Unusual problem with LH 2.2 system

[Seth Allen]

On Fri, 20 Jun 1997, John Bertram wrote:

> Date: Fri, 20 Jun 1997 21:22:53 -0700
> From: John Bertram <bertram at teleport.com>
> To: diy_efi at coulomb.eng.ohio-state.edu
> Subject: Unusual problem with LH 2.2 system
> 
> I'm wondering if anyone can come some ideas for troubleshooting a problem
> that I am having with my '87 Volvo 740 Turbo.  It uses an LH 2.2 system for
> fuel and an EZK 117 for ignition.  I am also running a Saab APC knock
> sensitive boost pressure control system which is totally independent of the
> fuel and ignition systems.  Here's the problem:  When I let my foot off of
> the gas under heavy boost (12-14psi) and reduce the boost to say about 3
> psi, I get black smoke out the tail pipe for 1-2 seconds.  I don't remember
> ever seeing it do this before.  If I lift my foot off the gas all the way,
> it doesn't smoke.  The car is equipped from the factory with a turbo bypass
> valve which I have tested and does work.  I have also tried a known good
> mass airflow sensor just for fun.  I have checked all of the grounds in the
> system, and the power supply to all of the components.  I have checked the
> NTC (pin checked from the LH ECM plug).  Anyone got any good ideas of what
> might be happening?  The O2 sensor seems to be working, but I haven't
> hooked it up to a scope to see how fast it's actually cycling (I was
> checking it with a slow DMM).  I replaced the fuel pressure regulator a
> couple months ago, but haven't gotten a chance to check it again.  The
> overall fuel consumption doesn't seem to be affected very much though.  The
> smoke is just embarrassing.  There are no noticeable drivablility problems.
> 
> Anyone got any ideas?
> 
> 
Here is a guess- when you drop the throttle completely, the injectors 
quit firing  when under decel.  This as a common strategy for efi.  When 
you lift, but not fully, either a value from the MAF (LH is hot wire, 
right?) or or the fact that a throttle closed microswitch is not 
engaged.  This results in the injectors firing, and the resultant black 
plume.  You were probably going quite rich at full boost to reduce NOX 
emissions.  Then you have excess fuel and exhaust in a hot fast moving 
exhaust.  Whn you lift the throttle partially, you have leftover unburned 
HC in the tailpipe and a suddenly slowly moving exhaust stream.  The 
result is condensing hydrocarbons and water from the now stoichiometric 
exhaust gases passing thru the catalyst.  The result is instant turbo smog.

This is just my first guess.  I would bet that everythig is fine.  And 
that you would never see this transient situation on the LA driving cycle 
emissions test for Volvo certification ;)


Seth Allen