bdw18_123

Well, my '93 LT1 coupe has been stranding me (cranks fine, but no fire) and I'm stumped as to what the problem is. Here is what it has been doing and the troubleshooting I've done (is kinda long, so bear with me):

First time it happened, started fine in the morning, went to a meeting for several hours, come out, engine cranks but doesn't start. I try several more times, nothing. Wait 5 minutes or so, try again, fires up. I'm able to make it back home.

Second time, same thing, starts fine in the morning, drives fine, turn it off, come back several hours later, cranks but won't start. Keep trying, still nothing. Wait 30 minutes, still nothing. Get a ride back home, come back 6-7 hours later, fires right up. Halfway home, let clutch out with not enough throttle, car quits. Won't start back up. I have tools with me, I remove ICM and cool it in the water at the side of the road, reinstall, car fires up and I make it back home.

So, I replace the ICM (got one from Autozone). Go to my meeting, car shuts off while driving on the freeway. No bogging, stumbling, or missing, just shuts off. Coast down the nearest exit, try to start it, cranks, but no start. Wait several minutes, try again, fires up and I continue to my destination. Come out several hours later, again no start. Get a ride back home, come back several hours later, car fires up. Half-way home, it dies while driving again. Won't start even after cooling off ICM and swapping back in the original ICM. Swap back and forth several times cooling the ICM's each time. Nothing, cranks but no start. Friend's house is nearby, I push car to his house. Get a ride back home.

Come back in the morning, after going to another Autozone to exchange the ICM for another one. Go back to car, install 2nd new ICM. Car fires up, get almost home, engine dies while driving AGAIN. Cranks but no start. I called my sister and she pushes me home with her car (had a pad attached to her license plate which contacted my rear bumper just below my license plate, worked well, no damage to my bumper at all). I even tried starting it while rolling, key on, in 2nd gear let clutch out, doesn't fire. Car is home now.

I tried starting it this afternoon and it fires up, let it idle for 10-20 minutes, turn it off, try to start again, starts to fire a little, but shuts off, doesn't start anymore. So I go to autozone and get a new ignition coil. STILL NO START. I know its a spark problem, i unplugged one plug wire, put in a loose plug laid it against car, no spark when cranking. I don't think the opti is the problem, it was replaced 2 or 3 thousand miles ago. Also when I put a spark plug in the main plug wire from the coil and ground the plug, there is no spark at the plug.

I followed the troubleshooting trees in the FSM, chart C-4, ignition system check. I end up at the box that says "faulty ignition coil module connection or faulty ignition coil module". I've checked all the wires from the ICM to their respective destinations with a DVM, they all check good. I know when I get up tomorrow the car will probably start, it seems to have no trouble starting when its cold. Once it warms up, it either won't start again after I turn it off and come back or it shuts off while driving. Could 2 ICM's from Autozone be bad in a row? But when cold the car will run on the original ICM too. I hope the ECM isn't going bad. I know the 92-93 ecm's are hard to find.

I just don't know what the heck the problem is, I'm getting very frustrated, any help/ideas would be appreciated!

TWISTERUP

Did you replace the ignition wires when the opti was replaced / i'm thinking about the coil wire itself as a possibility ? It's not unheard of for a "new" opti to crap out either. Have you checked for any codes in memory that may be opti related ?

jtc44

When no start happens are you getting fuel? Are you gettin power to the coil and icu? Look for any wires comming of the ing. sw. that are hot in run and start. No vete expert but many yrs. as honda tech. (hay its a job) Have had luck putting a led on each wire needed in ckt. in use tap in parrel with long wires so when in car and cranking you con see if all cks, are being fed. Joe

jfb

Engines require spark and spark at the right time, fuel, air, and in the correct ratio in order to start and run. When it won't start, THAT is the time to diagnose the problem. Use a timing light to see if you have spark and spark at the right time. Use a fuel pressure gauge on the shrader valve on the end of the right fuel rail to see if you have fuel pressure. Use a noid light (available at parts stores), or make your own out of an LED and a series 680 ohm resistor and use it on a disconnected injector plug and see if it pulses during cranking showing the injectors are being pulsed by the computer.

donaho

Are you getting a code 41? Code 41 will disable the fuel system for a few minutes. I have been trying to find my NO START problem for about a year. (1992 coupe) It let me sit two or three times this year, throws a code 41 every time. I let it set for about 1/2 hour and it fires right up. I know you think it is an electrical problem and it may well be. However, it sounds a lot like my problem and I`m sure it is related to the code 41 in my case. I just can`t find the reason I`m getting the code 41. In my case it only happens when the engine is hot. My car will fire then stall all in about three seconds. I`m sure it is because the code 41 cuts off the fuel. My search for an answer still goes on, if I ever figure it out I will post my findings. My next attempt at a fix will be to clean my grounding points. The thing that sucks, is I can go for months with no problem and just when I think it is fixed, it strands me. Good luck finding your problem, I feel your pain!

bdw18_123

TWISTERUP:
Yes, the wires were replaced at the same time as the opti. When I do the spark test, I'm using one of the plug wires from the ZR-1 (they're MSD wires), so I don't think the wire is the problem. I am going to look more indepth at the opti, the test the FSM has you do where you hook a DVM set to AC volts to pin B on the ICM connector and crank the engine, voltage is supposed to be between 1-4 volts AC (according to the FSM). When it's in fail mode, the voltage is usually 0-1 volts sometimes between 1-2 volts. It only went to 5 volts 1 time for 1 or 2 seconds. I read a recent post here where someone said that if that voltage is not 4-5 volts AC all the time while cranking, then the optical portion of the opti is bad. I'm going to check the voltage when the car starts and compare to see if the voltage is higher when it's not in fail mode.

jtc44:
Yes, I'm pretty sure I'm getting fuel, I can hear the fuel pump run when the key turns on, and while I know the fuel pressure gauge is the best way to tell, when I push the shrader valve, fuel comes out under pressure. Yes, both the ICM and the coil are getting power (checked while in fail mode).

jfb:
When I was doing the troubleshooting, it was in fail mode, I got it to fail in my driveway. I don't have a fuel pressure gauge, I'll have to get one. But I'm don't think its a fuel problem.

donaho:
There are no engine related codes being thrown at all. The only two codes displayed are history codes and they have to do with the lcd dimmer and lcd display. I think those were one time codes though cus I don't have any problems with the display.

santafe65

Check the coil resistance on your fuel injectors (should be 12-17 ohms) , especially if the fuel injectors are original. My 90 L98 has been doing the same thing. Just dies with no codes set, cools down and then starts fine until it gets warm again. Three of eight fuel injectors had a coil resistance of 4-ohms when cold.

Epimax

I'd check the resistance on your injectors, and it may also be coil related. I had a similar problem with mine not too long ago, and had to replace both the ICM and the coil..
No problems since..

pcolt94

The point you are looking at, at the pin “B” of the ICM is a square wave waveform. The faster the engine turns the higher the frequency.

This waveform which is a form of an A/C voltage should really be measured with an oscilloscope. The people writing these procedures know people don’t have scopes but probably have a DVM. DVMs are basically designed to measure A/C at 60 HZ sine wave (which is house voltage) for calibrated accuracy. A deviation from this frequency the accuracy declines. So that’s why the measurement has a large tolerance. Different DVMs will respond differently at frequencies other than 60 HZ and the results of different meters will vary.

It is written to find out if you basically just have something there. This waveform (point B) comes from the ECM which is driven from the opti (as the clocking pulse). The optical portion of the opti is much more reliable than out ECMs. The ECMs in those cars are known to have a high failure rate and strange problems. If the ECM fails to put out the signals to the injectors or ICM the engine will die.

If the drive (waveform) from the ECM gets to small, it won’t drive the ICM hence the coil and spark. I have not measured point B in a couple years and can’t remember exactly what I got but under 1.0 VAC I think would be to low. I don’t think I measured 4 volts but was somewhere mid range.

Think of the system this way. The ICM is just an amplifier. It takes 5 Vp-p and amplifies it to about 70 vp-p. The coil is a step up transformer which converts 70 VAC to the 25,000 volts for the spark. Its that simple.

Best thing to do is when the engine is running right, measure your point “B” so you know what the correct value is using your meter. Then when the problem occurs you have a comparison is a good idea.

onedef92

I'm sure you know this, but be sure and use the packet of thermal compound white grease that comes with the ICM, too. If you don't, the new ICM will stir fry itself to death in short order from the engine heat.

Elementaltoad

Listen for odd noises with the key in the run position, mine is doing this exact thing, only "no starts" after being heated up, turns out the computer is fried.

chevylu

I agree with santafe65 and epimax.

Bought Bosch Design III injectors for my 1990 L98 from Jon @ FIC: http://fuelinjectorconnection.com/sh...&productId=134.

For what it's worth, here is how I got to this point:
1990 Corvette 5.7L (VIN8 – L98) Completely Stock

Problem:
When it is cold, the engine consistently starts and runs reliably. After the engine comes up to operating temperature, when it is shut off, it will not start; it cranks but will not run.

Diagnostic Tests:
Following diagnostic chart A3 in the FSM
• No codes are present
• Good spark is present on all cylinders
• When the engine is cold, a noid light blinks reliably on cylinder 1; when the engine is hot, it will not blink
• When the engine is cold, a noid light blinks reliably on cylinder 2; when the engine is hot, it will not blink
• When the engine is hot, disconnecting the cylinder 2 injector will allow the engine to start; reconnecting it while the engine is running will cause the engine to stop running

Injector Resistance:
Cy- Cold - Hot
1 - 16.7 - 17.3
2 - 06.4 - 00.7
3 - 12.6 - 08.9
4 - 16.1 - 15.1
5 - 16.8 - 17.6
6 - 15.5 - 12.2
7 - 15.6 - 12.1
8 - 16.1 - 16.9

Conclusion:
Injector #2 shorts out completely when it is hot. When cold at 6.4 ohms, it will alllow the engine to start and run.

Check out the injector sticky at the top of the forum.

crunchy

Try replacing the fuel pump, takes about 15 minutes. My '90 was getting difficult to start, this fixed it.

93LT1

QUOTE=pcolt94;1570079583]The point you are looking at, at the pin “B” of the ICM is a square wave waveform. The faster the engine turns the higher the frequency.

This waveform which is a form of an A/C voltage should really be measured with an oscilloscope. The people writing these procedures know people don’t have scopes but probably have a DVM. DVMs are basically designed to measure A/C at 60 HZ sine wave (which is house voltage) for calibrated accuracy. A deviation from this frequency the accuracy declines. So that’s why the measurement has a large tolerance. Different DVMs will respond differently at frequencies other than 60 HZ and the results of different meters will vary.

It is written to find out if you basically just have something there. This waveform (point B) comes from the ECM which is driven from the opti (as the clocking pulse). The optical portion of the opti is much more reliable than out ECMs. The ECMs in those cars are known to have a high failure rate and strange problems. If the ECM fails to put out the signals to the injectors or ICM the engine will die.

If the drive (waveform) from the ECM gets to small, it won’t drive the ICM hence the coil and spark. I have not measured point B in a couple years and can’t remember exactly what I got but under 1.0 VAC I think would be to low. I don’t think I measured 4 volts but was somewhere mid range.

Think of the system this way. The ICM is just an amplifier. It takes 5 Vp-p and amplifies it to about 70 vp-p. The coil is a step up transformer which converts 70 VAC to the 25,000 volts for the spark. Its that simple.

Best thing to do is when the engine is running right, measure your point “B” so you know what the correct value is using your meter. Then when the problem occurs you have a comparison is a good idea.[/QUOTE]

Great Write up.. thanks

demon seed

Check out the videos on injectors also at the top of C4 forum.very good info....GOOD LUCK

bdw18_123

UPDATE:

It's defiantly a spark problem and I think I've found the issue (fingers crossed ). When said I checked the pin "B" for voltage in fail mode in my earlier post and said there was some voltage, that was wrong, there was no voltage. I had a brain fart . I was looking at the numbers after the dot: 0.05, which would be pretty much nothing. I didn't realize this until I tested it when it's not in fail mode and I got 2.5-3.0 volts AC. Also, I get spark at the coil when not in fail mode and no spark at the coil when in fail mode. Since I have already replaced the coil and ICM and still have the same problem, I suspected the optispark.

So I bit the bullet and removed the optispark. Good thing is, even though I did buy it off of ebay, it came with a limited lifetime warranty and it is a GM unit purchased from All-ignition.com for $200 around a year and a half ago. They actually have very good prices on opti-sparks now (even cheaper than $200 for stock optis). So I sent it in to them for warranty. Turns out the optical portion did go bad, so they replaced that for free. But they said that my cap and rotor got corrosion build-up from being sealed too well. I sealed the seams on the opti with RTV, but since the cap was not vented, the corrosive stuff that forms from the high energy spark stayed in and caused the build-up.

So I bought a new, later style vented cap and rotor from them for $50. I plan to install all the necessary vacuum hoses (going to buy the complete vented opti vacuum hose harness from GM, part #12558921, it looks like its still available for $18 at gmpartsdirect.com), following the how-to article in Corvette Fever. Here is the link if anyone else needs it:

http://www.corvettefever.com/howto/16758/index.html


And as I was writing this, my replacement opti with the new vented cap and rotor just now showed up! That was extremely fast shipping, they just barely shipped it out yesterday! Time to get to work, I'll post an update when I finish as to whether or not this solves my no-start issues. And thanks for all the detailed replies, I appreciate it!
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EDIT (10/22/13): I no longer own this car (sold it back in October of 2009), but I never posted the update I promised above so I'm just going to do it now for any others who have this problem.

The fix described above did end up fixing the "cranks, but doesn't start" issue. The fact that I sealed up the replacement opti without doing the vent mod upgrade described above contributed to the failure of the opti due to the buildup of the corrosive gasses that form because of the high-voltage spark discharges present inside the optispark unit.

The sealing of the opti needs to be done in conjunction with the vent mod. Even if you don't seal up the opti as I did, I do believe that is one reason why the earlier, non-vented opti's fail so much (the earlier opti's actually do have vents, which are 3 small holes on the bottom of the metal portion, but it is a passive vent system). GM designed an active vent system by routing intake vacuum through the opti to remove any moisture that could get inside.

The following is taken from the above linked Corvette Fever article:

________________________________________ ____________________________

EDIT (again!): I guess I did update this post back then, I just did it in a new thread. Here is a link to that post, it has pictures of my modded early version opti.

http://forums.corvetteforum.com/c4-t...e-is-back.html