO was indicative of catalytic converter, so high NO means your cat is bad or not warmed up or clogged from short trips.

Meanwhile, HC is related more to fuel mixture, so high HC means a bad mixture (though opinions vary whether it is from lean or rich running). This could be caused by a poor O2 sensor or other engine management issues. That's why my mechanic's saying is "weak O2 sensors wreck cats."

Why am I so wrong about this, and how could the same car get such different results on the same day? Do they ever calibrate these machines?

-Sean

krystar

i think u should just visit the second shop from now on heeh

Voith

Last year my bmw had way toomuch NO, and I passed since I am a friend of the shop owner.

This year, same car, same smog machine, every value was very good, and I did nothing to the car except drive it.

safulop

Yes, the phenomenon of NO being bad on unused cars is well known, and it normally goes away after a few miles down the highway or regular use. Good smog shops will ask people first if they have been driving the car a good amount, if it is properly warmed up etc. The real paradox with my car is that as the NO became good (an expected result) the HC (soot) became not good, which I don't understand.

-Sean

FRporscheman

HC is hydrocarbons, as in fuel or oil. Something made your car spit out more oil or fuel than before. Did you top off the oil or anything? When you went in for the second test, was the car fully warmed up?

safulop

The only I did after the first test was drive 40 miles up and down highway 99. The car was more warmed up for the second test, although you always have to wait a while so the car sits a while cooling off, but then they run it for a while, so this has never been an issue before. It is possible that the car burns more oil after a hard run, for some reason.

The second test was done with the AC running, and this also adds load on the engine and the old 944 rad fans run full tilt with the AC switched on, this can tend to overcool the engine when it is not hot out.

These are the only possibilities coming to mind, that could have raised the HC numbers so high after a few hours.

-Sean

kev951

give me all of your readings,
I am a Enhanced area smog tech here in california
I need HC CO CO2 O2 and NOX for both speeds

FRporscheman

Kev, I didn't know you're a smog tech! I'll bring all my business to you from now on! Seriously, my household has 10 cars, 9 of which need biennial smog checks.

safulop

The truth is, I don't have all the results for the passing test, only for the failed test. I have just sold the car, which was the reason for the tests.

I am still curious about the problem because I have another old Porsche that almost failed its last smog check, and I am concerned that these results are beginning to look almost random. It is like "if at first you don't succeed, try again." This seems ridiculous to me.

For the failed test, the numbers read:
15 mph RPM 2849 CO2 14.5 O2 0.24 HC 68 CO 0.21 NO 655
25 mph RPM 2862 CO2 14.5 O2 0.16 HC 23 CO 0.18 NO 750 FAIL

Later when it passed, NO is in 300s for both speeds, HC was 135 at 15 mph

OHNOES

Haha, no way! You're in the same city. I will definitely be bringing my car to you from now on!

FRporscheman

Hey ohnoes, I'm right around the corner, we should meet up and talk cars. I know a few 968 guys close to me but not enough 951 guys.

kev951

i am moving to SD guys, by late august!!! Building my 951 race motor as we speak, I need to post pics. Argggggh!!!!

OHNOES

Hey, absolutely. I love talking shop, sadly my baby is out of commision for the moment, but hey, I'd gladly Exchange info and meet up sometime.

And Kev, sorry to here you're leaving the bay, but hey, at least you chose a prettier destination. And yes!! You must post pics!