Failed Emissions Follow Up (long)

[William Conner]

I recently posted the following request for advice about a failed Maryland
emissions test:

> About the car....
> - 1993 Classic, 100K in generally excellent condition
> - Random Tech cat with 10K on it
> - HS Header with the AIV fitting plugged and AIV hardware removed (but
> EGR in tact), header wrap on the flex joint to reduce noise
> - JWT ECU with program for removal of AIV
> - AFAIK, the O2 sensor is original
> - Timing optimized on dyno after installation of JWT ECU -- probably
> 15-16 degrees BTDC
> - PR CAI, otherwise stock.

> About the test results....
> ...........MD Standard.....   ........Measured Level
> HC...........1.10.............................0.93
> CO..........20.0..............................6.55
> NOX.... .....2.30.............................3.51
> CO2.........N/A................ ...........328.85

First, thanks very much to all of you who offered advice.  Based on the
responses, I set out to focus on the EGR system because high NOX is
symptomatic of EGR trouble.  If I can't resolve the problem with EGR
changes, I will change the O2 sensor and see what happens.

To start checking the EGR system, I held my finger under the disc on the
EGR valve that raises as the valve opens while revving the engine to 2000
RPM.  The disc lifted but only about 1/8 inch and only momentarily.  At a
steady engine speed of 2000-2500 RPM the valve is definitely closed.  When
you rev the engine, the valve opens slightly as RPMs increase and then
closes quickly.  Does this sound normal -- or is the EGR not opening
enough?

I checked the tube from the bottom of the BPT valve to the EGR tube and
found a steady flow of exhaust at idle.  I attempted to clean the tube
anyway and may have increased the flow a bit -- it's hard to tell.  I can
blow air through the bottom BPT hose into the EGR tube to the exhaust
manifold with little resistance.  The tube may be blocked a little, but
it's not clogged.  Does this sound normal?

Then I put everything back together, retarded the timing 2 degrees and
went for a second emissions test.  The results were better but not good
enough:

HC.....0.89 GPM
CO.....3.99 GPM
NOX....3.05 GPM (failed MD standard of 2.3 GPM)
CO2....260.17 GPM

With hopes of a quick victory dashed, it's back to the FSM.  I removed the
CAI and throttle body (surprise: those are coolant lines fella!) so as to
expose all the vacuum hoses and the EGR/ canister control solenoid valve
located conveniently (not) under the throttle body.  Here is what I found
on this little diagnostic adventure:

- All the vacuum lines were in amazingly good shape -- and hooked up to
the right fittings.  No easy solutions there.
- The solenoid did all the right things with the 3 hoses that attach to it
both with and without power.
- After cleaning the throttle body and reassembling everything, I found
that the vacuum from the solenoid to the BPT valve comes on around 2000
RPM and goes off at 4000 RPM -- just like it's supposed to according to
the FSM.
- The BPT valve seems to pass the diagnostic test (holds pressure in the
top hoses when closed by blowing on the big bottom hose).
- The EGR valve opens easily when you suck on the vacuum hose to the
valve.
- When bypassing the BPT valve and feeding vacuum directly to the EGR
valve, the EGR valve opens a good 3/8 to 1/2 inch when revving the engine
to 2000 RPM --a very dramatic increase in amount of opening compared to
what happens with the BPT valve on line.

Also, JWT assured me that their ECU should operate the EGR valve just like
the factory ECU.  So, I'm not sure where else to go with the EGR system.

It seems to me that the EGR valve should open more, but I don't know what
else to try.  Your suggestions would be very much appreciated.  Thanks in
advance.

Bill Conner
--
billc@fred.net