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[T3] Carb vs. FI email threads

<x-flowed>Yah I guess I'll take a better look through the archives next time I ask a question. But since I asked the question, I found 2 informative email threads(from 97 and 01). In one of the threads it mentions the ability to tune the FI system. I didn't realize you could do that.



Fuel Injection. It's your friend!     By: Keith Park

Some pointers and explanation for the D Jet Type 3 FI System.

How many of us have a Type 3 where the FI has been ripped out or "Nobody can fix it". Doesn't it seem odd that the simplest production Electronic FI system has stumped the mechanic that just got that new corvette running again?? Why??

Well its because (and I may get in hot water here) most mechanics are just
trained chimps.. they can run the equipment and replace the parts that it
identified but they just don't understand how the systems work.  The D jet
systems are really Idiot simple, crude, but very reliable with only a couple
things that need to be kept up.  I don't pretend to cover all that is in the
Bentley manuel so if you don't have it get it as there are many variations
on the same basic theme but I will cover the basics and what will happen if
they are not there.

I will begin with the 1973 E System, I think it's the simplest most evolved
system and the most stable.  Everything centers around the control unit, NO
its NOT a computer but is VERY reliable and you can almost assume it is good
as long as no one has connected the battery backwards or something equally
as stupid. The Control units must have adequate voltage to them or the
systems run rich so make sure your charging systems is up to snuff and using
Bosch regulators.  This unit then connects to the engine sensors and
controls through a wiring harness which can be in rough shape after all
these years if people have abused it.  Each wire is numbered at each end to
make it easy to identify and trace (if the # isn't cut off) and the numbers
are identified in Bentley.  Some of the rubber boots are avail through WCM
and all the little spade connectors are removable from the plastic blocks
and avail through various suppliers.  If you recondition your harness
replace only the ones that are in need, DON'T lose track of the wires (1 at
a time) and SOLDER all the connections.  IF you are unsure of your skills
here have some electronic tech do it for you.

    Next we'll visit the Pressure sensor, it takes the intake manifold
pressure of the intake and through a bellows assembly changes it to an
electrical signal to the control unit which can then decide how rich to make
the mixture.  The engine will not run without it so if an internal coil
opens up (not likely) it will cause a no-start and the Bentley shows how to
test these.  What generally happens is that on the early cars people
overfill the air cleaner and the oil works its way into the sensor and gums
things up. The bellows can also develop a leak, and these cause the engine
to have poor transitional response, run too rich, or hunt a lot at Idle.
The 1971 007 sensors are particularly troublesome as the bellows are more
fragile and prone to failure.  Trouble shooting is to change them out, it's
the easiest and they are widely avail used and is a part you should have in
your extra parts box. Coils can be tested via Bentley and if they are good
it will at least start.

    Next we'll visit the Throttle position switch,  mounted on the right
side of the intake air distributor all it does is tell the computer what
position the throttle is in and in the later cars how quickly it changes
which serves as the accelerator pump on a carbed car does by giving
additional fuel if the throttle is depressed quickly.   With great care the
covers will come off and the contacts can be cleaned with a Q tip and
alcohol but BE GENTLE there fragile.  If improperly adjusted or loose they
can cause idle problems and dirty or broken contacts can cause throttle
response to be irradic.  These are generally very reliable and not a
problem. The 73 cars will idle without them attached at all.

    Now well visit the most common problem. the trigger contacts in the
base of the distributor.  All they are is a set of points that deliver a
square wave signal to the computer telling it when to fire the injectors.
Unfortunately points get dirty by nature and when they go open the engine
may cut in and out, run on 2 cylinders only, or not run at all.  Points can
be cleaned by using a business card soaked in alcohol dragged between the
points and released before the edge.  NEVER file them as they don't burn
with the little current they carry and it will ruin them.  With excessive
wear, like 150K mi or more, the nylon rider blocks can wear down and cause
the points to not open. This can be compensated for by bending the
stationary contact in VERY slightly, like .010"; then check with an Ohmmeter
how they all work in the distributor with the distributor removed.  On
occasion the rivets can go bad on the assembly and give a few ohms of
resistance so the ohmmeter should read no more than .1-.2 ohms with the
contacts fully closed.  Reassemble with just a thin smear or Bosch
distributor cam lube on the distributor shaft.  Do pay special attention to
the harness connector, they are critical here too and tend to get abused.
Make sure it is inserted in the proper direction as they can get stuffed in
backwards on occasion and this will give hesitations on acceleration beyond
half throttle.

    What's next???   Let's do the temperature sensors!  The control unit
only needs to know 2 temperatures, that of the engine which is done on the
3&4 head and the air temp of the intake air which is done on the intake air
dist.  When the head sensors go bad the car will track poorly during warmup
, run too rich or be hard to start.  Sometimes they can go intermittently
open and the car will cut out completely at speed then back in again.  You
can check it with an ohmeter or by subbing in the appropriate resistor to
check for intermittent's.  See Bentley for the values.  The other sensor
will cause more of a consistent rich or lean problem if bad, it will be more
subtle and be careful as it is easy to get its 2 pin connector confused with
the cold start injector connector.  Again check with an Ohmeter.

    The cold start system, on the later cars it is a simple thermo switch
that switches in the cold start injector below 32F and a cold start injector
that squirts in a little extra fuel.  This system is subtle too, I have
started mine pretty cold without it connected. The 68-9 cars had an extra
relay and a crude electromagnetic valve for the cold start injector. It
might be better to disconnect them in warmer climates in these cars as they
had the tendency to flood.

    Next well visit the intake auxilliary air regulator, all this does is
to let a little more air around the butterfly valve so the car will have a
high idle when warming up. Its controlled by the engine oil temp and is
adjustable so if your fast idle is too slow remove the Air reg and you'll
see a little slide adjustment, loosen the screw and slide it all the way to
the + and tighten it.
Other than needing this adjustment they are usually bulletproof.

    Now this is all there is to the electronic part of the system.. now Ill
go into the fuel section, without which the electronics is useless.  Keep in
mind that any vacuum leaks will drive the system nuts so everything must be

                     THE FUEL DELIVERY SYSTEM

    Well start with the safety aspect, the fuel pressure is 30PSI so every
fuel line should be 10 years old or less, if in question replace them all,
its easy and many a Type 3 has gone up in flames from an old leaky fuel

    With that said, the fuel pump is the heart of the system, very reliable
but sometimes get gummed up if sitting for a while.  It is operated through
a relay under the dash from the control unit.  You should get 1.5 seconds of
fuel pump pressure whenever the key is turned on, if you don't then trace
out why, just take a voltmeter and trace things out. Dirty trigger contacts
can cause this too. The pump only takes fuel from the tank and recirculate's
it through the system and through a pressure regulator.  This also is very
reliable but when mine went it let the pressure go too high on startup and
the car flooded easily.  That's all there is to it!  So if the car doesn't
run at all make sure you has fuel pressure before proceeding any farther
with the control part of the system.

    Oooops!  I did forget 1 important part, the Injectors themselves!  All
they are is an electromagnetic valve operated by the control unit that with
the 30PSI of gas entering them produce a nice fine spray of the appropriate
duration.  They are pretty reliable but when the cars sit the internal
rubber O rings can rust around them and leak.  The injector will then leak
gas between the plastic and metal sections and is not repairable.  The
injectors can also suffer from a plugged up screen in the nozzle, some are
screens and some are a brass casting but eather way you'll start off with a
low RPM miss on the affected cylinder leading to a dead cylinder as it plugs
up more.

    Now your ready to intelligently troubleshoot your D jet FI, keep in
mind that I have left out a couple valves and such that are used on the
Automatics as I have no experience with them BUT the BENTLEY must be used in
conjunction with  this article and they are covered in there. Just keep in
mind, this is NOT as complicated as it looks and since you have already made
the purchase of a Type 3 your intelligence is NOT in question! Relax, and
check things as much as possible before replacing them and feel free to ask
questions of the listee's  (Vwtype3.org).


On Wed, 28 May 1997, Shannon wrote:
>his former owner felt the same as i do about fuel injection.......it sucks.

I have heard this a lot...when the FI works it's great...when it doesn't
it's al living nightmare.  And I believe it too because I had all kinds of
heartache with my 70's FI system.

	In my opinion, if you can use a multimeter & read a fuel pressure
gauge, fuel injection is no more difficult to work with than a carburetor.

	In fact, in some ways a carb is equivalently complex.  The basic
design concept of a carburetor is inherently flawed, since the amount of
fuel drawn for a given amount of air taken is varies with the air speed.
Much of a carb's design is a bunch of features--bandaids, workarounds--
that attempt to correct for this behavior.  This is the source of its
relative complexity, IMHO.

	What carbs *do* have going for them is that lots more people have
worked with many carburetors for many, many decades (over a century, eh?)
and so the body of practical knowledge re: carbs is much more widely spread.

Are there any special maintenence tips to keep your FI system alive?  Any
fuel additives that you fellow T3'er swear by?

I think that studying up on D-Jetronic fuel injection is the absolute best thing one can do for their injected T3. I highly recommend "How to Tune and Modify Bosch Fuel Injection," but read everything you can get your hands on.

	I had *lots* of problems with my fuel injection when I first got
my squareback.  The first problem was one of my own:  "if anything goes
wrong with the injection, I'm throwing on carbs!"  My injection system
caused my car to crap out once, and I was ready to lay down the cash for
a traditional induction system.

	But guess what?  My so-called (by me) "injection" problem was an
ignition problem.  It had *nothing* to do with the injection.

	See, the f.i. was a big mystery, so any difficulties I had with
the car I immediately chalked up to the injection system.

	My next few "injection" problems also had nothing to do with the
fuel injection!!!

	Now, I'm not saying that it's perfect, and I have had difficulties.

	If you swap to carbs, you're trading one set of failure modes for
another... know anybody who's had trouble sync'ing dual carbs?  Stuck float?
Wrong jet?

	D-Jet really isn't that bad.  I wonder if anybody can honestly say
both "I understand f.i. as well as or better than carbs" AND "I hands-down
prefer carburetion."

	If you are well versed in carbs, guess what?  D-Jet has a system
that performs virtually every funcion in the carb, conceptually speaking.
Check it out:

	choke = temp sensors
	fast idle cam = aux. air regulator
	accel. pump = throttle position sensor
	pumping accel. on a cold day = cold start valve

	Many of the D-Jet components don't play a heavy role in steady
cruising; they just augment performance under extraordinary circumstances
(e.g. very cold temps, starting, punching the gas pedal).  Here's what's
left at the heart of the system:

	injectors:  they open & close to squirt gas behind the intake
valve.  When more gas is needed, they stay open a bit longer each time
they pulse.

	trigger points in distributor:  there are two of them; when one
opens it causes one pair of injectors to fire, when the other opens it
causes the other pair of injectors to fire.

	manifold pressure sensor:  a crude (but more efficient than carb!)
measure of the amount of air entering the engine.

	That's really it, folks.  The brain looks at a combination of
inputs to determine the length of time the injectors are open, but that's
about it.  Cold out?  Hold the injectors open a bit longer--gives more
gas like a choked carb.  High manifold vacuum because butterfly is closed?
Must be idling or on decel; shorten injector duration.

	I think that D-Jet also suffers from the old guess-replace-repeat
repair philosophy:  "I guess that X is the problem, so let's replace X."
Doesn't fix the car?  Guess again:  "Maybe it's Y; let's try that."  This
can get very expensive very quickly!  I would caution folks to not pay to
replace any f.i. component unless it is confirmed to be faulty via electric
or other tests.

	I also understand that if, say, somebody has a bad brain, a clogged
injector, worn distributor shaft *and* a bad pressure sensor, the carb
swap may save you lots of dough.  My personal insurance against such a
situation is the collection of f.i. spare parts.  People practically (or
literally) *give* them away in many cases!  Try low-balling at swap meets
on f.i. parts; you'll come away with loads of spares.

	It took me a *long* time to try to be rational about my injection
system, and it still takes diligence for me to stay rational when there
are mysterious problems.  If you're having D-Jet difficulties, grab a
book--or a couple of books, like "Tune & Modify" and the Idiot book--
and have at it, slowly & carefully.

	...and if you decide to pitch your f.i., drop me a line & I'll
give you my shipping address.  I'll even pay the postage. :)

'71 squareback
'63 Beetle

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