chstitans42

My '96 LT4 has developed the dreaded P0300 code. I have done some research here on the forum and have figured out that this code can come from a number of places (fuel, spark, compression etc.) or in other words one of those ingredients is missing randomly and causing the misfire thus causing the code. Here is what I have done so far:

Checked a few spark plugs: They are AC Delco platinum plugs, and look fairly fresh. The two I pulled looked awesome, no fouling, oiling or running too lean etc.

Checked Spark Plug wires: I sprayed water on them and then ran the car, and I did not notice any arching from any of them. They are NOT the factory original wires, but look to be a replacement set. I noticed that the spark plug wire boots have numbers written on them so someone has been in here before

Checked fuel injectors: With the engine running, I unplugged each injector one by one, and all 8 made a difference in how the car idles. Every time one was unplugged it idled more rough (obviously a dead miss)

Vacuum Leaks: I looked at all the vacuum hoses that I could see on the engine and cannot seem to find anything leaking. At idle the car runs smooth. I also took a temp gun to the exhaust manifolds and checked temps there. All 8 cylinders would heat up the manifolds evenly.

How the car runs: I have had this car for 2 weeks, and during the test drive and also during the 200 miles I have driven the car, it has ran just flawlessly. this recently came up about a week ago. The strange part is at idle, it runs smooth as a top. Once you get between 1500-2500 rpm you can hear and feel the miss, engine runs rough, not much power at all. Also today while doing a test drive I heard a little popping coming out of the tail pipe. Basically I have a miss under load.

I am looking for a scan tool I can borrow, but I do not have the $$ to invest in one at this time. I figure that would be the easiest way to find the miss, but I am old school and figured I would come here and ask for advice first.

I would assume that if it was the optispark, it would throw the code for that? I am pretty sure that this opti is original seeing how dirty it looks. If it ends up being the opti what brands should I be looking for?

Any help is appreciated. Thanks!

antfarmer2

When you scan look at the o2s how many miles?

chstitans42

Car has 125k. I guess if it IS the opti, it is about time?

antfarmer2

I would change the o2s they are due normal maintenance to me others think they last forever. If money is tight do the front two but I would do all to match.

chstitans42

Would those explain my symptoms and problems? and cause a misfire?

antfarmer2

My 94 would bog and pop with little power then run fine till it did it again. Never put a code out. Best to run your scan to be sure. It could be many things but this fixed mine. Might try your fuel filter first. I would also check your wires on the coil and icm. And your crank position sensor.

Red 91

I would check fuel pressure at idle and running down the road, could be as simple as a fuel filter. Good luck!

antfarmer2

I fogot to mention the pressure gauge is always a good first step.

chstitans42

Will this work to check fuel pressure?

antfarmer2

Yes but if the filter is old you can change it before it comes. Take the line off the back and break the return line by it get the release tool from parts shop and release the lines from the top and pull the filter from there. If not really low on fuel clamp or pull the fuel line by the cap or you will get a bath.

RUU

Miss under load usually means opti - but you may want to reset the code and fun a fresh tank of gas through it in case you just got bad gas at your last fill up. If the opti that's on it now is the original and not full of oil and the shaft still moves smoothly, you might be best to throw a new cap and rotor on it instead of having to figure out who makes a good new unit.

chstitans42

Oh I also ohmed out all my fuel injectors and they all show 12.3

H Watkins

I agree with RUU. New cap and rotor.

1stVetteFinally

note about the O2 sensors, normally a bad O2 sensor causes the car to blow black smoke out the exhaust when under load. My LT1 blew a lot of black smoke when it's O2 sensors went out. And it did act like your's is. Idle was fine, but load had a bad miss and power loss was significant, but mine also blew black smoke when accelerating.

antfarmer2

Mine never blew the black smoke but ran like crap.

lt4obsesses

P0300 is the random misfire code. It can typically be set by the knock module. A lot of things can effect this.

Check warm fuel pressure, particularly how long it holds after engine is off, key off. Check your fuel pressure regulator. The fuel pump is another possible cause, thus check fuel pressure warm, give it some revs and see how the pressure reacts.

another common culprit on these engines is the ignition control module. They can get hot and malfunction.

Yes the opti can be going, but not where I would start. But it might not be a bad idea to check the opti harness connector. It's location is dubious. The connector goes to the opti on the passenger side. Stuff can get in there. Typically opti issues will throw a high/low resolution code. I wouldn't mess with it until that happens, it seems the aftermarket hasn't mastered this part yet.

On a side note, and my story...I had a Hotcam installed at around 86k miles. No tune, no other mods. I would get the 300 code fairly frequently. Fast forward, at about 135K I replaced the clutch and flywheel, the dual mass was toast. Replaced it with a steel flywheel. I never got another 300 code after that. Thinking that the flywheel was out of spec and causing the knock sensor to trip the 0300 code.

chstitans42

No, this is not my first rodeo lol.

antfarmer2

Excuse me?

antfarmer2

Texas compared to what over 100,000 sq miles bigger than California.

chstitans42

New update: the Fuel pressure regulator is leaking gas onto the intake. What issues would this cause. When the fuel pump is primed, the fuel pressure goes up, but falls slowly