Re: Winter wondering

["Scott M." <scottmo@teleport.com>]

Jeff,
> 
> It's a rainy Sunday and I've just been doing some archive
reading to get smarter about my 86 5kTQ, <snip> I have been
puzzled about two topics:
>   1.  Fuel octane. <snip> As a validation of this
empirical data, the max. boost (1.3 bar) is the same with
87 or 91/92.  If the computer were to decide that it does
not like 87, I think the first thing that it would do would
be to cut the boost (e.g., to 1.1 or 1.2) to control
pre-ignition.  

The Engine Control Unit (MAC11 ECU) has two timing maps
programmed into it, one for premium and one for regular
fuel. When you first start the car it begins by using the
premium fuel map, if sustained pinging is heard enough to
have the ECU retard the timing by three steps of 2.6
degrees each, it switches over to the regular fuel timing
map. The boost doesn't get messed with until it pings long
enough to have the timing in the regular fuel map retarded
by three steps of 2.6 degrees, Orin can likely add some
additional details on this process, he has the timing
instructions fully decoded and well understood in the
MAC11. :-)
> 
> 2.  Boost pressure.  The highest (and commonly seen)
boost pressure on the display is 1.3 bar.  Is this normal
(for a stock vehicle)?  

Normal "stock" boost is set at 1.42 bar with the boost map
inside the ECU, the stock overboost fuel pump cutout is set
at 1.6 bar. Often times the stock boost gauge is not
accurate and it also takes "seconds" to respond to boost
changes so you may not see 1.4 bar on the gauge. The Waste
Gate (WG) spring tension is set to provide ~1.3 bar, the
ECU then adds pressure on top of the WG to get an
additional 0.1 bar at sea level. (More at higher altitudes)


You may want to check your throttle switch for correct
operation at idle and at Wide Open Throttle (WOT), often
times the internal solder connections are cracked. You can
replace it or cut off the plastic cover and resolder the
connections. Sometimes the WOT contacts are corroded as
well. Correct operation of the WOT switch is required for
the ECU to know you have the throttle open past the 2/3
point so it can regulate the boost. I suspect there are
many '86-90 5000/200TQ's that are running around with a
non-functioning throttle switch and have low boost. You may
want to run the self diagnostic system and see if there are
any other problems. The Bentley repair manual describes
this procedure.

There are several people on the q-list who can provide ECU
mods for getting ~1.8 bar, send me an email if you have any
questions.  :-)

HTH
Scott M.