From: Fred Rourke [rourkefg@omantel.net.om]
Sent: Thursday, July 06, 2000 3:19 PM
To: Pirtle John
Subject: Re: intake removal

Hi John,

Well there seem to be quite a few clues in what you have written. Let me
give you a few thoughts and judge for yourself.

Your symptoms almost certainly indicate problems with the fuel system
pressure circuit. This canbe cuased by faulty check valve on the pump or
more likely, the regulator or dampers.

The way to test the fuel pressure regulator and dampers is to put a pressure
gauge on the end of the fuel injector rail. I bodged a set up but Dave
Roberts does a neat package for $30. You would do well to get one for
testing purposes. When hot, the system should hold a pressure of around 54
psig for an hour without any significant loss.  When cold, the temperature
sensors tell the computer that things are cold and so the computer outputs
longer opening times to the injectors to compensate. Hence when cold, low
fuel pressure is not too much of a problem and cold starts are relatively
easy. When hot, if there is little fuel pressure then it does not want to
play. Turning the engine over for a few seconds and then trying to fire it
again is a pretty sure sign of this. Cranking the engine helps build up fuel
pressure so that when you fire it again hey presto!

When my regulators were goosed, warm starts were very dodgy and it would
fire and die. When it did start the tickover was erratic. The local agents
told me the idle speed controller was goosed but I reckoned otherwise. I
replaced the regulator and the two dampers and everything worked just fine.
It sounds as though this may be one of your problems. One test I devised to
check these three items is to run the car, switch it off, leave it a few
seconds and then take the vacuum connection off the top of the regulator
[and dampers one at a time]. Push your thumb pad onto the top for a second
or two, take it off and see if you can see or smell petrol on your thumb. If
you can, then the diaphragm is goosed. If one unit is gone the other two are
also gone or about to go [unless the PO replaced one unit]. You may also see
or smell fuel in the vacuum lines.

With regard to the mass air flow sensor connection did you have a look at
the terminals? Oxidation of the terminals can cause problems so removing and
replacing can help polish the tangs a little. A little scotchbrite rub-up
may help. If the MAF goes down the car can still run but it does so on a
pre-programmed basis that sets the air-fuel ratio for about 1/3 throttle
opening. Presumably this means that you run too lean if you pump it and too
rich if you feather the throttle but it gets you home.

I also recommend that you check the throttle settings. The 928 has a nice
throttle action but it is very easy for it to be set incorrectly. First of
all get the nearest and dearest to sit in the car [engine off] and floor the
throttle. When in the fully open position grab the throttle linkage and see
if you can open it some more, if you can something is wrong in the set up.
Under the manifold lies the throttle switch. This has two settings. One
tells the computer you are trying to screw the nuts out of the engine [full
throttle] and so the computer throws in the kitchen sink. The way to check
this is to remove the ignition brain plug and put a multimeter over the
specified terminals. At full throttle the circuit closes. The same goes for
the idle switch when the throttle is closed. If this does not work the
computer does not know you are trying to tickover. I I have enclosed a scan
of the diagnostics for this test. Note: you can carry out these tests on
either the ignition brain or the injection but the terminal numbers change.
You must also ensure the ignition is switched off when you pull the plugs on
the computers.

I see nothing in your notes to indicate the brains are fried. Try the simple
things first.

Power Loss.

If the power is well down then in all probability one or both of your anti
knock sensors are goosed. If this happens then the computer by default
retards the ignition by 5 degrees, definitely not conducive to high
performance. The other main culprit for loss of performance is the ignition
leads. As I said, if they have not been replaced already, then they will
need replacing now. You can look for sparks tracking at night time, that can
tell you something but not everything. The most probable area of leakage
will be down the plug hole slot and you cannot see that. Other important
item are the distributor cover and the rotor.

As regards the nitrous setup all well and fine if one wants 20 seconds of
glory at the traffic lights. If that is what people want then I think they
need a Z-28 with Carlos Fandango super wide wheels and an Albert
Schwartzenegger super-duper-charger. For me the 928 is about performance
with finesse and style. A corvette with class if you like. I do not know
what plumbing has been inserted but make sure it does no harm if the bottle
is disconnected.

The other thing to check is the famous [infamous] resonance flap. You should
see this flip over when the engine starts. If it does not, then you will
lose a whole load of mid range torque. You should check that the vacuum
reservoir is holding vacuum by putting a vac gauge on the nylon line leading
to the solenoid valve located at the front of the engine. The vacuum should
hold constant at 600mm Hg [mercury column]. Unfortunately when Porsche
designed this sub-system only the toilet cleaner was on duty. When this
solenoid is electronically actuated by the computer it opens and allows
vacuum onto the flap actuator, which is located under the manifold. Any
failure in this sub system and the flap does not work. Prime cause of
failure to this system is if an a/c system actuator fails. This is a crazy
system design on a $100k car.  To minimise leaks I replaced the crimped
connections on the hoses around the throttle actuator with jubilee clips.

The bottom line is that all the above items can be replaced for around $750.
Leads $300
Rotors $40 [for 2]
Caps  $120 [for 2]
Pressure dampers/regulator $200
Coils $100 [2 No.]
Fuel pump check valve $10.

I did the lot. Most of them were gone at the same time [ the fuel dampers
went a little later].

Make sure your timing belt and water pump have been replaced. Find out when
from the service records. Ignore what Porsche say and change the timing belt
after 45,000 miles or 5 years whichever comes first. That belt goes and you
can kiss goodbye to Father Christmas for a year or two. Also note that
history tells us it is wise to change the water pump when you do the timing
belt. Reconditioned ones run at $110.
Follow these basic tips and your bills will be relatively modest for running
a supercar.

When these things are running well you know it. The local Porsche agents
rate my S4 as the best 928 in the country including the dozen or so GTS's.
One chap who has an S4 of your vintage brought his car round to me a few
months ago. I tested his car and then took him out in mine. Afterwards he
reckoned his needed to see the doctor! Mine is a 1990 model and has the PSD
system [traction control]. I can spin the rear wheels in first and second if
I floor the throttle and that is on an automatic!

I felt the same as you. Where is the urge? Mine has the urge once more. Lets
hope I get it back from the garage soon after my recent rear end damage
[second time in 8 months].

Hope the above of some use to you.

Regards


Fred R
Oman