My 1984 XJS (152,000 miles) failed its California smog test, as expected.
Hydrocarbon emissions had been creeping up to the limit (140 ppm) during
the past two tests in '97 and '99 and were well over the limit (163 ppm)
this time. NOx was also creeping up (725 ppm versus limit of 1070 ppm).
Only CO stayed well below the limit (0.40% versus limit of 0.74%). The
advice from my local Jag mechanic was “change the catalysts”. Since I had
not changed the plugs, cleaned the contracts on the rotor/distributor cap
or oiled the distributor for ~18,000 miles, I opted to do these as well to
be on the safe side.
The initial plan was to only replace the front cats, but one of the two
rear cats was partially plugged so I had it replaced as well. Cost was
$160/cat, but I had to remove and replace all hardware (~8 hr). While
installing the front cats, the local muffler shop removed and did not
replace the “precats” in the downpipes leading into the front cats. The
tuneup cost was ~$100 for cap, rotor and plugs, and, of course, required
the necessary ~4 hr time to complete the task. I made no attempt at any
other changes.
The end result on retest: hydrocarbons = 0.0, NOx = 125, CO = 0.0, which I
consider a pretty drastic reduction. I suspect I could easily have spent
half my investment in cats alone having a mechanic fiddle with the tuning
while the car was on a scope and still not have made the limit. An added
benefit is that there is a slight, but noticeable, improvement in mid-rpm
performance that I ascribe to a net reduction in obstruction of flow in
the cats. Thus when all else fails, “change the cats”.
Dan Jensen
San Diego, CA
''83 XJ6 105K
'84 XJS 152k
'96XJ6 56K
The end result on retest: hydrocarbons = 0.0, NOx = 125, CO = 0.0, which I
consider a pretty drastic reduction. I suspect I could easily have spent
half my investment in cats alone having a mechanic fiddle with the tuning
while the car was on a scope and still not have made the limit.
I think your new catalysts probably contribute less to your newfound smog
certification than does ignition work. No doubt, the catalytics are
probably wrenching that last little bit of HC’s out of the emissions, but
the big stuff is all a function of your ignition (esp. considering you had
caps rotor and plugs all replaced, and not just misc tweakage).
If the engine is running proper, the catalytics should be unnecessary in
order to pass smog.
Wow! I gather you believe the dramatic (that word is not really big enough!)
reduction is due
to the cats? The conventional “wisdom” is to ignor them and pass with clean
plugs etc.
My car is still in pieces. I will learn is a week or two if it passes.
I think your new catalysts probably contribute less to your newfound smog
certification than does ignition work. No doubt, the catalytics are
probably wrenching that last little bit of HC’s out of the emissions, but
the big stuff is all a function of your ignition (esp. considering you had
caps rotor and plugs all replaced, and not just misc tweakage).
If the engine is running proper, the catalytics should be unnecessary in
order to pass smog.
Perhaps you are correct. I would point out, however, that I did a similar
tuneup of the ignition prior to my smog check four years ago and it in no
way reduced the levels to those recorded this time. Thus on my car, at
least, I conclude the cats had a big effect.
Dan Jensen
San Diego, CA
''83 XJ6 105K
'84 XJS 150k
'96XJ6 50K
'81 XJ6 (with the kids) 150K
If the engine is running proper, the catalytics should be unnecessary in
order to pass smog.
Sean Straw '88 Jaguar XJSC 5.3L V12 (LHD) ‘Black Cat’ 63K
Perhaps you are correct. I would point out, however, that I did a similar
tuneup of the ignition prior to my smog check four years ago and it in no
way reduced the levels to those recorded this time. Thus on my car, at
least, I conclude the cats had a big effect.
You both might be right. It could depend on the overall condition of your
engine and the particular limits your locality sets. NY, for example,
has very loose limits.
It might be interesting if we could compile a comparison of the
testing limits for, say, a 1985 XJ-S HE for different states.
Perhaps you are correct. I would point out, however, that I did a similar
tuneup of the ignition prior to my smog check four years ago and it in no
way reduced the levels to those recorded this time. Thus on my car, at
least, I conclude the cats had a big effect.
If you’re still pumping all that much HC into them under normal driving
conditions, then they’ll probably eventually plug up like the originals
were showing signs of.
I don’t discount that catalytics can make the difference between pass/fail,
but given the choice between an ignition overhaul and a catalytic swap, I’d
put more weight (and spend less money! ) on the ignition having a more
significant impact on your change in emissions.
There are little things which might be tweaked between tuneups: perhaps
your previous tuneup was accidentally performed with the wrong gapping
(pre-HE gaps were 0.035", while the HE, which is a higher compression
engine, is 0.025" - too large a gap would lead to misfiring as well as
other ailments), or an injector plug was jostled loose before but has been
properly reseated after the new plugs were installed. Unplugging an
injector has sort of the opposite effect from what one might at first
expect: you’re removing HC’s from the ignition equation, but the ECU will
sense increased O2 levels via the oxygen sensors and enrichen fuel mixture
to compensate for what it believes is a lean condition - replugging it
restores that cylinder to order AND reduces the emissions.
Did you examine the old plugs – both gapping and condition?
My car is still in pieces, but in a week or so I plan to take it back for the
2nd smog test.
About all I have done regarding emission levels of any real consequence is
cleaning up the
fold plugs on A2-A5. Is there anything else I should/could (i.e, w/o extensive
test equipment)
do on ignition before testing?