haxxx

I developed a really bad random misfire last weekend and all my testing is pointing to the optispark. This annoys me because the previous owner has a pretty pricey work order where it was replaced about 15k miles ago. This included new plugs wires, water pump and thermostat. I'm guessing the modification to add a hose to the water pump was not done.


This video is cheesy but it was the most helpful in completely understanding the unit. I noticed I had no vacuum on the inlet line. I checked vacuum on the line coming out of the intake manifold (after the check valve and filter) and had about 18psi. I also tried pull vacuum on the opti and it did not hold vacuum at all, which I'm guessing it should.

Should I just condemn the stupid thing at this point? Is there any further testing I can do on the unit?

1993C4LT1

What year is your car? If not a 95/96, then it will have no vacuum line going to the opti. Unless it was converted to be vented, but that doesn't seem to be the case. I would start by seeing if you have any stored codes. Even if the check engine light is not lit.

haxxx

It's a 96 so it is vented. It's a P0300 which is obvious from sound and driving. Now that it's dark, and just to make sure, I sprayed the wires with water while running. All I have to say is HOLY CRAP I can't believe how much they are leaking. The wires were replaced with the opti and still look brand new. They are the AC Delco ones. What gives?

MatthewMiller

FWIW, I know some people have written that the opti should hold a vacuum (i.e., you could measure vacuum on the inlet hose to the opti). But unless someone sealed the pass-through where the wiring harness goes into the cap with RTV or at least silicone, I don't think it will. I wouldn't assume that is a clue to your problem.

Not to discount your wire arcing findings (because that sounds like a real problem!), but you should also check your O2 sensors. Your car should throw a code if one of them goes bad, but it isn't guaranteed. I don't have any great answers as to why your wires are arcing.

1993C4LT1

If your wires are arching, I would for sure change them.

haxxx

I bought a new set of wires. They all appear to be leaking all over the place and the wire to the coil was leaking out of the boot which was probably the final straw.

In my search for wire sets there are different sets for the LT1/LT4 but I'm not sure why would they would be different. The only difference I could find on the AC Delco ones was the LT4 were red and the LT1 were black. I bought LT4 ones anyway to keep them red, but I'm wondering if there is any real difference.

MatthewMiller

There should be no functional difference. The two engines used exactly the same ignition parts AFAIK.

81c3

Be sure to use dielectric grease on each plug boot at the cap and the plugs..... it will help seal the boot and help prevent spark arc.

lt4obsesses

this is just personal experience...

I had an issue with a flashing MIL with a P300 code. It was intermittent for about a week. The true cause came to light when the fuel pump crapped out.

I also had a hotcam swap done and never got around to a dyno tune. It would throw a p300 code every once in a while, which, from research was typical. However, when I replaced the dual mass flywheel with a steel single mass, I drove the car for a year and never got another 300 code. My only guess is that the DMF was so out of spec that the vibration, combined with the lope of the cam triggered the knock sensor.

Replace those wires, and check the plugs while you're at it. Check ignition control module, the thermal grease can wear out and it gets hot. Check the pigtail connection at the opti, I had to replace mine as the connector was starting to corrode. (however, yours was probably replaced with the opti) Of course, check that vacuum line and maybe replace the check valve. If this is clogged, there can be a build up gasses inside the opti causing it to arc and mis-fire.

haxxx

My LT4 is stock so no hotcam. I have already replaced the fuel injectors last year and that cleared my first P0300 though it wasn't noticeable. Now you can feel the uneven power distribution in the low rpms. I'm getting the new plug wires in today and hopefully will have that done this afternoon. Looks like a real PITA because of space restrictions. For the opti, I have good vacuum after the check valve and filter, just not getting any on that smaller line that goes before the throttle body (but thats probably because the opti is not holding vacuum). ICM and coil check out, pretty confident it's the wires. Will know for sure this weekend!

What flywheel and clutch did you put in? I'm looking at doing that soon as well. My flywheel has been getting very noisy.

1993C4LT1

Passenger side is easy, the driver side is the fun part.

Klondike

I didn't think there was a difference either till I changed my wires. The coil wire is a couple of inches longer on the LT4 set than it is on the LT1's. WHY? I haven't a clue, but my coil wire from the LT1 set was too short. MSD sells an LT4 set and I think they also sell just the LT4 coil to Opti wire.
The vent tube really shouldn't pull a vacuum on the Opti, just pull a breeze through it. If the inlet is plugged and the Opti is sealed up, pulling a vacuum on it will collapse the cap down onto the rotor and destroy it. It doesn't take much vacuum to move a large surface, that's how the power brake booster works, and you know how strong those are!

haxxx

It seems as though my opti isn't sealed so the vacuum is just leaking out and not really going through the tube that connects back before the throttle body. Either way it works I guess.

The drivers side wires to the opti is the most uncomfortable and frustrating thing I have ever done on a car. Holy crap. I have thin small hands and arms and I really struggled so I can't imagine how anyone bigger manages. For the opti wires on the drivers side I had pull on the tensioner and I propped it up with another bar just so I could fit my hand in there.

I used this Taylor set:


They are way cheaper than the AC Delco set and appear to be better quality. They are thicker and have kind of a glossy and tougher feeling insulation. The AC Delco measured right at 7mm and the Taylors measured at 7.9mm and just barely fit in the wire looms.

Anyway, after realizing I broke the ECT sensor wire bracket and got two wires switched it is finally running perfectly! Goes to show you really can't rule anything out without testing. Just in a span of 20 minutes the wires degraded to a point where the car was running OK to really terrible.

1993C4LT1

I haven't done this in a while, since I am on LTCC(lt1 coil per cylinder) but, I remember taking that tensioner off, serpentine belt and maybe the alternator for extra room. I used Taylor wires as well. They never gave me a problem. Props to you for diagnosing this

Wait, what wires degraded on you?

haxxx

All the spark plug wires were leaking spark all over the place. I sprayed them with water in the dark (it needs to be REALLY dark to see it). The wire from the coil to the opti was leaking out of the boot to the coil.

The ACDelco wires I replaced only had 15k miles on them but looking back at the work order they were replaced in 2009 so age seems to be more of a factor than anything. Previous owner was only putting about 1,000 miles a year on it.

MatthewMiller

Glad you got it fixed! I think I have Taylors on mine, too, and they work well and have stood up to time.