Engine Misfire Code300

AMiller · December 6, 2003

Having a problem with my 96 Silverado, 5.7L.

I had a cap cross fire between #2 and #4, those 2 cyl weren't firing and threw code 300. Replaced the cap and rotor. Cured the big no firing problem. But light returned in a couple days. Was still misfiring, felt like the engine would buck a little at operating speeds, never hesitated or missed through the accleration to highway speeds.

Replaced wires, no change. Replaced plugs, no change.

Took into shop, they ran in on a snap-on computer, told me that it was misfiring,(knew that much)and rerouted coil wire. (they said the coil wire was wrapped around the trans dip stick and was causing the misfire).

Was fine 4 days, then code and light came back on.

Took the truck to a buddy that had a snap-on computer and went for a drive, found that I'm having a misfire on #1,7,8.

Would have bought off on the coil wire BS but you'd think it would miss on all 8 and not just on those 3.

These 3 cyl are beside each other on the cap, it there anything I'm missing or not thinking of?

02 Silverado Z71 · December 7, 2003

Firing order on your truck is 1-8-4-3-6-5-7-2

If all of your ignition components are new, and INSTALLED CORRECTLY, then I would have to lean towards a fuel-related problem, causing a misfire.

How old is your fuel filter??

BUT, before you throw any more $$$ into this, do a quick check. Remove your cap and rotor, and inspect the base of the distributor for any hairline cracks. Any cracks found on the baseplate will cause a misfire. Just something to verify; I have seen a few of these drive people CRAZY. Just check it quick.

If all ok, then start looking towards a fuel delivery problem. A few weeks ago, another member had some info that GM is "secretly" replacing all 8 injectors on these trucks; though there are certain things that must be diagnosed first. Do a search. Keep us posted. And DO NOT forget the basics!

AMiller · December 7, 2003

Forgot to list that I'd had replaced the fuel filter. But wouldn't a fuel deliverly problem give me misfires on every cyl rather than just a giving 3.

But thanks for the advice, I'll have to look in there again in a few days, I sprayed out the everything inside the cap with contact cleaner, thought the cam sensor might be doing it. Inspected the coil and plug wires, rerouted 1,7,8 and we'll see what happens. The light hasn't come back on, but the last time it took a 4 hour drive to show its face.

PunchT37 · December 7, 2003

Run a good professional injector cleaner through the eng. See if that helps.

Then check the dist. shaft. See if it moves any sideways or too much up and down.

One other thing iv`e seen is the fuel pressure regulator leak. Since it`s under the plastic plenunm it`s hard too check. If it leaks,it drops fuel on top of intake

inside and causes a rich condiction. Check the plugs on the missing cyls. for

loading up.

350vortec · December 7, 2003

I had that same problem.I did a complete tune up and cleaned the fuel injectors and did not fix it.Come to find out it was the fuel pressure regulator.It is located under the upper intake maniflod.It has to come off in order to get to it.Hook some gauges up to the fuel line and that will tell you if it the the regulator or not.I would lean toward that direction.

AMiller · December 9, 2003

Still it seems like I would be getting misfires on all cyl instead of just on 1,7,8.

vortec350 · December 9, 2003

I did not get but 3 cylinders misfiring,thats what the scanner said.

AMiller · December 10, 2003

Thanks for the help, I appreciate it. No one else I've talked to has ever heard of such a thing.

Thanks again

tmoreau · December 10, 2003

I have been having the same problem for the two years that I have owned this truck. Every time I brought it to the stealer they would check and replace something and that would clear it up for a couple days. My check engine light never comes on but I can feel the engine running rough and they always pull the p0300 code (random misfire). Ususally they clean the injectors or replace the cap and rotor then it stops misfiring so they give the truck back to me.

Anyway, what type of gauges do you need to check the fuel pressure regulator? Is it something I could check myself if I pick up a gauge?

AMiller · December 11, 2003

They sell a fuel pressure tester at autozone for about $35, I just looked at the prices on the computer but I'm sure they all have it. Heard it was pretty easy to check it out, but I haven't gotten around to doing mine.

I'm still getting around 17mpg, and the truck runs good, all but the misfire.

If I get around to testing it out, I'll list my findings for you, or vis versa if you don't mind.

PunchT37 · December 12, 2003

The fuel pressure won`t always be off.Look down the throttle body and see if

it`s wet and smells strong fuel. Should be dry[injectors don`t spray up].

carl722 · December 12, 2003

I have a 97 Chevy Silverado ext cab 5.7 Z-71 and I am having misfire trouble also. I put it on a computer and it didn't show any particular cylinder. But it misfires on Idle, cruising and low acceleration you don't notice it much at all under heavy acceleration. I have changed the distibutor, cam, lifters, spark plugs, wires , rotor button, distributor cap, fuel filter and 2 of the O2 sensors. I still have not eliminated this problem. I bought a coil last night but it is too cold to go put it on today. The coil is the last thing in the ignition system yet to replace. Anyone had such trouble finding a misfire before?

AMiller how are you getting 17 mpg I can only get 12 mpg with the 3.73 gears. I do have a K&N air filter too but it didn't make any difference. My 5.3 Trailblazer only gets 15 to 16 mpg in town and 19 mpg on the interstate it also has 3.73 gears.

carl722 · December 13, 2003

Wel I cleaned my garage out got my truck in it and changed the ignition coil and it still misfires. Now that is a new distributor, rotor button, distributor cap plugs and wires and the fuel filter was changed and I even have premium fuel in it and it still continues to misfire.

carl722 · December 16, 2003

I went by a garage this morning and the mechanic told me the truck was misfiring because I put a new distributor in and didn't set it with a scanner. He said it had to be within 8 degrees before the computer would run the fuel injection otherwise it would just randomly fire the injectors thus causing a misfire. He is going to set it for me. Then we'll see how it runs. I found out that Superchips will custom program a programmer for your vehicle if you have changed things like heads cams or things that affect your computer.

AMiller · December 16, 2003

The fuel pressure won`t always be off.Look down the throttle body and see if
it`s wet and smells strong fuel. Should be dry[injectors don`t spray up].

I looked down into the throttle body, and smelled strong in fuel and had a bunch of coated on build up in there. The butterfly had a some hard coated build up aswell. It looked really nasty in the intake as far as I could see.