Re: mechanics stumped

[KSMITH1@mailgw.sanders.lmco.com]

Greg Woodard <gwoodard@ndsuext.nodak.edu> requested:

     I really need help with this one.
     
     1987 1.8 4cyl.
     
     My son can start the car in the morning and drive it to school (15 
     miles)- no problem.  Start it after school and and drive it to work 
     (15 miles)- no problem.  Start it 2 hours later and drive it home- no 
     problem.
     
     But if he drives around town, stop and go driving, lets it idle for a 
     few minutes.  It will start to miss.  He can limp it home, but it 
     takes over 1/8 of a tank of gas (14miles).   You can smell raw gas 
     almost running out of the exhaust.  The plugs will be completely 
     black.  If we clean the plugs and leave it sit overnight, he can 
     drive to school- no problem.  
     
     We have replaced nearly everything with known good (ie borrowed from 
     my 1.8).  I'm stumped, my sons stumped and several mechanics are 
     stumped.  The car is getting really close to the crusher, would be 
     now except that my son really likes the car.
     
     HELP

OK, since I have never seen this engine, I feel eminently qualified to 
guess at your problem.  I'm assuming it has a Motronic injection 
system that controls the duty cycle of the injectors.  I also assume 
that the distinguishing difference between "normal" driving and idling 
about town is the decreased natural air flow through the radiator and 
engine compartment.  This is usually mitigated by the radiator fan.  
Are you sure the fan works when the coolant temperature rises?  If so, 
perhaps the engine compartment temperature rises anyway, and causes 
some electronic part (a transistor comes to mind) to have high 
leakage, stay on, and cause 100% injector duty cycle.  An oscilloscope 
on the injector drive would be revealing, perhaps.  Ignore this if 
anyone who seems to know responds.

***                 ...Kirby    (Kirby A. Smith)                 ***
***                      2 X 1988 90q                            ***
***              ksmith1@mailgw.sanders.lmco.com                 ***
***              [=]   kirby.a.smith@lmco.com                    ***
***  Opinions expressed herein are entirely those of the author. ***