92VX High HC

[GasSavers_vxhatch]

I have trouble getting my VX thru Maryland emissions.
Emissions are every 2-years here and this is the 2nd time the VX has been tested. First emission test, right after I bought the car, it was "driven" by the emissions station attendant on a chassis dyno, following a speed profile (multiple accelerations, periods of cruise and decelerations) presented on a screen to him. Before the test completed, the screen changed to read, "Sample dilution detected - test aborted." After this happened a 2nd time, the operator tapped on a keyboard, motioned for me and to my surprise said you passed.

Flash to this year?s test, no chassis dyno. I was asked to stay in the car and to hold the RPMs at 2500 in Neutral while tailpipe emissions were monitored. Failed ? high Hydrocarbons. Came back the next month on a warmer day, after driving the car hard. Passed, but just barely. IIRC, measured was 199, limit is 200.

Anyone have any suggestions as to what may be causing this? High HC is unburnt fuel, right. Could a faulty (stuck-open) thermostat affect HC? Car gets decent MPG, averaging high 40's and runs great. Valves have been adjusted, right plugs, timing set, etc. EGR ports never cleaned and thermostat never replaced, however.

I prefer not to be a polluter and fixing a high HC issue should improve MPG, right?

I look forward to your thoughts, comments and suggestions!

Doug

[jadziasman]

All gasoline engines produce unburnt hydrocarbons - it's just the way they are. A small amount of fuel just will not burn during combustion because it exists in the quench zone at the periphery of the piston at the cylinder wall. What usually happens is this unburnt fuel makes its way down into the crankcase where the pcv system can reintroduce it into a subsequent combustion event. However, some of this unburnt fuel is pushed out the exhaust valve and into the exhaust system. It is the catalytic converter's job to "convert" these HC emissions into H20 and CO2.

What might be happening with your VX is the catalytic converter is no longer capable of handling the amount of HC it did when it was new or is incapable of converting a greater level of HC the engine is producing now that it has been driven for 200K plus miles and 18 years. How can this happen? Well the short answer is the HC emissions are often caused by leaking exhaust valves at top dead center of the compression stroke. It is possible that the exhaust valves and/or guides on your VX have worn to the point where a leak down test would show your engine doesn't hold pressure as well as it did with 100K miles. That would account for the extra HC.

One possible solution to your emission test problem would be to add a second CAT below the existing one. The second catalytic converter would probably take care of the HC the OEM one cannot.

[GasSavers_vxhatch]

Originally Posted by jadziasman

Well the short answer is the HC emissions are often caused by leaking exhaust valves at top dead center of the compression stroke. It is possible that the exhaust valves and/or guides on your VX have worn to the point where a leak down test would show your engine doesn't hold pressure as well as it did with 100K miles. That would account for the extra HC.

Wow, I hoped for something easier and less expensive, but I will get a hold of a leak-down tester and compression gauge to see what they reveal. The engine seems to run perfectly, plenty of power, smooth idle, lean-burn operational and avg MPG around 45-48. Do CAT converters fail or wear out? As far as I know the exhaust system is original,all 18-years and 175k mi old. The muffler rattles, when I wiggle it as it has some loose chunks of metal in it. My Google research indicates that high HC can be caused by lean-misfire. Any chance that the no-load 2500 RPM emissions test is a tough test for a lean-burn engine?

Bottom line is I did pass emissions and will not have to retest for two years. My hope in posting was to possibly find out that others have had similar experiences with the D15Z1 and know how to troubleshoot/fix.

Doug

[theholycow]

Catalytic converters do fail, rot, and/or wear out. Not always, but it's not too rare.