Subject: High HC emissions (was: we made it with miles to spare!!) From: "Dr. Bob" Date: Thu, 9 Sep 1999 16:09:35 +0000 X-Message-Number: 17 Jason Butler wrote to the list: <> >> Just in case you all are thinking that my luck must have >> no end, I failed my emissions test with HC readings nearly >> triple the legal limit at 2500 rpm. In speaking with >> Ken Postma, we will check the Oxygen sensor just in >> case it died at sea level and hasn't adjusted the mixture. <> Jason: Just a reminder of what causes different emissions readings and what to do about them-- A high HC reading is the result of poor combustion. The most typical cause is a poor ignition system. HC is nothing more than fuel that makes it out the tailpipe without being burned. So, your first step on the way to compliance will be a complete ignition tuneup: plugs, wires, caps and rotors. These parts are quite reasonable at any of the Big Three suppliers. The only exception to that might be the wires-- still "reasonable" but maybe just a little price shock. Resist the urge to spend on exotic multi-electrode plugs too; their only purpose is to provide extended life on new cars. The Bosch platinums will work just fine. Don't succumb to the temptation to get "bargain" wires either. The care and time required for wire replacement make installing good wires the best investment. Some folks have replaced aging ignition coils as part of the process too, while everything is out and accessible. The CO reading tells you about the fuel mixture. For the most part, HC and CO are unrelated, unless the mixture on a cylinder is lean enough to cause a misfire. Lean conditions result from vacuum leaks at hard/cracked/broken hoses, leaking gaskets and boots around MAF/MAS sensors, and partially clogged fuel injectors. Old vacuum and emissions hoses undoubtedly need replacing just due to age on many cars, so getting all new ones and installing them in one session is not a bad way to go. A poor oxygen sensor might cause a lean condition severe enough to create a misfire, but an engine with reasonable ignition will easily fire with that slightly lean mixture. So, your marching orders would read ignition tuneup as described above, a visual inspection of hoses for vacuum leaks, followed by a listen and spray, and finally a test with a good high-impedance DVM for output from the oxygen sensor. You should also be aware that excessive HC passing to the exhaust is death on a catalyst. They are only rated to react a certain mass of fuel, so extra fuel translates to extra heat, which leads to (hopefully just) a failed substrate, but too often also results in a fire. (!!) No matter-- if you have the high HC readings at the tailpipe, new catalysts are in your future if you want to have the car clean. Often, a carefully tuned 928 will pass without catalysts working, so start off with the basics and see how you do in pre-test mode at the smog check station. Then do the cats as your budget allows. You'll breathe easier. Many owners will tell you that it's a good idea to do a wholesale replacement of maintetanance items when you first get the car. It gives you a new baseline for everything, and will help you avoid a lot of the sniggly little problems that often plague aging cars no matter what the make. The hoses and belts and fluids all fall into this category. If you don't have a solid history of timing belt replacement, now's the time. Don't forget to change the brake fluid too. Good Luck! dr bob From: Dr. Bob [dr.bobf@worldnet.att.net] Sent: Wednesday, August 02, 2000 3:23 AM To: 928 Subject: [928] re: emission test failed les gordon wrote to the list: > >Greetings: >I just returned from my N.J. emissions test and i failed the HC portion. > >My results are as followed: >gas standard reading result >NO 1046 480 pass >HC 140 166 FAIL > >Could someone point me in the right direction. The car runs great all >the time. I really haven't had much problem with it so far. I put new >plugs(platinum} maybe four k miles ago. It could use a new air >cleaner. Could it be a bad o2 sensor? Any and all help will be greatly >appreciated. >I forgot to mention that i have a small(?) vacuums leak but i suspect >that would make it run lean rather than rich. > >Thank you all >leslie gordon >1984sAUTO Les: The HC portion of the test indicates how good the combustion is. Poor combustion passes hydrocarbons unburned (sorry for the redundancy...) to the exhaust. Principal causes of poor combustion are ignition problems and mixture problems. Since CO indicates the mixture, a low or high CO reading would be a hint that the mixture is too lean or too rich, either of which will cause a misfire and resultant high HC reading. Remember to check the CO --upstream-- of the catalysts if you are using it to tune. If the car seems to be running OK without lean surge or flat spots, and the CO reading is acceptable but not too low, focus your attention on the ignition system. You've taken the first step by changing the plugs. If they still look OK, work back to include the wires, caps and rotors. Old wires are a common problem, to the point where 50k or 5 years might be considered a reasonable service life. I know- there are many that have a lot more miles and years than that, but those make a good target nonetheless. Another response suggests the catalyst may not get hot enough during the test cycle. Might be worth a sniff at the sample tubes (if your car has them) to see if the HC and CO before the cat are similar to the readings at the tailpipe. New cats will often help hide another problem long enough to pass a test, but spilling much unburned fuel into them will shorten their lives as well as risk a small fire. So, start with the rest of the ignition parts. It's probably been a while since they were serviced. dr bob