Is there a way test the Opti still in the engine on a 1994 Corvette?
My car cranks normal — at the turn of key — but after a few miles,
it appears to idle a "little rough" especially at low speeds in 4th,
5th and 6th gears. This just suddenly started to happen. Opti was
replaced a little over 2 yrs. ago. Thanks

 

Get the car warmed up and put it in 6th (or your highest) gear. Try and go up a hill if it stumbles or cuts out then its bad.

The car has to be hot for this to happen.

Btw. I'm told the tach signal comes from the opti. So if the tach acts weird (like doesn't bounce around as you crank it) then the opti is bad. Of course, in that case you'd have a no start condition which you don't have......

 

Donne Trav,

Before you do anything, read the codes in the memory of the PCM using the paper clip method or a tester that can read OBD 1 diagnostics. If you have no codes that reference the Opti, then look at the plug wires and any vacuum hoses coming from the intake manifold under the plastic covers. You cand check for broken wires and arcing in a completely dark garage with the engine running. Cracked hoses can be checked by feel and looking for hardness rather than flexibility. The Opti catches a lot of grief from those who don't understand it. Yes, you do not want to wash down the front of the engine and Yes, you want to replace a leaking water pump, BUT other obvious things can be wrong such as the Ignition Control Module (ICM), coil, or a break in a wire or a loose connection. Check the obvious first,

Good luck.

Mark

 

The "rough running" at low speed in the higher gears is an indication of ignition problems, but not necessarily the Opti, as Mark stated above. Try his suggestion about checking for arcing in a dark garage...you may be amazed at what you see. A small spray of water from a bottle will also help track down an elusive arc.

 

Will the "Service Engine Soon" light appear if the Opti is bad? When I
had an obvious bad Opti, I never noticed that warning before. And a
BIG THANKS for the replies.

 

It depends on where the fault is. If there's a fault with the low-resolution optical sensor, the engine will not run, and it will set a code. If there's a fault with the high-resolution optical sensor, the engine will still run (although not optimally -- no pun intended) and it will set a code. If the fault is in the high-voltage distributor section, it will not set a code related to the Opti.

Your symptoms do sound ignition-related, but that includes more than just the Opti. Could be the coil is starting to fail, could be a plug wire, etc. I would recommend you get it to a good diagnostician before you start throwing expensive parts at the problem.

Live well,

SJW

 

Hell, my opti blew up and didn't set a code.

 

My very first thought was the ICM. It may be starting to fail when it gets warm.

Plugs, wires, coil, icm, clean maf, these are all things I would check before even considering the opti.

 

I have a 95 LT-1 Coupe. I chased a number of what I thought were Optispark probems last year. The uphill test in high gear is the best way to test and do it several times. Now from a practicql point because of the heat from the head, replace both ICM and Coil with new genuine A C Delco modules. Now if your MAP sensor (top forward right on intake manifold) is more than 3 years old, replace it with A C Delco MAP Module. This device tells the ECM the absolut atomospheric pressure. It is a gel cell with a pizzo-electric transponder attached and the engine heat drys the gell to HARD. They gennerally fail to 0 or 6 volts (the 2 end conditions).You can check this with a voltmeter. The diagnostics is very clear in the FSM. It just doen't tell you how much it effects the ECM when failed. The atmosperic pressure changes behind the throttle body dramatically during hard accelleration. The new module cleared a number of small but troublesome problems in my engine particularlly during warm up and open to closed loop transfer. The Optispark diagnosis becomes much easier with these items no longer in the equation.

 

Where is the MAP Sensor actually located on my '94 LT1? I did notice
a "connector" attached the passenger-side / middle of the Throttle Body
with about 12" of wires that go directly to the Opti. Other than that, I
don't see anything in the location you describe. I also dis-connected each end — along with the many other connectors — inspected them,
re-connected all, but still get "Service Engine Soon" message. i have
no means of actual testing, ie: pulling codes, etc. Thanks a bunch!

 

Passenger forward on top of Intake Manafold jut behind the throttle body. Its a black rectangle, mounted with 2 screws and has a connector on the bottom. Save the plastic gasket device that goes around the port that comes out of the bottom and into the Intake cavity. If its missing or unusable use RTV but give it 24 hours to set up before stating car. The reason I know it gets hard and can not detect atmospheric pressure changes is I took mine apart, and since A C Delco (Amazon) sold me the wrong one and would not take it back, I cut it open also. The gland in it was soft and plyable. MAP Sensor, ACD# 273-3205, GM# 1906672.

 

[QUOTE=Donne Trav;1581995544.... I also dis-connected each end — along with the many other connectors — inspected them,
re-connected all, but still get "Service Engine Soon" message. i have
no means of actual testing, ie: pulling codes, etc. Thanks a bunch![/QUOTE]

Hi Donne,

seeing no additional posts, hopefully you found your answers. However, as you originally did not mention a 'SES', but later on do. If it was not on originally but is after disconnecting all the connectors, if done with the key on, just the disconnect/reconnect could create errors/codes.

Either way, you only need a paper-clip to pull the 'codes' which can define why the 'SES' light is on and diagnose some codes which DO NOT illuminate the SES light.

your 94 is OBD I, but has a OBD II 16 pin DLC connector (same as all cars starting in 1996).

To pull the codes, a paper clip to 'jumper' between pin 12 & 4 (12 = CCM Diagnostics enable, 4 = ground -- slightly wider top row is 1-8 left to right, narrower bottom row, 9-16 left-to-right) and then turning on the key (but don't start) will display the 'codes' on the dash's digital display (do a search to understand how to read).

 

My 96 is doing the same thing and even get like a arcing sound from front of the motor too. Ran a scan and it comes up muliple misfire.

Changed my plugs and didn't help at all. Going to look into other info on this thread and see whats happens, before i change out the waterpump,wires and optispark.

Thanks everyone..

 

[QUOTE=theadmiral94;1582123657]Hi Donne,

seeing no additional posts, hopefully you found your answers. However, as you originally did not mention a 'SES', but later on do. If it was not on originally but is after disconnecting all the connectors, if done with the key on, just the disconnect/reconnect could create errors/codes.

Either way, you only need a paper-clip to pull the 'codes' which can define why the 'SES' light is on and diagnose some codes which DO NOT illuminate the SES light.

your 94 is OBD I, but has a OBD II 16 pin DLC connector (same as all cars starting in 1996).

To pull the codes, a paper clip to 'jumper' between pin 12 & 4 (12 = CCM Diagnostics enable, 4 = ground -- slightly wider top row is 1-8 left to right, narrower bottom row, 9-16 left-to-right) and then turning on the key (but don't start) will display the 'codes' on the dash's digital display (do a search to understand how to read).[/QUOTE:
Posted New Thread: "Opti-Testing: Update". After making an appointment to take my problem to the local dealership, I purchased a
$9.00 bottle of Lucas Fuel System Cleaner. Like magic, problem solved!
Next time I'll start small, before over-reacting and assuming the problem
is bigger than it really is: ie., Opti and other electrical problems.
Thank you All!

 

optispark wont always throw a code. Mine didnt and heres what it looked like inside.




Again, follow my advice about getting the car hot, putting it in 5th/ 6th gear and try to lug it up a hill. If it doesnt act smooth, cuts out for a second or so. Also sometimes Id get a stumble during WOT dragstrip style run. Only if it was hot. Once the car just wouldnt exceed 105 MPH or something like that in high gear (due to wind resistance creating a load). Weird stuff.