[v12-engine] RE: Failed Emissions Test [ High NOx ] cont. Vac Readings

I ran the idle adjustment bolt all the way in & 600 - 650 rpm is the
lowest it gets. At this rpm the timing is 8-9 degs BTDC vac line
clamped, unclamped 22 degs BTDC. I also removed the B side air filter
& placed a cap over the hole leading to the AAV, no change in rpm, no
air felt or heard from said hole.
If this is the correct location for this procedure I thought that the
engine should die when this is blocked IF there are NO vac leaks.
Correct ?

RPM’s @ 600, I took vac readings clamping & unclamping the vac advance
line in open & closed loop.
Why I don’t know.

clamped A side open loop: 17" Hg.
" " closed loop: 16 - 17.5" Hg. idle fluctuating
500-600 rpm rhythmically, 1-2 sec duration.

clamped B side open loop: think I forgot to take this one …
" " closed loop: ~ 15.5 - 17.0" Hg. again cycling
in ~ 1-2 seconds

vac @ ECU closed loop: 15.5 - 17.0" Hg. no measurement for open loop on
this one.

Voltage readings @ feedback socket, closed loop: GW 1.98-2.77v; GR
1.89 - 2.78v
" " open loop :
" " 2.519v " " 2.481v

unclamped, A side open loop, RPM ~ 650-675: 18" Hg solid.
" " closed loop," ": 17

  • 18" Hg. I’m starting to confuse myself here, at this point I created a
    vac leak on the B side by pulling the cap off the CC tap on the over run
    & the gauge reading steadied @ 18" Hg. Of course the rpm increased also.

unclamped, B side open loop, " “:18” Hg. slight
" " closed loop: seems I was too busy creating
a vac leak to get this one…

Here are the feed back socket voltage reading with a vac leak on the A
side this time while in closed loop, RPM ~ 950:

GR 2.86- 3.81
GW 2.85 - 3.95.

This is the hot crank over vac reading: 4" Hg.

And finally I blipped the throttles with the gauge tapped into the A
side: The gauge went to 0" then up
to ~ 24" Hg.

So based on these findings it looks like my cats are not melted or
clogged. The rings / valves are in good order. The line to the ECU is
now clear & full manifold vacuum is reaching the MAP sensor. The O2’s &
ECU are trimming the A/F mixture correctly. Timing is perhaps a deg or
so retarded & I have a vac leak most likely on the B side some where.

Now if the MAP line has been obstructed & full manifold vacuum was not
present what would this do to the emissions ? Would this cause high NOx
content in the exhaust gasses ? Would it cause the ECU to lean out the
mixture causing the high NOx reading ? Would a vac leak & low manifold
vacuum @ the ECU combine to cause this reading ?

At the least this post answers the question of what vac readings are
produced by the H.E. engine.

Comments welcome.


Bob. 84 XJ-S H.E. with GOOD manifold vacuum…if nothing else.