Hello! Yes, I am a new guy.
Just picked up my 94 C4 yesterday and am quickly falling in love with it!

It made a 50 mile trek home without issue but I noticed on my second drive that the engine was skipping occasionally and the tachometer would jump erratically at the same time. I assumed this must be the famed optispark that I've read about. I'm more familiar with the GM DIS setup and the best I can relate this issue is that it feels like a failing ignition module or a bad crank sensor/wiring... that said, I'm not sure if the LT1 uses an ignition module or if it is built in to the optispark. I'm sure it must have a crank sensor for the fuel injection but does it use it in relation to spark timing as well?

I want to replace the optispark, coil, wires and plugs in the near future. I've read that the vented MSD optispark replacement is the preferred choice. I've seen them labeled optispark and optispark II? Is there a particular one that I need? What about coil and wires, MSD a safe choice here as well? Plugs? I'm a fan of AC Delcos but open to whatever works best.

I'm not new to wrenching around a car/engine and dare I say I'm pretty good with a wrench. I have plenty invested in tools, compressors, etc. to get just about any job done. I'm new to the LT1 though and if there is a how-to written up on how to replace the optispark I would sure like to see it.

I guess this post can't be complete without at least 1 pick of my new car... here is what I think is one of best looking parts of this car, its butt:

 

Check your plugs and wires, first. These HEI ignition engines are very sensitive to bad coil and plug wires. A bad wire or wires can easily cause symptoms you're describing.

Open the hood at night or in a darkened garage, start the engine, then pull the hood lamp fuse. Look for any telltale electrical arcing and/or "creepycrawlies" along the wires.

What shape are your plugs in? I'd consider swapping them out, too. NGK TR-55 Double Platinum is an excellent plug.

 

Thanks for the response. I don't know the age/condition of the plugs - just got the car yesterday. I can see how a ba coil/plugs/wires would cause the skipping but could it also cause an erratic tach. That said I do plan on replacing plugs/wires and coil. I guess I'll do those first and see how it runs before replacing the optispark.

 

I just got a 95 last week and it looks like a job just to put new wires on.I have oil around and on my optispark so I think I have to replace my intake gasket
Mine has 71,000 and I don't know if the plugs have ever been replaced.Wire look original.
Mine seems to have a small mis.

 

OK, looks like we have two different posters with different issues.

jmgtp - With no codes its more likely to be wires, ICM, or coil. The ICM and coil are mounted on the passenger head.

tblt44 - oil behind the opti means the seal is probably bad in the timing cover, not intake unless you can trace it all the way back.

When I had to replace my opti I had a small leak at the waterpump seal. Just replaced it when I did the opti and waterpump.

For vented stock opti an AC/Delco replacement is fine. If want to spend more money MSD or Dynaspark are an upgrade. Non-vented opti go with the MSD or Dynaspark. I put a Dynaspark in because the only choices at the time were Dynaspark and Accell. No way would I put an Accell on, I didn't want to have to do it again.

 

That sure is a great looking car! hope you get things ironed out with it soon.

 

Here's a coil and ICM pic for you guys as a visual reference.

 

Thank you all for the responses and the pic.

My coil does look to be in beat up rusty condition. I am surely replacing it and plugs/wires.

Sometimes it runs great, other times it barely runs. What bothers me is that the tachometer behaves erratically. I imagine the output signal for the tach comes from the optispark. For that reason alone, it makes me want to think that my optispark is the problem. I don't mean to step on the toes of what you guys have suggested but this is just what is running through my head, is my logic sound?

The below link is a video of the car at idle. I am not touching the gas pedal at all. You can see the tachometer do some weird spikes. While the 'spikes' report an increase in engine speed the engine is actually misfiring and losing rpm then the idle air controller tries to compensate and it revs up a little. Could a bad coil/wires/plugs cause the tach to do this? The optispark is new to me, I'm familiar with GM DIS systems like the back of my hand but the optispark is a whole other game.

Link: http://www.youtube.com/watch?v=JLoqKq33Ws0

thanks for looking!

 

Embedded to help a Brother out....

I'm wondering if you might have a bad voltage regulator in your alternator? Do you get spikes on the voltage meter on your dash readout, too.

 

I'd be doing a whole lotta "Grounding" checks, and cleaning right outa the gate, and as previously recommended, check your Alternator output with a hand held DVOM.

Oh ya...Buy a FSM, they do help.

 

Thanks for the replies and keep them coming!

I haven't noticed any voltage spikes that accompany the tach jumping/engine stutter. The gauge remains steady and the digital output near in the speedometer screen seems steady at ~14.2v. I'll throw the meter on the car next time I'm at the garage to verify but as it sits I don't suspect the alternator.

I'm going to change the coil with an MSD unit and replace the wires/plugs as well. That will be my first step, if the problem persists after that then I'll let you guys know!

Thanks again, John

 

I had this happen on my daughter's Dodge Avenger recently...I finally traced it to the main ground to the engine being loose. It caused all kinds of misleading codes to be thrown, and the idle was doing excatly what yours is doing.

BTW, check for codes. You can see how to do this with a simple paperclip on your 95. The DIC will then show you if any codes have been thrown.

Check your grounds

Also, if you plan on doing your own work on the car get the 2 volume service manual....If I remember its about $160 from Helms

 

I've got a 94 that I'm i love with too. Great car. It was missing a bit at first, and got progressively worse. A friend recommended putting Techron in on top of a full tank of Chevron, as the injectors could use the cleaning, which helped ALOT.

I'm sure my opti will need changing too as I've noticed a very small drip of antifreeze under the car where the waterpump is but it is runnng better now with the fuel treatment. I will be watching this thread though to see how you manage it.

Congrats on the 94!

 

I assumed the paper clip trick wouldn't work to read codes on the 94 ecm. I have a 91 Grand Prix that I use this method. I was under the impression that the OBD 1.5 introduced in 1994 eliminated that option as well as making tuning much more of a pain. I have an OBD I cable that I use on the Grand Prix to interface to my laptop through TunerPro RT. I also have all the hardware and capability to burn new eeproms which I do for the Grand Prix - if that is possible with the 94 Vette ECM that would be great.

The check engine light isn't on, but I suppose there could be codes stored.

I'll have to do some searching for a service manual, $160 is a bit steep right now given that I just purchased the car, paid registration, and I bought a coil/plugs/wires this evening for just under $200.

If it matters... I went with an MSD Street Fire Coil, AC Delco Rapid Fire #14 plugs and AC Delco Wires.

 

Paperclip still works on a 94.

See the bottom of this link:
http://www.corvettebuyers.com/c4vettes/codes.htm

I'm an interested observer on this one, as I'm having a similar unresolved issued with my 93 even after replacing the ICM.

-John

 

chevron gas already has techron in it

 

If the tach is jumping around it's probably a 97% chance the opti is going out. You might even get some codes (using the paper clip method).

Since it's a new car for you, a new set of wires and plugs would be money and time well spent.

Optis have come way down in price, last time I did it, I was happy with $250. Now ebay has them for $90 shipped!

Doing the venting thing is a good project as well. While you're in their take a good look at the vacuum lines. 15 years is a really long life for rubber hoses and fittings.

I'll be replacing my opti pretty soon (again!). It's a full day job the first time, about 3-4 hours the second time. The hardest part is figuring out "how the -f- do you get a wrench on that m-f bolt!!" Take heart, you can get to all of them.

Cheers - JR

 

I too thought that the tach jumping had to mean a bad opti. However, I installed new AC Delco Rapid Fire 14 plugs, AC Delco wires and an MSD Street Fire coil and it runs like a champ. No more skipping and the tach behaves normal all the time. I also discovered that my throttle was not opening 100% - the adjuster near the ASR took care of that and now the LT1 is running like it should. I am very happy with it right now!

Thanks to everyone who responded, my problem is solved!

I did the intake mod while I was in there and removed the fins off the airbox lid. I don't know if there were any real world gains from it and I don't think I notice any difference in sound. It does have an aging paper filter that should probably be replaced. At $80 I'm wondering if the K&N is really worth it or if I should just go with an aftermarket intake system for a bit more? If the aftermarket intakes are only worth a few ponies then it would be hard to justify the cost and I'd rather put that money towards exhaust or gears.

 

Just put a new paper filter in there.