The car is a 94 Z28 and just recently started skipping and spark knocking at RPM's less than 2500 and throttle less than WOT. The car has a Paxton SC but I've disabled the blower drive, MSD box and the SC fuel management system so non of that should affect things. In fact I've disabled all of that before and driven the car signficant distances with no problems. There are no trouble codes and I've replaced the EGR. The problem is gradually getting worse and I'm afraid it will strand me soon. The car has 65K miles and everything, excluding the EGR, is original.

A GM Dealer said, "No Joy" and claimed the Paxton stuff is interfering with their diagnostics and trouble codes. I say bull.

What's the chances it's the OPTI? Any help is appreciated.

 

If you can hear "knock" you definitely have a serious problem. The knock sensor/knock retard system is able to pull up to 15deg of timing to solve knock issues, and prevent you from even hearing the knock. If yuo hear it, either the knock retard system isn't working, the knock is so bad it requires more than 15deg retard, or the "knock" isn't from detonation but from some mechanical problem.

When you say "skipping"...... do you mean "misfiring"? Severe cross-firing in the wires can obviously cause misfires, and can cause sever knock. It could be an Opti problem, since the dielectric properties of the cap can break down, and allow the flat conductors embedded in the cap to cross-fire. But it could also be cross-firing on the wires.

What is the general condition of the plugs and wires?

 

I have not closely inspected the wires etc., and as I said they are original so they are in question. The "skipping" is definately a misfire and in multiple cylinders when it happens. When a knock is herd is is assicaiated with misfires. The car runs much better when cold. It has progressed to an inconsistant misfire at idle now. It apperars to run fine at WOT especially above 2500 RPM.

If it is the OPTI, will a vented OPTI for a 95 work on the 94 like..
http://www.jcwhitney.com/autoparts/S...searchbtn.y=14

Thanks Again!

 

You can't put a vented Opti on your 94 without changing a lot of other parts:

http://para.noid.org/~muttvette/opti.html

 

Thanks for the OPTI clearification. Does the problems discussed lead one to try replacing the OPTI next or do you suggest a different sequence of thing to try.

 

I would rule out everything else before going to the opti.


Injuneer, you say it can pull up to 15 degrees of timing so does that mean 1 degree of timing for every degree for knock retard?

 

OK, one more "stupid" question. In the dark I can see a faint blue glow around the coil during the misfire and spark knock events. I'm thinking plug wires and coil now. Any thoughts?

 

I would replace the plugs, wires, then coil in that order.
They are all easier to do than the opti.
If this does not help, then you may have to check/replace the opti.
The dealership can scan your car with a Tech2 even with the Paxton in place to check for codes and misfiring....he is feeding you a line of baloney saying he can't...I have a "modified" 95 Z28, although it is not SC, I scan it quite often with a Tech2....I happen to be an Electrical engineer at the GM CorporateEngineering Technical center Headquarters in Warren Mi....you were right on the money saying bull that it interferes with DTCs...that dealership is either lazy or full of nonsense....find another dealership.

By the way, what octane fuel are you running ? Needless to say with the Supercharger you should be running at least 93 or 94 octane fuel....it will never run right with 91 or worse fuel.
Could you have gotten a bad tank of fuel ?

 

The fuel is the best Shell offers for the last couple of tanks. I even purchased from diffrent stations to rule out the idea of bad gas.

As far as the octane and for this issue, I've got all the SC stuff unhooked including the blower belt so the SC and associated aftermaket stuff is out of the picture.

I've put a friends hand held scanner on the car, and there are no trouble codes.

I'm convinced the dealership/tech are just lazy.

Thanks for the help!

 

Both spark advance and knock retard are expressed in the same "degrees". "2deg" of "knock retard" means your timing advance is being reduced by 2deg. When you look at a scan, if there is any knock retard, the ignition advance will show that many degrees less. Add advance and knock retard to get the advance value the PCM would call for without the knock retard.