mattjohn245

Okay hey everyone, i have a 1995 LT1 in the 1993 silverado Indy pace truck. The other day my Dad was driving the truck and it back fired. So i reserched and talked to many people trying to figure out what could becausing it to not run. sorry for jumping... but as my dad was drving it, it backfired and that's all she wrote. It wouldn't start after. I read that it is more than likely the optispark. So i replaced the whole opti and module. I replaced the water pump shaft seal and the one ehind the opti also in the block. I hooked it all back up and still will not hit a lick. I have no clue what else to do besides replace the coil, and or the ICM. The motor ran amzing before it back fired and wouldn't run. I have not had it very long and i havent ran it like a mad man. I stepped down on it to do a few burn outs and it back fired but only once and keept running fine. and then that one day it just pooped out on us. Can someone please help me, im about sick on trying to figure it out. I just want to be able to drive my new truck.

I have checked for spark, And i have no spark at all i also tried spraying starter fluid before checking for spark and still nothing. I need help

highway man

I am really sorry about the trouble you are having with your truck. If you have no spark at the plugs I would have the ICM checked. Most auto parts stores can do this. If it checks out okay I would check the coil. I don't know that either one of them would cause an engine to backfire but a lot of strange things happen. Look for breaks in the coil wire to see if it has gone bad on you. Where I work some of the guys after starting their vehicle would turn the key off and then back on producing a backfire so there could be something in the ignition that has gone south on you. This may be a longshot but while you are at it you may as well check to see if the engine is getting the proper fuel amount at the right pressure. This may not be much help but I am not a professional mechanic either. If this doesn't product results for you and you can afford it you may be better off to take it to a reputable shop and have them look at it. Good luck!

pcolt94

Did you check for spark before you replaced the opti?

Since you have a 95 engine and the opti is the updated one, cam alignment is probably not a factor and besides that, there is not spark at all now and alignment would not affect that.

There is a good possibility the ICM could be the problem. A lot of the early symptoms could be those of a bad ICM. You could either replace it, or measure the drive to it from the PCM.

If there is drive then replacing the ICM would be a good bet. If no drive then you have to look to the opti, PCM and related power and wiring (connectors).

Here are a few tests to get you started on the problem. Be the utmost of careful not to short anything to ground. ING key off, remove ignition module connector (ICM) and check the ground connection on pin C - BLK/WHT wire, (resistance about 0) .

Next, “Key on”, check for 12 volts DC at pin A - PNK /BLK wire and pin D – Dark green wire. If no voltage check the coil fuse 10A, (#25), or wiring.

ICM drive
If all is good, set meter to read AC volts and connect probe to pin B - white wire. Have someone crank engine, meter reading should be between 1 to 4 volts. Key off! Put connector back on. If OK, this indicates opti and PCM are probably working. If not, opti or PCM could be source of problem.

I would also check to see if the injectors have drive. Use a noid light on one of the injectors to check for this. This would indicate the opti is working. You need the opti low resolution output to drive the PCM to create drive for the injectors.

In the future or if you have to go there now, the engine should be started before the water pump is installed back on.

derekguzz

Had the same problem. My ICM was bad....Take the ICM to autozone and have them test it. Make sure they test it about 6-10 times to get it warm then if it is bad it will fail.

They test it multiple times to get it warm/hott like it would be in a car.

I went threw 3 of them...

I replaced my opti, water pump, coil and had no fire, then replaced ICM and she fired right up !

mattjohn245

Well, I have tried taking the ICM off and took it into autozones and a bunch of different places and see if they could test it. All of them didn't have the connectors to test it but advanced did. The guy there told me he would get the part for me and if it didnt run i could take it back and get my money. So im wondering if i should do that. Also My ICM is the little mod. right above the coil? And also, my coil is looking bad, it has alot of wear look to it (a little rust and grime) could that be the culprit? I hate to spend all the money on thses two parts and that not be it.

mattjohn245

when your ICM went bad did yours just quit running?

mattjohn245

Well yesterday i pulled off the wire that goes from the coil to the opti, when me and my Dad were changing the opti he used a mallat the get the harmonic balencer back on and hit the end of that wire breaking the ceramic inside it. And soon later i found a bad spot on the wire close to the boot but i don't think it would cause it not to run. Maybe just to miss? But like i told the other guy, my coil doesnt look in the best of shape.

mattjohn245

I didn't check for spark before changing the opti but we are sure it more than likely wasnt getting spark. And I was going to put the water pump back on but left it off to see if it would start.

pcolt94

Usually a mallet is used to get the balancer off from underneath the car. But is should just slip right on after that. The balancer is held on by 3 bolts. The hub is held on with 1 big bolt. Only the balancer has to be removed with the 3 bolts. Did you remove the hub also?

What ceramic piece did you break? Is it the hot wire input from the coil on the top of it. If you did break the cap who knows what it could do inside aside from possible arcing.

You need to check for spark at the coil for now, forget the plugs for the moment.

The coils are pretty good in these cars, looks don’t mean much.

Sounds like the problem is being compounded. You need to make some of the measurements suggested because I feel like we are throwing darts now.

It's probably not a bad idea to try a ICM is possible with your deal.

mattjohn245

We just wiggled the balencer off we didn't remove the hub. I did move the engine when removing the bolts and putting them back on. But when we put it back on it was snug so we used a rubber mallet to bump the balencer back on. A guy that lives next to me is pretty skilled at knowing a lot so he came out and tested some stuff. He said its not breaking the current to the coil if i recall right. He also said the first thing he would try was the ICM and we went to different stores none of which had it in stock but one guy said he didn't think that was bad. And the wire from the coil to the opti, if you look down in it it looks like a cap but with a ceramic cap around the connector. That whole piece slides down in to opti to connect.


Now i just got a text from my Dad.. he said he went out and started it and it fired up. He didn't leave it running long though since we have the water pump still off. I'm guessing when I put the new optispark on, i didnt put the coil wire back on very good not getting a good connection to the port on the opti. But when i put it back on last night it poped on when i pushed it down in that hole. But Im going to go and get one of those wires and just replace the whole plug wires and all while im at it so i no longer will have to worry about that half of the engine. But i have one more question, My starter started acting up after we put the new opti in. We turned it over and then it made a pop/crack like when your battery or starter is going out and it would not turn the engine at all but you here the starter turning. Maybe a stripped gear? But when you tunr the key back and try again its fine.

derekguzz

Well I would be just getting home, go inside, come back out to leave again and nothing at all. Then I would wait a couple hours and come back---then it would fire. I had a failing ICM. Shop told me 2,000K to replace opti, water pump ect. I said no and did it myself. Like I said: I replaced everything but the coil and ICM.....Still did not start. Replaced the ICM and she fired right up.

You have to get a spark tester and start at the beginning. Check for spark at the coil- if not spark then there is where you start.

My money is on a bad ICM...


Have advanced run it a couple times to make sure. Make sure to use the grease that will come with the ICM.-> and yes your ICM is right above the coil.


"They don"t look bad"- but they can be...

I had autozone test my new one before I even took it home because I bought one, went home and it lasted a week. Landed up being a bad ground connection blowing my ICM.

mattjohn245

Thanks for that info. Im at school right now and my Dad texted me and said he went out and turned it over and it started. He didn't live it running long since the water pump is still off. But i guess that wire going to my opti from the coil needs replaced. When i put it on after putting the new opti on i guess i didnt have a good connection and when i puut it back on last night it felt like it poped on better so i guess that might have been that problem. Now i hope to get it all put back together tonight!!!

pcolt94

What you should have done is to use emery cloth and sand the inner surface of the balancer and the outer portion of the hub. Then lubricate it and that problem will not happen again.

This is 60s lingo and technology of points and condenser theory. The points claps and energizes the field in the coils primary which hence makes the coil produce spark. Things are done quite differently today and basically similar in most ignition systems today. A waveform (square wave) is produced by the ECM/PCM which drives the ICM. The ICM is nothing more than a amplifier and in the corvette it takes a 5 volt P-P waveform and amplifies it to about 70 volts P-P. This is what drives the primary of the coil and hence the secondary of the coil is where the high voltage (spark) is produced. Same basic principal of yesteryear but done differently.


There is still no assurance that the original opti was bad. It could still be a possibility the ICM is defective.

The end of the wire that snaps into the opti has a rubber boot on it. Not sure what ceramic piece you are trying to describe. However wonder if the 95 is different from my 94 coil wire.

Possible bad starter. The Bendix might be sticky and not engaging. Just make sure the battery has a good charge on it. It could be low due to all the work being done and not being run every day.

mattjohn245

ICM was the problem, replaced it and it started right up. But now the engine is squealing when you give it gas. I have not put the water pump back on yet though. And i only had it running for very few seconds. Maybe that is going along with my starter or belt problem that i told you about?

pcolt94

Good deal on the ICM.

Do you have the belt on at this time? If you do, take it off and see if it still squeals.

FYI- With the water pump off , you probably don’t have the sensor in the pump connected. Without it the engine will run very rich in a few seconds and then stall eventually. You can stretch to wire up, lay the pump on the top of the air ducting and connect the sensor. You can then run it for a short time normally.

mattjohn245

Im glad we got it going! Just to bad we spent alot on trying to figure it out and it be that simple! The belt is on but its more like a griding sound but witha squeal. Tomorrow im going to put the water pump back on but I would like to figure out why it might be making that noise.

pcolt94

Just a suggestion.
If you did not have the noise before you changed the opti, I would take a look there. Always look back as to where you have been or touched.

mattjohn245

Im going to put the water pump back on today, maybe its just the water mump shaft thats making that noise.