Ever get that sinking feeling?
Gearheads do, all the time.
For many it's almost like ESP. We
just know something's off even if we can't quite put or finger on it. Maybe it's subtle. The occasional misfiring ignition, a hard
start condition or maybe the not so subtle hint that's something gone wrong
when your car just stops.
Any of those symptoms can indicate a problem with the
ignition system but when it's a car equipped with the GM Optispark optical
distributor you're going to be questioning the "excellence" in the
"GM mark of excellence"
What's an Optispark?
The Optispark is a unique ignition system found primarily on
GM's venerated LT1, LT4 and L99 V8's from 1992 to 1997. This was the second generation of the GM
small block V8 motor and the first major design change since 1955. Unlike its predecessors, the Optispark
distributor is an optically triggered ignition system much like those seen in
high performance and race vehicles.
Optisparks showed up in everything from Corvettes to Cadlillac
Fleetwoods and they all have the same design flaw. That being, putting it in the absolute worst
place you could possibly locate a component sensitive to heat, moisture and oil
contamination.
Put yourself in the following scenario. Imagine having a job where you were forced to
work next to a blast furnace every day with the threat of a huge water pipe
bursting over your head.
That's pretty much the life of an Optispark
distributor. Buried deep under belts,
hoses, pulleys and a water pump, the unit can be barely seen and hard to
service. Replacement is always a major undertaking involving the removal of the
aforementioned bits and pieces as well as assorted brackets, wiring harnesses
and other assemblies depending on the vehicle being serviced.
So just what's different about an Optispark?
Other than the look of it, not much really. It's still a distributor that requires a
rotor, Ignition module and coil to provide spark to the spark plugs at the
right time. The primary difference is
that timing is determined via an optical signal instead of the position of a
reluctor or cam follower. Normal distributors
usually have shaft with a rotor on one end and a gear on the other that meshes
with its counterpart on the camshaft.
Where the only indication of incorrect timing on a regular
distributor is a poorly running engine, the Optispark ignition provides
feedback to the engine's PCM. Using data
from other sensors at its disposal, the PCM can then adjust the ignition timing
to optimize engine operation.
It utilizes an optical pickup assembly that reads a thin rotating
disk with two rows of perforations punched into it. One row is comprised of 360 slots on the
outer ring with another row of 4 cutouts of differing size on the inner ring corresponding to 90 degrees of engine rotation. The disk rides on a bearing assembly mounted
into a base plate that engages with the timing assembly on the front of the
engine. As the engine rotates the
optical pickup reads the position of the disc and reports that information back
to the PCM where it can adjust the ignition timing according to current
conditions. The major components of the distributor are fairly simple consisting
of:
- ·
A cap
- ·
Rotor
- ·
Plastic separator
- ·
Metal separator
- ·
Optical pick up assembly
- ·
Timing wheel
- ·
Wheel support disc
- ·
Bearing assembly
- ·
The man body or base plate
Aside from some sealing gaskets and the wiring harness
there's not much more to it.
Depending on the year of the engine, the distributor engages
the crankshaft timing gear with either a splined shaft or on later engines (95+)
a large bearing resembling a hockey puck with cutouts to engage a pin on the
engine's timing gear. An issue with
condensation and ozone buildup in earlier models of the Optispark (92-94)
caused premature failure. GM redesigned
the distributor for later models in an attempt to eliminate the issue. The easiest way to tell the difference
between the two is the presence (or absence) or two rubber hoses connected to
the distributor. One is connected to
vacuum while the other usually connects to the intake ducting of the engine.
The Optispark distributor allows for both fine and coarse
tuning of the engine timing and has a certain amount of redundancy because of
the presence of those two timing rings.
Should something short of catastrophic failure (which usually happens anyway) interfere with either timing signal,
the engine can continue to run at reduced power levels. It usually sets a code in the engine's PCM
that will trigger the Service engine light.
Most common codes are:
P0372 - Loss of the high resolution timing signal ( not reading any of the 360 slots)
* 92 to 95 engines will set an OBD1 code of 36
P1371 - Loss of low resolution signal ( Not reading the 4 cutouts)
* 92 to 95 engines will set an OBD1 code of 16
The design allows finer control of the timing by reading not
just what cylinder is firing in relation to mechanical engine timing ( what valves are open, where the pistons are
in their stroke etc.) but exactly what degree in rotation the engine is currently in. In this way
the Engine's control computer or PCM knows exactly where the timing sequence
was in relation to a specific condition and how long it lasted. For example, If a condition such as a misfiring plug were indicated, the PCM would be able to either correct the condition or set a trouble code.
Sounds great, so what can go wrong?
Oh wow, let me count the ways...
I remember when optical pickup distributors were cutting
edge. Using light to time an engine
might as well have been rocket science to most shade tree mechanics.
Usually, optically based ignition systems are among the most
accurate and reliable ignition systems available. Even modern so-called "Distributor less"
ignitions use "flying magnet" or Hall effect sensors that operate
similarly to optical triggered ignition systems but use magnetism instead of
light to determine timing.
BUT...
Unlike most ignition systems that are generally designed to
keep the most sensitive components away from harm the Optispark stands in stark
contrast.
It sits low on the front of the engine between the water
pump and crankshaft pulley and sandwiched between a rats nest of cooling hoses,
brackets and engine accessories. It's at
just the right height to get drowned by the undercarriage spray at the local
car wash and sit's below a notoriously leaky water pump. Lest we forget seals for both the distributor
to timing cover and water pump shaft whose inevitable failure are sure to coat
the Optispark in a greasy mess.
There's usually some degree of warpage on most Optispark
distributor caps which allows gaps in the seal between cap and
distributor. That makes it even easier
for infiltration of water and oil.
Then there's the issue of varying quality of
replacements. I've yet to find any
source for replacement of just the optical pickup assembly which is the most
critical component of the Optispark.
That means failure of it requires replacement of the whole
distributor. With prices ranging from
$50 for a unit guaranteed to leave you stranded to $900 for a GM aftermarket
unit that may be no better you could go broke before you found a reliable
source. Nothing like doing the same job
two or three times before you finally get one that works.
We've talked a lot about failure of the Distributor itself
but don't forget about the wiring. Mid
90's wiring harnesses tended toward the flimsy and after 2 decades they can get
a bit crispy. It's always good policy to
check the wiring before going to all the
trouble and expense of replacing an Optispark only to find out a $20
Distributor harness would have fixed the problem.
A good source for troubleshooting Optispark ignition issues
can be found at SHBOX.com. While focused
on 93-97 Camaro and Firebird models the troubleshooting steps will work on
LT1's installed on other models. If you
have one of these engines, this is a bookmark you'll use often.
So if you've made it this far you'd probably like to know
how to fix your Optispark problems.
Aside from the link above we'll leave that topic for the next article.
Labels:
2018,
auto repair,
backstory,
cars,
classic car,
firebird,
fix,
formula,
stories,
transmission,
used cars
If that's not possible then you're only going to get a general idea that there's a problem. In which case you'll have to take into account other factors. In my case I know the Fuel Pressure regulator is good ( brand new ) so I'm fairly confident that any pressure loss seen at the rail is going to be isolated to a component of that rail. Note that all of this testing is done with the engine off. With the engine running other factors like the Fuel Pump, Injector pulse time and engine vacuum come into play which can mask a basic fuel pressure problem. 
The MAP sensor's job is basically to read engine vacuum and send a signal back to the PCM. With an LT1 the PCM takes that vacuum value and compares it with the Air flow data from the MAF to see if the two values complement each other. That allows the PCM to make more minute adjustments to the fuel mixture for performance and emissions. It's also a sanity check for each sensor since the failure of one can be compensated for by the other. Either one can provide enough data to allow the engine to continue to run but not as efficiently.
The procedure was fairly simple once I figured out what vacuum
port to use. Access to anything under
the hood of a 4th generation Firebird is difficult. Finding the right vacuum port without
breaking parts that have been cooking under it for 20 years even more so.
.JPG)
