Narrowing down my start issue (84 4kq) [with suggested procedure]

[Ben Swann]

Cody, 

I used to think that, but is more of a common mis-conception.  Differential pressure is
a different use of fuel pressure than the Control Pressure.  They have a similar
effective end result, but the implementation is entirely different.  There is actually a
reason they give them different names, since you could concievably use both differntial
pressure and control pressure in the same unit and they would be different pressures.  I
believe they didn't because they were trying to simplify things and this was in effect
one step closer to electonic fuel injection - just using port injectors with a
continuous pressure instead of pulsed port injectors.

Off the top of my head:

The WUR is basically used to effect the control pressure (kind of back pressure) to
create a set regulated fuel pressure that is constant overall, but a little lower
control pressure will give higher regulated pressure for richer warmup.  As the engine
warms up, the control pressure is raised making the fuel pressure lower for leaner
operation at temp.  Some WUR/CPR units have other influences, such as vacuum/boost, but
the ouput effect is the same.  The control pressure is provided into the top of the
metering assembly to balance the system pressure and give proper base pressure to the
injectors.  Later CIS Lambda uses fuel frequncy valve to add fuel pressure
electronically - O2 sensor is primary ECU input.

The DPR or Differential Pressure is kind of looking at control pressure from a different
angle.  For one, it is electronic, and sensors (temp, Oxy, accel pot, TPS, others?)input
to ECU can be used to effect pulse width modulation on clamped orfices (DPR Mechanism)
that allow more or less fuel to pass into the metering head.  That is where it kind of
looks/acts like control pressure, but if you look at the diagrams there are differences.
I think the diff pressure offsets pressure to the metering orfice fuel in the chamber
and result is injector pressure. Control pressure on the other hand, counteracts the
system pressure by applying force above the metering orfice rod.  The DPR as I
understand pretty much effectivey replaces the combined use of WUR and Fuel Frequency
valve on the metering head.  There is still a fuel pressure regulator to make base fuel
pressure, and then DPR is modulated to give the precise correct fuel pressure to the
metering orfices.

Devils in the details, but the internals of the CIS-e usually aluminum/no paint are
completely different than CIS painted black, but the internal differences are far more
different than paint.  I have tried to swap parts between them for similar diagnostics
or to change to later style injectors, what have you.  You can't do it without breaking
things.  You can't readily interchange CIS-E metering head with CIS or vice versa.  If
anyone has done this, I'd sure be intereste in the details.

Basically I found that all the working fuel pressure control is inside the metering
head, so fuel pressure tests are pretty much useless.  You can measure pressure from the
pump before the regulator and pressure at the regulator output.  I don't know any sane
way to effectively/meanigfully  measure Differntial pressure, but assume you could
confabulate something to tap into the metering head and measure that, heck there
probably is a procedure - just don't know what it would tell you and probably would vary
far to fast to mean anything - it is electronically controlled.  If you narrow things
down and find the metering head is bad - simply replace it.

So that is why I recommend fuel pressure testing on CIS-E as somewhat a waste of time.
There are easier ways to get the answers to the question - Is the metering assembly
getting enough fuel (pressure/flow).

Ben


-----Original Message-----
From: John Cody Forbes [mailto:cody at 5000tq.com] 
Sent: Tuesday, August 17, 2010 12:34 PM
To: Ben Swann
Cc: <audi at humanspeakers.com>; <quattro at audifans.com>; Johnny B
Subject: Re: Narrowing down my start issue (84 4kq) [with suggested procedure]

Correct me if I'm wrong, but isn't the DPR essentially a WUR in a small plastic case and
mounted to the metering head?

-Cody (mobile)

On Aug 17, 2010, at 12:21 PM, "Ben Swann" <benswann at verizon.net> wrote:

> Cody,
> 
> This is probably why they did away with the WUR and went CIS-E. 
> Indeed, the WUR is the main reason these fuel pressure tests were 
> used.  CIS-e does not use it (WUR) and basically there is less need to 
> do fuel pressure tests.  BTDT thinking I'd trouble shoot CIS-e the 
> scame way as CIS, only to find out it became an irrelevant waste of time, and all the
connections were competely different from CIS.
> 
> This reminds me of the story about the old man called out of forced 
> retirement to fix a critical system so folks could have their Electiricity back.  He
used to work on the
> system that he helped design and implement.    The officials stood around watching as
he
> climbs down a manhole with a large screwdriver and comes up less than 
> a minute later after turning a control switch with the screwdriver.  
> He hands them the $500 bill and they complain - Why so much? since all he did was to
turn a screw to reset a breaker.
> He replied, " Yeah, but I know which screw on which control panel and 
> in which manhole in which street."  "Come to think of it, don't you 
> think I ought to charge you more - how much is the outage costing you per minute?"
> 
> Ben
> -----Original Message-----
> From: John Cody Forbes [mailto:cody at 5000tq.com]
> Sent: Tuesday, August 17, 2010 10:14 AM
> To: audi at humanspeakers.com
> Cc: Ben Swann; quattro at audifans.com; Johnny B
> Subject: Re: Narrowing down my start issue (84 4kq) [with suggested 
> procedure]
> 
> I have two five gallon buckets filled to the brim with bad warm up 
> regs diagnosed with a simple fuel pressure test that takes 5 minutes. 
> Changing parts on hunches and well meant educated guesses may work for 
> those with parts cars or a fleet of similar cars, but in the 
> professional world my customers don't want to hear "yeah I changed the 
> temp sensor, see if that works". There is a test procedure for 
> everything that can lead to fixing problems by only changing the 
> things that are actually broken 90% of the time. I understand that not 
> everybody has all of the equipment that a shop does, but in this case 
> a fuel pressure gauge set is very inexpensive (cant they be had at 
> Harbor Freight for like $40?) and would take a minute or two to 
> install and KNOW what is going on with the fuel pump, injectors, DPR, 
> and others. CIS is not like a carb or EFI system where the system pressure is
important, but can be 10% off before it's noticeable to the driver. The system pressure
on CIS is not even very important, but the control pressure that controls the movement
of the metering plate is *critical*.
> 
> Now with that all said, is it possible this problem is ignition 
> related? OP mentioned that the PO changed many things in the fuel system and had it
looked at many times.
> Could it possibly be that he was barking up the wrong tree? IIRC the 
> car in question has a basic ignition system (vacuum advance 
> distributor, no ECU), right? It's sort of unlikely, but maybe the advance is sticking
all the way retarded for a time?
> 
> Also, a smoke test would be pretty useful just to be certain of the 
> vacuum system. I've made a "smoke machine" before out of a quart paint 
> can with the lid soldered shut, some WD-40 or ATF as smoke fluid, two 
> pipes soldered to the sides to let air in and smoke out, a propane torch to make the
smoke, and an electric tire pump to do the air moving.
> Sounds silly, but I discovered and fixed a few small leaks on a car that way. 
> 
> -Cody (mobile)
> 
> On Aug 16, 2010, at 2:35 AM, Huw Powell <audi at humanspeakers.com> wrote:
> 
>> 
>>> Now that you have gotten to the point of things that Cody and Huw, 
>>> and others are pointing out, they may want to suggest clever ways to 
>>> do these tests.  Although I have done most of these in my ownership 
>>> of CIS cars, in most cases I never needed to go to the point of fuel 
>>> pressure tests  since I usually got things sorted by then.
>> 
>> I have never - NEVER - seen any need for fuel pressure tests.
>> 
>> Vacuum leaks and the resulting maladjusted static fuel mixtures, 
>> endlessly.  On almost every CIS engine I have encountered.
>> 
>> --
>> Huw Powell
>