[v12-engine] 78 XJ12 Passes California Smog Test

Paul Runneals, “You da man!”

After hauling my XJ12 2,000 miles from Missouri to California, I was very
unhappy Friday when it failed emissions testing. I will limit information to
15 mph readings – The test has 15 and 25 mph scales. The car’s HC was 241
(201 max) and CO was 2.42 (1.21 max). NO was 318 (1248 max). Timing was a
few degrees advanced. (I had done that intentionally, but couldn’t believe
they actually checked it as it is something of a pain to do.)

Over the weekend, I cleaned the intake butterfly areas, changed the 82C
stats to regulation 88C stats, cleaned the auxiliary air valve, set the
timing to 10 BTDC and then did Paul’s magic trick (from the archives). I
turned the manifold air pressure sensor (MAP) adjusting screw between 1/8
and 1/4 turn clockwise (leaner). Basically, you take a car that runs very
well and turn it into something of a dog. The readings today were HC at 142
and CO 0.14. NO had gone to 357 (1248 max). Paul’s numbers were a little

Unfortunately, and as is typically the case on this list, I could not afford
a systematic approach. However, it is my opinion that the MAP adjustment was
key. It clearly made a marked performance difference. Unlike Paul’s, mine
had never been adjusted so I could just use his advice about rich to lean
being within a quarter turn. Frankly, I wonder whether a 78 XJ12 just off
the showroom would pass 2000 emissions tests. The best part is that I can
turn the screw back to its original setting and recover the lost
performance. Not that I would do such a thing of course.

So, Danny and John, thanks for offering to keep the XJ12 in Missouri and to
even warm it up once in awhile, but no thanks. I’m back on the road!

Steve C.
78 XJ12
83 XJ6

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