Alright, I apologize for the long explanation in advance, but really want to get some ideas on what is going on.

I have 2003 Silverado, 4.3L, automatic transmission, with about 115,000 miles on it. I bought it brand new in late 2002.

About 4 months ago I was driving from home in Phoenix to San Diego. It is a drive I have taken often. About half way through the trip my check engine light starts flashing. It flashed for about 2 min and then went away and I forgot about it. Then 3 months ago I was taking long drive from San Diego to Barstow and again the check engine light starts flashing, but this time it stays on permanently for about an hour then went away. Same thing happened when I was driving back to San Diego. It went away after a short time and again didn't think much of it again. Now last month I was driving home from San Diego and again I go the flashing check engine light, but it went away. When I got to AZ I did some research and learned that a flashing check engine light means there is a reoccurring misfire. I told my Dad (who is pretty knowledgeable with vehicles, especially Chevys) about it and thought it was no more than a fuel filter problem. The next day I was driving to my girlfriends house, who lives about 30 min away, and the check engine came on permanently. I took it into a mechanic. They did a code check and that showed a "300," random misfire. They continued a diagnostic and found that there were issues with the cap and/or rotor, which I had changed out about a year and half ago when I did a tune-up. I took the vehicle home and switched out the cap and rotor. The Cap did have a decent amount of wear on the posts. The following day I noticed my vehicle was running smoother. That night again driving to my gf house I got a solid check engine light again. I took it to Autozone and they did a code check. Again I got a "300." Within 12 hours the check engine light went away, but I checked all the plugs and wires, all were still in really good condition. Two days later I got a check engine light flashing and then solid. I took it back to the mechanic the next day, and by that time the check engine light went away. He did another code check and again a "300." He cleared the code. And yes, you guessed it, three days later I get another check engine light on. I take it back to the mechanic and again they are getting a "300." This time the mechanic spent a half a day trying to figure out why there was a random misfire. He could not find anything. My Dad talked to mechanic and the mechanic recommended that I try changing the coil. If that doesn't work he told me that it might be best to wait to were it happens more frequently, thus making it more obvious. The next day we changed out the coil, and we also noticed that a clip on wiring harness which looks to be connected to the computer chip had been busted off. My Dad secured the wiring harness to ensure a bad connection was not causing these check engine lights from appearing.

Two days later I am driving back to San Diego and half way there the check engine light starts flashing, then stays permanently on through the rest of the drive. BUT I did notice that when I was going down hill, at which I take my foot of the gas pedal, the check engine light would start flashing as soon as I tapped the peddle slightly, once I started driving on flat rode and accelerate, the check engine light stops flashing and stays on solid. This happened about 6 or 7 times an only when I was going downhill. I tested it going up hill, but was not getting the same results. The check engine light stayed on until the following morning and then went away.

So here are facts:
1. I am either getting a flashing check engine light or a solid with a "300" code.
2. I am only getting these after a longer drive, making me believe that heat is partially the cause of the malfunction.
3. I was getting a check engine flashing when I was going downhill when I was not in full acceleration.
4. Check engine light goes away after about 12 hours, usually sitting overnight.
5. I did a tune-up at about 90,000 miles with AC Delco parts.
6. Changed out the cap and rotor, plugs and wires are still in good condition.
7. I changed out the coil
8. Changed out the fuel filter.
9. About 2 1/2 years ago I had to change out the fuel pump
10. About the same time I also had the catalytic converter replaced because the old one was bad.

Now, I have done research on the internet, to include this forum and every time read about a similar issue someone is saying that there is a different cause. I have read that if this continues a random misfire could f' up my pistons. I don't know how true that is, but I need to keep this truck for about another year at least, so I need to get this issue (if there is one) fixed.

Thanks for the help in advance.

 

the 300 code is random multiple missfires...when the light is flashing it means that the miss fire is happening...........plugs, wires, and some ignition maintenance seems to be in order

 

When you pull the plugs do they all look the same? Have you cleaned your throttle body and injectors? Some of the older (early vortec) motors had issues with the distributor gear wearing out which would cause a random misfire code. Just a couple of ideas, good luck!

 

I recently had the same code PO300 trigger the Engine Service Light. The GM dealership recommended: R & R intake manifold plenum & replace both knock sensors. Also, a fuel injection service. As follows: 2 sensors, 1 gasket kit, 1 sealant = $296.00 parts -- labor $325.43 Grand Total $602.03. Guess what? As I was driving it that very afternoon 1/8/11 the Engine Service Light starts flashing like a strobe light for about 20 miles and then went solid. Checked the code after I got off the highway... PO300 is back. I am very frustrated with GM vehicles and you can understand why if you read my most recent post on "Broken Brake Cable" 1-7-11. And, P0300 is still on today 1-22-11 Good Luck with your repair.

 

Chris - The plugs and wires checked out fine. I know what the "300" is and what it means when the check engine light is flashing. What are some issues with the ignition that could cause this?

Shamrock - When I pulled the plugs they all looked the same and they all still had the correct gap space. As for the throttle body and injectors I have not cleaned them, but I did have some work done on the injectors. I have to go back through my receipts and see what kind of work was done.

Skoot - It looks like me and you have the same issue. I didn't think my issue was uncommon, but their seems to be different answers to the million dollar question.

 

With 115,000 miles on the truck the plugs are due to be changed if they haven't been. How did you check the wires? The distributor gear is also a possibility still. As is the way the coil grounds....sometimes just removing the coil and cleaning the ground helps alot.

 

Just an idea here, a few of my friends were telling me cleaning the catalytic converter or even replace it may increase performance and can even solve a problem like a misfire. Not sure if this sounds right or not. Let me kno what you guys think

 

Ah sorry, should have read more carefully

 

I temporarily resolved my eye catching, annoying dashboard service light on icon. I covered it over with a piece of black electrical tape. I don't feel so bad when I can't see it.
darkyellowsky - If you continue any more attempts to repair the engine service light issue please post your results. Thanks, Skoot

 

I have a 2001 2500HD with 157k on the odometer. I am having the exact issue as the OP. The 'Service engine soon' (SES) light starts to flash indicating a misfire. It happens when at highway speed and seems to happen when coasting or going down hill. I have also had it happen recently when hauling in the tow/haul mode. I pulled the code - P0300. Not much help. The light has not stayed on as of yet.
The plugs were changed at about 90k. The coils and injectors have had no service other than cleaner run through them. It would seem that perhaps this in not a factor however. I have also cleaned the TB with no change in results. The truck runs fine when this is happening - no stutter or odd behavior - other than blinking light.
I did some research as well and thought perhaps intake gaskets as this is a known issue but one earlier post has me rethinking this.
My next step is to take it to my mechanic and have them take a look but I was looking for any other insight from the forum and potential issues or if you have resolved the same problem.

john