best_vette_yet

The customer brings a 33 street rod into the shop that’s powered by a 96 LT1. He is complaining about lower than normal power, backfiring through the throttle body on quick snaps of the throttle blade. So, we go through the usual injector tests, and fuel pump test, nothing. The car seems to be mechanically right. It was tuned by a reputable ecm programmer. So, we pull the tune to see what he changed. Normal stuff, but the customer had mentioned the car had a 97 tune in it originally and the tuner had changed it to a 96. So, we flashed the ecm with a 97 tune. Nothing, no change. Now we know the car is idleing with a bit less vacuum on a vacuum gauge but everything else is dead on. All the sensors are working and in range. Everything looks perfectly normal. Except for the slightly low vacuum which points to a worn engine but the customer insists it’s a 60,000 or less mile engine. We were seeing 45-48 KPA at idle. So, we bring the car to top dead center on number one. Mark the balancer and the block. Fire the car up and throw a timing light on it. The scanner is showing 25-27 degrees of advance. The timing light is showing the plug is firing at top dead center. What? Now that’s strange. We break out the lab scope and watch the hi and low res siginals out of the opti. We watch the signal out of the ecm to the ignition module, everything is normal. So we change the scope over to viewing the ignition event against the low res signal showing top dead center. Everything points to the opti being installed incorrectly. We pull the opti and find it was installed correctly. We install the degree wheel to see if the car had jumped time on the timing gears. Perfectly normal. That brings us back to the opti. But remember we were getting the correct signals from the opti. As luck would have it we had another opti laying around. So we compared the clocking of the rotor to the index pin hole in the back of the opti. One was quite a bit different from the other. We tried to grab the opti and move the rotor but it was solid. All we could figure was the heat and vibration caused the opti to rotate off of its original position when it was manufactured which was retarding our timing. This was the stock original opti. We installed our spare opti and fixed the problem. I never would have believed it if I hadn’t seen it myself.

The quick short check is find TDC, mark your balancer and somewhere on the block, start your engine, watch the timing on the timing light against what your scanner is showing. I wonder how many are just off by five or seven degrees and no one knows anything about it? We were actually off by 28-30 degrees.

cv67

Must have been a nightmare to diagnose.
One more reason not to own an LTx too many potential issues with that fubarred ign system. At least they got the intake right post L98. lol

Not bashing your Ltx guys just a poorly thought out system imo.
Otherwise Id own one

1985 Corvette

I hear more cons about the opti's finicky nature than praise. Maybe an opti expert can enlighten me on the design's intent. The system was still gear driven but was it suppose to offern an advantage over a traditional distributor? Since GM went coil packs they haven't looked back at either designs.

The old cam gear driven distributor is archaic by comparison but proven.

93Rubie

This is what they where thinking.
http://www.youtube.com/watch?v=lFCja...layer_embedded

Optical Sensors with a slotted disc are very accurate. Nissan used them for a long time. GM's only mistake was putting it behind the water pump and not venting it. Many Opti-Sparks live longer and give better service than comparable HEI units. GM used it because it was cheaper than coil packs and such.

The vented ones do just fine as long as the water pump doesn't get coolant all over it. Or you don't power wash the engine.

I personally think that about 3/4 of the Opti-Spark problems are as follows:

Owner washing engine
Water Pump Failure=coolant on Opti-Spark, NOT the distributors fault.
Cheap Opti-sparks get placed on a vehicle, owner sells, new owner has problems has to replace.
Other Ignition Problems that are NOT caused by Opti-Spark but because NO one can diagnose anything properly it gets replaced, then the car still runs like poop. They still blame Opti-Spark.

The other 1/4 are legitimate Opti-Spark failures.
Age takes it toll, 20+years on same cap and rotor/internals is a long time. Seals go bad, rotor screws come off,etc...

If you have a Opti-Spark replace only with GM or Petris Unit. Retro vent if not already done so to vacuum. Only use GM water pumps, they are higher quality than many others.

Personally, my LT1 runs great no issues thus far with the Opti-Spark.
I did have a issue this last winter/early spring. It would start to miss (intermittent) once it got warm but only at idle. I could not drive it so I could only rev to see if it still missed, it did not. Nothing wrong with Opti-Spark at all. I suspected the ICM and pulled it off, found little/no thermal grease behind unit. Replaced ICM with new grease. Its has been 100% since. How much you want to bet a lot of folks would have replaced the Opti-Spark???

dizwiz24

so you are saying that somehow,the cam's dowel drive slot position (on the 'cloverleaf' shaped rotor on back of the optispark) got misaligned from the optical disk?

Is that just an interference fit? Or is there a keyway?

Ive never had this part apart.

I would imagine there would have to be a keyway, which you think would make it impossible for that to happen.

The fact that this occurred on a stock optispark (ie. the best optispark) is disturbing. If this was a rebuild-in-China, or MSD optispark, it wouldnt surprise me.

1991Z07

I'm wondering how many people have had an Opti unit last 100k miles...

My guess is not too many.

They should have sent the signal from the optical sensor to an external firing mechanism like MSD does with their crank-trigger systems. If it was only low voltage inside, those units would have been a lot more robust.

pcolt94

We have heard of this before.

Just last month similar symptoms lots of posting and several phone calls including myself and others. The opti was a cheap replacement and the rotor was out of position to the shaft. We all worked hard on the solution and went thru a lot of troubleshooting but in the end a proper operating opti fixed the problem.

Here is the link: Pages 1-4
http://forums.corvetteforum.com/c4-t...ease-help.html

I am a bit unclear if the problem opti was on the engine originally, or had been replaced before best_vet got the car.

best_vette_yet

It was the original opti that had been removed and reinstalled by another shop because it had gotten wet when the owner washed the engine.

Man after reading that post it's obvious you guys had a lot more fun than I did. I still think there are a lot of people out there with lost power due to their opti's. I'll try to get the customer to bring his old opti back so we can take it apart and press the shafts off of each other and look for any alignment marks that may be there. I like to see what caused the problem in the first place.

93Rubie

See my post, totally called that one. Point PROVEN.

PLRX

I bet if they stay dry (no water, no oil and no condensation) it would last 100+K miles.

My MSD on Betty Boop was installed in May 2008. The original opti was working good at 72,000 miles and I made the biggest mistake on C4 history. I bought The Dynaspark, even China made fun out of it. Betty Boop has been across country four times and the MSD still working fine.

Phat98

The Missus' '94 had 139K on it, Opti was running fine but we replaced it when we replaced the water pump because it had been pissing on the Opti.

My '96 LT4 has 109K on it now, still running great. I'll probably replace the plug wires before too long...which means I'll probably replace the water pump and check out the Opti while I'm in there. I have a MSD rotor/cap kit on hand if the rest of the Opti looks good.

aboatguy

I had almost double that on the opti in my 95 SS before the car was totalled.


Mike

2VettesMike

126K on it when the original failed. Have almost 60K on the GM replacement.

Thanks,

Mike

CCarter8192

159785 miles on mine.. original opti.. But, guess what.. sputtering/missing and im suspecting its the cause.. gonna check into and diag this weekend