Emissions Failure - High HC

Emissions Failure - High HC

Jason Butler wrote to the list:

>> 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.


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

Dr. Bob's post on this was very enlightening... wanted to add my own experience... I recently had a fuel brain crater on me, and it caused exactly the problem dr. bob was advising the other lister on, namely triple the regulated emissions of HC... we tried everything else BUT the brain, with almost no results, and I was told by an old timer at a dealership shop that the oxygen sensor failing would NOT cause HC readings to triple... so, after doing all the other stuff, like mass flow meter/plugs/wires/rotors/caps/etc, we stuck a new brain in there and PRESTO! instant compliance, with readings below half of what is necessary in Harris County/Houston, Texas. Turned out my lambda sensor worked fine after all...we unplugged it during the post brain test and it did change emissions for the worse, plugged it back in again and they went back down...

I'm no expert like dr. bob (I had to read his post three times to get it. My brain is failing too) but if your emissions are three times what they should be in an electronic injection car, and it has 100,000 miles or thereabouts on it, your fuel brain is probably dead. I'm told most of them fail at around this mark. I am also told they have a default setting they go to when they fail which is very rich to ensure the car will run under all conditions and that this is what causes the emissions to go so high.

Less than a grand if you shop around

Dave Perkins
'87 928S4

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