MaximumJavelin

I did some searching about my problem and will be going over shoeboxes Opti testing and the regular checks tomorrow when it's light out, but had a few unanswered questions that I didn't come across searching.

Here's the situation, 95 z/28 110k mostly stock, ~5k on new MSD coil and ~30k on plugs and wires, stock original vented opti. When I was driving the car every day, I never had a problem starting up regardless of the weather. Once I stop using it daily, it will not start if it's been raining out. Fuel pump primes and shuts off, car turns over at normal speed, I can smell fuel out the tail pipe, but it won't fire (I have tried the clear flood pedal to the floor with no results). However, as soon as I release the key from start to on, it will sputter and cough erratically, sounding like it's running backwards. If I remove something along the air intake track, like the air filter, smoke comes wafting out. The only way I can get it to start is if I open the hood and let everything dry out for hours; once dry it starts like nothing ever happened. If it's still somewhat damp it will cough to life then be clear after only a few seconds and run like normal.

Those are the conditons, and I have a few questions:

Does the tach receive an RPM signal while cranking? i.e. if you grounded the ignition lead and let the engine crank, would the tach show ~400-500 rpm while the engine is spinning, or does the ignition not output the signal until it's in the run stage? I noticed this today that the tach indicates 0 while cranking but no ignition. I normally never notice because the car will start right up and I don't get a long look at the tach.

How bizzare is random ignition after the key is turned from start to run? The opti must be sending some sort of ignition signal in order for there to be spark, it's not like a hot engine that's dieseling. There's plenty of raw fuel floating around in the intake and cylinders so it would just take a spark and a flame path through an open intake valve.

I've been reading some good info about the optispark (I'm a scientist so things like spark scatter, leading and trailing pulse resolution, and optical resolution are interesting to me ) and it seems like a brilliant system. What confuses me is that since the computer knows down to 1* the position of the crankshaft and which cylinder is approacing TDC on the firing stroke, and in addition to that there is a cap and rotor system which physically connects the route for the spark pulse... how can there possibly be a random misfire like the one I describe? The computer knows it's at cylinder 5 and the cap and rotor physically engages the coil circuit with the spark circuit for the #5 cylinder, so how could a back fire occur (which would need an open path from the spark, out the intake valve, and into the manifold?)? Could there be arching under the cap? If moisture gets in under the cap, does it interefere with the cap/rotor contacts or the optical pickup sensor or condense on the wheel or optical sensor and block signal input/output? To me that seems the most likely considering the almost on/off nature of my problem.

I figure you opti experts can give me some pointers so I have a good place to start troubleshooting from tomorrow.

pizzi-man

reading your post reminded me of when I was trouble shooting the same type of problem. The test that did it for me was the spark test. I had spark and fuel but upon further reading the I found that if the spark was not steady or weak then it was the same as no spark. Found that the cap had deposits on it's conductors as you would find on any distributor cap that hadn't been changed in a long time. New cap and rotor and has been fine for the last2 years.

Guest47904

cap and rotor

MaximumJavelin

I was thinking of replacing the cap and rotor, I'm shocked that they can last 100k+ miles and some people have double that. Are these supposed to be replaced at regular intervals like normal caps and rotors, or just on a "when they fail" basis? The general consensus here seems to be that if you're already swapping the cap and rotor, "might as well" replace the whole thing. I don't like that reasoning, if it's not broken I don't want to throw money away "fixing" it. Should I pull the cap off and visually inspect it for wear and fouling, then replace as necessary? While I'm at it, I should put new plug wires in and spark plugs. To get the plugs out, I should toss the manifolds and put some shorty headers on, new y-pipe with new cats and O2 sensors... well, you know how it goes. Throwing money at parts on a whim is a big no-no when a cap and rotor costs $150.

Injuneer

The back 1/2 of the Opti has a fairly cheap bearing in it. At 60K, the bearing in mine was shot. Its not going to last forever. When the bearing gets sloppy, the rotor and the optical timing wheel start to wobble. That's the logic in replacing the whole thing.

Appears you may have cross-firing. Either the signal from the Opti is corrupt, or the spark from the cap is being directed to the wrong cylinder. Could be the result of problems with the wires or the Opti. Look for corrosion in the plug and/or wire towers on Opti, weak insulation on the wires, etc, that is sensitive to moisture build-up. Also check the Opti harness connectors (there are two) for corrosion and/or bent pins. That would account for no start, and cross-firing that is putting smoke into the intake. I'd check that before assuming the Opti has to be replaced.

Not to say it isn't the Opti. The flat conductors in the cap that route the spark from the cap button to the correct plug wire tower will cross-fire when the cap materials lose their dielectric properties.

The tach gets its signal from the PCM. The PCM reads the low resolution pulse from the Opti to determine RPM. You should have an RPM signal while cranking.

Fatdog2

I'm no expert,but previous posts have suggested putting the old coil back on,with a new coil wire and see if that helps.Or,check the coil terminal and coil wire for nasty stuff.Hope this helps.(do the cheap stuff first).