CIS Diagnostic, help

[Bernard Littau]

Hi Steve,

I'm no expert, but I have read the Probst book on the Bosch CIS.  I have
also had to deal with a low fuel pressure problem on my car -- with major
help from local listers, including the loan of the Probst book :-)

> A bit more info on my 86 5KTQW that won't start (or stay running the few
> times it does catch)...
>
> Starts/runs if you feed it ether.  Checked for vacuum/hose leaks, but
> did not find any.  Spark plugs are sooty, from too much fuel not oil.
> Good compression, and crank, valves and dist are all in sync. Fuel pump
> works, but when I took a pressure reading, I got 40-50#'s; with quite
> alot of jumping around (that is it did not settle down at a steady
> pressure).  This is with the engine off and the fuel pump jumpered to
> run.

Fuel pressure should be much higher than 40 or 50 pounds; closer to 90.
Where are you measuring the pressure?

> And FWIW, the air plate in the fuel distributor seems hard to move
> up, but comes down easily; up resistance goes away when fuel pressure is
> removed.

This is correct behavior.  The fuel pressure pushes on the plunger, making
it harder to raise.  They could have used a spring, but using fuel pressure
instead of a spring helps to compensate for minor fuel pressure differences,
and saves needing another calibrated spring.

>
> I'm trying to add these all up to come up with a likely diagnosis, but
> seems like they are pointing in different directions...  The low/erratic
> fuel pressure would seem to say clogged fuel filter or bad pump.  But
> the sooty plugs and up pressure on the air plate seem to point to too
> high a fuel pressure.

Plugs take some time to acquire a rich or lean condition.  It's likely you
were running rich before your no-start situation.  A few cranks of the car
won't change the plug's look.

As above, the pressure on the plate is normal.

I think you need to not link the fuel pressure to the plug condition.  The
fuel pressure is now, the plug condition is historical.

>
> Anyone in list land an expert on the CIS system?  Could low or erratic
> fuel pressure cause rich running sometimes (sooty plugs) and no fuel
> situation others?  Other possibilities that fit these symptoms?

Low fuel pressure is not going to make the car run rich, in fact, it will
tend to make the car run lean.  This lean tendency can be compensated for by
the ECU using the O2 sensor, so a car will typically run correctly even with
somewhat low fuel pressure under moderate throttle conditions.  At high
throttle the car will stumble.

Rich looking plugs are most likely to suggest a leak in the air intake
system -- the typical Michelin Man Hose leak is an example.  It seems to me
that it would be difficult to check for small leaks in the vacuum or intake
system with the car not running :-)  You may want to address this again
after the car is running.

Do a fuel delivery test to determine how much volume of fuel is delivered
through the CIS system.  See:
http://www.sjmautotechnik.com/ficis.html#fuelp

(There is lots of good info on Scott's site that may be pertinent to your
situation.  Most of what I know comes from Scott's site, the Probst book and
the Bentley manual.  Send $10-20 to Scott using Paypal if his site helps get
your car running again :-)

Scott does not detail the fuel delivery test, so you need to check the
Bentley manual.  Basically, you measure the fuel delivered back to the gas
tank after the pump, filter and CIS.  A clogged fuel filter will typically
affect flow more then pressure; low pressure tends to suggest a bad fuel
pump.  It would take a seriously clogged fuel filter to show pressure drops
like you are seeing.  Do a fuel delivery test and see what it shows.  If
your flow is low, I would replace the fuel filter and try the flow test
again.  You can get a $20 cheap FRAM fuel filter at almost any FLAPS.

Poor flow with a new fuel filter will suggest a bad fuel pump.

If you have good flow, and low pressure, you may want to consider messing
with the fuel pressure regulator spring.

My guess is that a sudden onset problem like yours is way more likely to be
a failure of the fuel pump than fatigue of the Fuel Pressure Regulator
spring.

Your pressure readings, assuming they are being taken in the correct place,
are close to half of normal.  I'd guess half-normal fuel pressure would make
the car not run -- the injectors should stay closed during the residual
pressure phase, which Scott mentions is around 43.5 psi.

Best,

Bernard Littau
Woodinville, WA
'88 5ktq