topnotch at nycap.rr.com
Thu Dec 30 18:51:04 PST 2010
Thought it was a good time to post this.
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
Well it's 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 manual 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, or
"computer" but its 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 are 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 erratic. 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
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 warm-up
, 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 ohm meter 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 Ohm meter.
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 we'll visit the intake auxiliary 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. The 70 and
newer Automatic cars used an electrically heated auxiliary air regulator,
which if binding will cause the same idle problem but this one isn't easily
disassembled. You can check the heating element with an ohm meter, T4 cars
use similar units that *May* be able to interchange if the hoses clear.
Correcting idle speed problems with later automatics may be done with
"restrictions" added to the hose to reduce airflow if you need to slow the
fast idle speed, just make darn sure the car can't have the restriction for
lunch by its coming loose.
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 tight.
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,
it's 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 have 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 either way you'll start off with a
low RPM miss on the affected cylinder leading to a dead cylinder as it plugs
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).
Top Notch Restorations
topnotch at nycap.rr.com
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