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87 930 87 930 is offline
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Join Date: Nov 2007
Location: Hesperia, Ca.
Posts: 156
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bcoats; We need to know your HC, CO, NOx, CO2, and O2 readings to get a complete picture. Here are some general thoughts on high HC with normal CO conditions.

Excessive HCs are caused by incomplete combustion where not all of the fuel (which is liquid HC) is consumed. Most often it's an ignition system problem (EG; ignition timing, coil, wires, cap, plugs, etc.). It can also be a low compression problem (worn engine, mistimed valve events, improper valve lash, worn cam lobes, worn guides) where low compression fails to "heat up" areas of the air/fuel mixture and the flame gets extinquished too early and leaves behind unburned fuel. Are your cams timed correctly?

Along with ign probs, a huge cause of high HCs is a misfire. A misfire is when proper ignition and complete combustion don't occur. Some combustion does occur; it just didn't finish completely. Misfires have many causes. Misfires dump HCs into your exhaust stream, often with excess oxygen. A vacuum leak is a very common reason for high HCs because the AFR will be lean, and the lean mixture resists proper flame propagation and causes a misfire. CO readings are often okay.

I'm betting that your problem lies in a misfire condtion cause by...1) a vacuum leak that's leaning out the mixture a bit and causing a slight misfire, or MORE LIKELY, 2) when they adjusted your CO, they leaned it out so far as to induce a slight lean misfire. What are your O2 readings? They're your best indicator of rich or lean. Your O2 sensor would help adjust for a small amount of excess O2 (lean), but it only "fine tunes" the a/f mixture. An improper idle mixture setting can't be addressed by the O2 sensor. If your O2 sensor sees excess O2, it may add a tad bit more fuel keeping your CO level normal.

Are your R or L? If your air pump is still hooked up, the oxygen readings should be around 5-6%. If you don't have an air injection system, a bit lower. If you're too rich (too low on O2), don't go chasing after a fuel problem yet! Your CO was just adjusted and should be fine, and CO problems are fuel issues (meaning you're rich or lean).

ALWAYS, fix CO fuel issues first, then pursue HC issues. HC levels react to CO levels. (HCs usually go up when CO goes up, but can go up when CO goes down too far.)

High HCs with normal CO and excess O2 (or maybe close to normal O2) usually means ignition problem, misfire, or mechanically maladjusted or tired engine.

When you make adjustments to one of the gasses, you change the others.

Give us more info so we can narrow your problem down. I've grossly simplified my explanation based on the limited data that you've given. (And thanks to the three people that are reading this boring reply!)
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Scott - 1987 930: 3.45L Supertec engine w/GT3 crank, California Motor Sports upgraded 930 trans, modified suspension, Zuffenhaus 9's and 11's, black on black..."The Black (Financial) Hole"

Last edited by 87 930; 11-06-2008 at 07:42 PM..
Old 11-06-2008, 07:36 PM
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