personal D-Jet opinions (long) [was: HELP! My '73 squareback died!]

["Gregory M. Merritt" <gregm@engin.umich.edu>]

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
	etc.

	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. :)

Peace,
-Greg
'71 squareback
'63 Beetle