View Full Version : Smog fail: Hydrocarbons wayy high during idle, Knock sensor code (34)

08-16-2012, 07:23 PM
1992 bone stock (KA24DE), 175K.

Car runs fine besides typical annoying ass nissan timing chain guide rattle. Car had a CEL for a while, checked it and it was code 34 (Knock). I assumed it was just a bad sensor, as they are known to fail and the car was driving fine.

Smog tested it, passed on everything but HC (during idle test) which was literally 2x the pass rate... luckily NOT a gross polluter. Car passed on HC during the 2500 RPM test.

Some of the more likely causes for HC failure that I have seen are:

1) bad cat (I have a CA legal replacement that I am going to try)
2) bad temp sensor, or no signal from temp sensor
3) bad injectors or bad spark plugs

So the problem is that I'm running rich (or something) during idle.

Other notes:

1) recently changed the oil with 5W-30 synthetic.... its pretty thin oil and this is an old car.... Could it be oil burning?

It is logical to assume that oil could cause a Hydrocarbon failure, but I'm not knowledgeable enough about the actual specifics of the testing to be sure.... I would assume that the fix is a drain and fill, and fill it with a heavier weight oil.

2) All other metrics on the test appear to be good, or certainly within spec for a car of this age.

Burn information:
750 RPM CO2 = 14.1%
750 RPM O2 = 0.8%
2500 RPM CO2 = 14.1%
2500 RPM O2 = 0.5%

750 RPM = 245 (max = 120)
2500 RPM = 67 (max = 140)

750 RPM = 0.25% (max = 1.0%)
2500 RPM = 0.60% (max = 1.0%)

3) I recently changed the radiator and Temperature sensor.... Perhaps the temperature sensor is/was bad.

4) OCCASIONALLY, the car's idle will drop from a normal 500-600 RPM down below 500 RPM... This doesnt happen all the time, but it does happen occasionally.

In the past I have associated this with loose/poor electrical contact with certain sensors (because it typically fixed the problem by literally unplugging and firmly reseating the plugs in question; the MAF plug and the plug on the CAS).

Any suggestions would be appreciated.

My plan is to double check all wiring (specifically wiring associated with the engine temperature sensor) and the knock sensor (its a huge pain in the ass), and also to replace the catalytic converter.... Even though it probably won't help for an Idle failure.

08-17-2012, 12:13 AM
Change your cat, don't change it to a used one, just go buy a new one as they are under $200. And 500-600 is not normal idle, it needs to be 700-800. Check your EGR and BPT, change your cat, fix your idle, make sure your spot on on timing no +/- at all and try again. Oh and

08-18-2012, 06:42 PM
Rubbing alchohol in gas tank
Open fuel injector circuit with electrical tape.

Unless they scan OBD 1 where you live, you'll pass.

Be careful how much alcohol you put in your tank though. Have half-1/4 gas and two bottles of rubbing alchohol should work. Make sure to fill your 240 up with E91 right afterwards though.


08-24-2012, 02:35 PM
what does the rubbing alky do again? i am going to visit the smog man in about a week so any help i get is much appreciated. thanks KOME