My Emissions Story Options

[pbanders]

Well, I always dread December here in AZ, because that means I have to get the 914 smogged. If I'd only stayed in CA, I would be home free by now!

In the past, I have never trusted the AZ MVD test equipment, which as you'll see, was probably a mistake. Always seemed high on CO and HC to me. As some of you may know, I have a garage full of 914 D-Jetronic test equipment, and I also have a CO meter of my own.

I didn't play it smart by checking my CO first, I just went and warmed the car up by driving for about 30 minutes, backed off the ECU idle mixture knob a few clicks leaner, set the idle speed and took the test. Here in AZ, they test CO and HC at load (about 2000 rpm on the dyno in 3rd) and CO and HC at idle after the load test. The limits for my year at load are HC>500 ppm / CO>4.20%, and at idle HC>500 and CO>5.50%.

The car has less than 2K on a full stock 2.0L rebuild. Passing those standards should be no problem. Well, it failed big time. Loaded HC=115 ppm / CO=4.85%, and idle was HC=890 ppm (!) / CO=5.85% (!). Waaaaay rich, wow. I groused at the guy that it couldn't be that rich and drove it home.

First thing I did was check my timing, it was spot-on. Next, the CO. Yep, I got just below 6%! I could fix that by adjusting the ECU, and I figured the high CO at load was because I'd swapped in an new MPS this past year, must have been richer.

I took a few minutes to compare the MPS's calibrations with my tester before I swapped them. Both were identically calibrated, with 8.9 ms injection pulse width at 0" Hg vacuum (wide-open throttle conditions), 6.3 ms at 5.0" Hg vacuum (moderate part-load), and 4.2 ms at 10" Hg vacuum (light load, near idle levels). Since they were the same, it made me concerned that I'd fail again for loaded CO. But, hey, I didn't trust their numbers, so I figured I'd just hit a different test lane this time and it would pass.

I did the swap, set the idle CO down to 3%, and went back for the retest. And as you might suspect, while it did better, it did fail as I feared for loaded CO. Results were loaded HC=127 ppm (nearly the same as last time) / CO=4.57% (fail, and nearly the same as last time), and idle HC=335 ppm (MUCH better) / CO=2.71% (MUCH better, and the same as I read on my meter). OK, now I know what to do.

Went home, yanked the MPS back out. The one I'd put in was one that I'd removed the epoxy seal so that I could do my own calibrations. I removed the full-load stop screw (the big screw slot in the end of the MPS) to expose the main load adjustment and part-load transition screws. While connected to the tester, I held the part-load transition screw fixed with one screwdriver, and screwed in the main load adjustment screw while watching the injection time on the tester. I dropped the full-load duration from 8.9 ms down to 8.5 ms, which resulted in the 5" Hg reading dropping to 5.9 ms, and the 10" Hg reading dropping to 3.9 ms. I figured this would make it so lean that it would drive like crap, but would pass the test easily.

Stuck it back in, set the idle CO to around 2.5%, set the idle speed. Surprisingly, the car ran fine. Drove it hard back to the test facility. You know what they say, "third time's the charm".

Guy who tested it this time owns an '80 911SC, so he was really interested in seeing if it would pass. And it sure did! Loaded HC=57 ppm (Wow!), / CO=1.58% (Wow!), idle HC=113 ppm / CO=0.99% (!). Got my cert and left.

Obviously, I went a bit too far with my adjustment, as I'd like to see the loaded CO at about 2%, and I'm going to bring my idle CO up with the ECU knob a few clicks. But I learned a few important things. First, something I've always known, which is to set idle and loaded mixtures correctly, you've got to do it on the dyno. Guessing with road results doesn't cut it. Second, the MVD's numbers were right and I should have paid more attention to them in the past, as they've always shown me to have quite a rich loaded CO. Third, the base calibration on the 2.0L MPS's is quite rich. I've got two NOS MPS's and both are closer to where I started from.

Long story, hope it had some useful info in it and wasn't too boring. Now that my car's registered for 2010, I'll be having a happy New Year, and I wish all of you out there in 914-land a Happy 2010!