Won't start in the rain - Impala Tech
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post #1 of 5 (permalink) Old 04-28-2011, 04:46 PM Thread Starter
Join Date: Apr 2011
Location: Milton MA
Posts: 1
Won't start in the rain

I have a 95 Chevy Caprice Classic wagon. It won't start after it rains or when there is a lot of moister in the air. Can anyone help me out with this mystery?

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post #2 of 5 (permalink) Old 04-28-2011, 05:38 PM
Join Date: Jan 2006
Posts: 12,455
Welcome to the Team Steve!

Ah the joys of the Opti-Spark ignition. Here is one solution to the problem that I whole heartedly endorse.


You will not have any problems with your ignition after this fix is installed.

Big Dave
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post #3 of 5 (permalink) Old 05-28-2011, 12:24 AM
Join Date: May 2011
Location: yourmomshouse
Posts: 23
turns over but won't run ?
or does not turn over?
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post #4 of 5 (permalink) Old 05-28-2011, 11:06 AM
Join Date: Jan 2006
Posts: 12,455
The Opti_Spark has two expensive issues with moisture. The low voltage (ignition control module or ICM) is very sensitive to moisture so Chevrolet put it under the water pump weep hole and in the perfect position to get wet from road spray and rain coming through the radiator. The replacement ICM is about $75 each from GM (Chevrolet Parts Service) and I suggest you buy at least two to carry a spare as that is the most frequent source of the continual failure. The other problem is Ozone build up and moisture causing corrosion on the high voltage side of the distributor which results in no spark to the plugs but spark from the internal coil.

Here is a list of Chevrolet LT1 Engine Ignition Part Numbers broken down by years:

1993-1994 optispark (non-vented style, spline driven) 10457702
1995-1997 optispark (vented style, cam dowel driven) 1104032
1995-1997 vented optispark vacuum hoses 12555323
1993-1994 optispark electrical harness 12106559
1995-1997 optispark electrical harness 12130319
1993 ignition coil 1115315
1994-1995 ignition coil 10477208
1996-1997 ignition coil 10489421
1993 ignition control module 10483131
1994-1995 ignition control module 10483139
1996-1997 ignition control module 10482803

You can read up on on how GM wrestled with the problem of reliability here:


Here is another replacement option for the Opti-Spark, but this company (or it's predecessor) has since gone out of business unless I am mistaken.


Either way the Opti-Spark is a headache and the reason few use an otherwise really great LT-1/LT-4 engine that Chevy dropped like a hot potato the instant the LS-x was introduced because of all of the warranty complaints. The cheapest solution is to bolt on a Edelbrock RPM Air Gap manifold and a Holley 650 cfm Street Pro vacuum secondary carburetor and then add a Street Billet MSD HEI distributor.

Now that you have a carb instead of EFI you can add a much more aggressive cam to wake up the potential of the small block two reverse cooled engine design, that was much more tolerant of detonation than the earlier small block every one has. The reverse cooled heads where required by the dropping quality of gas that we are saddle with today. So a LT-1 with a carb is the best small block made after a little head work.

Big Dave
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post #5 of 5 (permalink) Old 05-28-2011, 12:57 PM
Join Date: Jul 2009
Location: Canada
Posts: 1,399
ALL B body optis were the later (better) vented version, even 94s. Also, the ICM and the coil are on the driver's head, not under the water pump.

To fix your problem, just replace your opti vent harness. Cheap, and easy.

Two doors, four doors, wagons, and ragtops.
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