No more stumble at high boost + high rpm :-)

[Bernard Littau]

Thank you to all of you that sent advice.  I checked out almost everything
that was suggested.

Ignition, wires, etc all checked out fine.

Turned out the problem was fuel starvation at high boost and high rpm.  I
was very, very, fortunate in that a local lister has, and graciously allowed
me to borrow, an instrumented MAC-11B chipped ECU that outputs interesting
data to a serial line that can be logged with HyperTerm.  The log data
showed that the FV duty cycle was higher than normal, and that the O2 sensor
showed way lean at high rpm and high boost.

SJM's site has a good write up of this stumble problem, by the way, see:
http://www.sjmautotechnik.com/ficis.html#cutout

Interesting that no one pointed out this area on Scott's site.  It pretty
much nails the problem.  FYI.

Anyway, the first thing I did was replace the fuel filter.  At 10:00 AM on
Sunday all I could get was a FRAM.  Blecch!  I hope I don't get thrown off
the list for installing the FRAM filter...

Filter change resulted in no change in the stumble behavior.  Next test was
the fuel pump.  Specs say 760 ml in 30 seconds with 12V on the pump, less ml
at less volts.  No good way to measure the voltage on the pump, and Bentley
suggests pump is about 2V less than battery post at 12.5.  Well, no need to
have worried about the voltage.  Pump put out 1300 ml in 30 seconds.  Well
above spec.

That left fuel pressure.  I had no way to measure the fuel pressure, but I
figured I could change the pressure and see the effect.

I pulled out the little spring assembly that does the control pressure
regulation in the fuel distributor.  I managed to find some washers that
were ever so slightly smaller than the washer on the assembly.  I had to
drill out the centers on the drill press to make the hole big enough.

I shimmed with one washer, and took a test drive.  Stumble seemed gone, and
the numbers looked good on the data log.  Idle is around 40% duty cycle.
Mixture stays rich at boost (0.80 - 0.90 V on O2).  Good deal :-)

I decided to try two washers shim.  This was a mistake.  Nothing inherently
wrong with two washers, but when I installed the little assembly, the o-ring
must have ripped.  I could not start the car.  When I pulled the spring
assembly out, I got maybe 1/3 of the vitron o-ring, in small pieces.  I
feared for the worst as to where the other 2/3 of the o-ring went.

I dug around for o-rings, but had nothing that was even close.  I drove down
to B&B Auto in search for o-rings.  They had a limited selection, and I
picked up two candidates.  In the end, the smaller candidate was still too
big.  I used a file to downsize the o-ring so it would fit into the orifice
and seat.  All the while I was worried about those extra vitron bits.  I
finally fit the assembly back in, this time with two additional washers.
After much cranking, a sputter.  Then she started.  Yay!

The log numbers for two washers are not significantly different than from
one washer.  I guess once you have enough, more does not help.  I'll leave
in the two washers, as my ECU goes to around 2.2 bar, so I figure the extra
potential can't hurt, might help.  Who knows.  Comments and criticism
welcome.

This morning when the car was cold, I noticed it runs a bit rough while
cold.  Maybe the second washer was indeed too much.  I'll have to measure
the fuel pressure and fine tune it a bit more.

Thanks again for the help, all of you kind souls.

Best,

Bernard Littau
Woodinville, WA
'88 5ktq