I could use some help with ideas why my SES light keeps coming on. The car received a new water pump and distributor cap/rotor, and new plugs about a year ago. The car has 40K on it and is stock. Ever since, the SES light will come on, usually after the car has sat for a few days. The light comes on blinking at first and then stays on steady (it will go thru this cycle a few times). Usually, it goes off by the end of the day and may or may not do it again the next day. The only code it ever reads is 0300 - random misfire. My Vette shop has more disgnostics stuff than any Chevy dealer and specializes in C4s. I trust them completely. He has run it hot, cold, and with diagnostic equipment attached while driving. Nothing comes up that is an obvious cause of the misfire. Everything has been checked multiple times. He has a complete computer baseline on the car so he can see if anything changes; and it hasn't.

Thanks, JCS44
96 LT4 Roadster, triple black

 

Opti? It seems to me that if the ses didn't come on before the work was done, and came on immediately after, something was not done properly. I'm just guessing here b/c the ses light can mean so many things. But our opti's are vented with air from the intake. Is it possible that there is either a clog or it wasn't hooked back up correctly? If this were the case, during the cooling process there could be condensation forming inside the opti causing a misfire until it warms up and the moisture evaporates? I would take it back to the folks that did the work, I'm suprised they didn't catch it before you did. They should have checked it all before taking your money.

 

I have not worked on the 96 but this is how I would approach the problem to begin.

1 - I would find out what sensor or sensors monitor this function. Don’t know if they could even be checked or if anything would even show up on this kind of problem. If feasible, replace the sensor(s) and see if problem clears.

2 - Check the books for theory of operation to see if there are any additional inputs to the PCM for this code.

3 – Consider that the PCM is indicating the code on its own and might be defective.

OK – Re-think, I read the post incorrect!

Since it happened at that time of opti replacement, check to see if there is any arking from any plug wires anywhere to ground. Coil wire also near water pump. Make sure the wires are plugged in to opti all the way. Look for a wire that might have got knocked off or a damaged sensor. Re-seat and check connectors on PCM and other connectors that might be on side or front of engine

 

I've also wondered about moisture somehow getting into the opti. This makes sense with sitting for a few days and then going away when its been warm for a while. Plug wire have been checked.

Thanks, John

 

Is the car actually misfiring, can you feel it? If not you could be getting a false code.

Paul

 

Here's some good advice. Take it to a tuner and have them turn that code off so that it does not trigger the light anymore. It should take them about 5 minutes. Done!

 

has the car been on a scope? If so it should show you what cylinders are misfiring. The last time I had this code it was on my wifes sebring and it was a PITA. since almost everything was in the distributor I replaced it and it did not correct the problem well after two months of trouble shooting I decided it was the distributor and it was, I got a defective part the first go around. If I remember correctly the basic things that can cause this code are Spark plugs, plug wires, Cap, Rotor, Coil, Opti, CPS then the less likely parts are CPS, or something in the FI system. Since this started after you replaced the cap, rotor and plug wires I suspect you problem is one of those parts. I am sure you have checked but first check your plug gap and read the plugs, second I would check the fire with a spark tester on each plug and coil.

 

Here's a thought...How old is your battery? In your post you do not actually say that your vette has a misfire. Just that the code seems to indicate one. I know that in the earlier LTx motors a SYS light would flash when the battery was weak. You did state that it happens "after it sits for a few days" then it goes away after being driven.

Sounds like after sitting your battery may weaken enough to set off the SYS, then you drive her and it charges the battery back up.

This hapend to me and replacing the battery solved the SYS. I also use a trickle charger for long periods of inactivity. These cars have alot of electronics that will use power when the car just sitting. Good luck!!

 

I know you didn't mention that you put one in, but an aftermarket stereo makes this light go on too....I put an alpine in my 96 and have had the ses light ever since. There is a work-around involving a resistor or something, but I decided just to live with it until I have to have the dash apart again.

I mention this because when I put the motor in my car, I did the stereo upgrade at the same time since I had to wait for parts and there was a lot of downtime. My assumption would have been that the motor was causing the sys light had I not read the stereo stuff in advance on this forum.

good luck.
sam

 

Looks like there's some confusion from a few of the posts. The SES (service engine soon) light and "SYS" indicator on the LCD screen are two different things. The SYS indicator would point to an electrical, radio, battery, etc problem with the chassis. His problem is with the SES light and OBD2 codes.

If memory serves, the computer uses a combination of the optispark optical sensor as well as the reluctor ring on the front of the crank to 'sense' a misfire. Basically if the engine pulses during a specific cylinder firing event that causes the computer to think there's a misfire. The computer does not 'know' what caused the misfire. However, with a basic logging tool, which I'm sure the shop has, the computer will record each instance of the misfire event, and what the operating conditions were (temp, RPM, etc).

The problem with this method reporting a misfire is that a rough idle (for example caused by a cam) can make the computer think it's misfiring. It doesn't sound like that's your situation, though.

Does the car seem to run properly? Does it idle or run rough? If so, that would be an indicator that the problem is real and not just a finicky sensor.

A few things to try (considering this all started when they did the work):
1) Make sure the crankshaft position sensor is plugged in and the wire isn't damaged. It's possible they unplugged it when they did the work. The plug is located on the bottom, passenger-side of the timing cover.
2) Check all of your plugs for condition and proper gap. A fouled plug or incorrect gap when they were installed could cause an intermittent misfire.
3) Check all plug wires (same comment as above)
4) Check all optispark connections (there's just the coil wire and the weatherpack connection at the top). If these get wet, they're prone to corrosion or could just be a little loose.

It would also be a good idea to cross-post this in the computer tuning section of the C4 forum -- some of the guys over there are VERY knowledgable.

Good luck!

 

EXACTLY ! I have a 96 LT1 that I race... and have installed a switch to run the fan when the motor is off... to cool it down between rounds... If I'm not careful & run it too long I get the SYS light..

I installed a new battery .. problem fixed !

 

To expand on the theory that moisture may be building up, then dissipating from the opti. There is a check valve located on the drivers side of the intake manifold on the vac line. Check to make sure it has not been reinstalled backwards. This turns your opti into a non vented unit and causes all kinds of issues.

 

You could have a bad Opti Spark unit.
Just because you replaced the cap & rotor does not mean your unit is ok.
My opti went out at 55,000 miles. I replaced the opti spark unit and 3 weeks later my car would mis fire & cut out. I have a friend who works at a GM dealer and told that I had a bad opti spark. Sure enough, I replaced it again and it was bad. I know it sucks but ZIT happens.
My friend told that it has happened to him 3 times on the LT1 motors.

 

Many thanks to all. I think Mark B. and Dr Evil are on track here and I'll pursue that advice. Thanks also for clearing up the SES vs SYS confusion that some have. I've had SYS issues before and it has always been related to the battery. With regard to the SES issue, the misfire is subtle and not always that noticeable. It idles normally, does not run rough, and will run up to the redline easily. The car has been run on a scope.

John