P0300 won’t go away!!!

[1998OBS]

1998 Chevrolet 1500
Automatic transmission
5.7L Vortec

I have a P0300 code…

So far I’ve done a NEW:
Distributor, cap, rotor, wires, plugs, crankshaft positioning sensor, mass airflow sensor, EGR valve, fuel filter & battery. I’m pulling my hair out.

On acceleration my check engine light blinks and before the new distributor all of the other new parts I mentioned were installed and it was still idling really rough, it sounded very muffled and choppy. After the distributor it sounds somewhat clearer and I smelt rotten eggs like the cylinders that weren’t firing before started too (?) but it’s still choppy and it has a mean vibration and yes… P0300

Is there something that I’m overlooking? What could it be?!

I was told that I need to time it.
I was told it could be the catalytic converter…
I was told it could be the temperature sending unit…

How do I narrow this down?

 

[Ron88]

I won't take a guess at what your problem is. I can tell you that when I searched PO300 in the search box more than a few guys have had this problem one guy after changing like everything turned out to be his distributer cap which he had already replaced. I am not trying to be mean, I know after 40 years of fixing heavy trucks what it's like to bang your head against the wall. Hope this Helps.

 

[1998OBS]

I won't take a guess at what your problem is. I can tell you that when I searched PO300 in the search box more than a few guys have had this problem one guy after changing like everything turned out to be his distributer cap which he had already replaced. I am not trying to be mean, I know after 40 years of fixing heavy trucks what it's like to bang your head against the wall. Hope this Helps.

I understand… I spent all day yesterday researching and found out it’s a huge issue with these trucks. Short story I’m a muscle car guy, when I was 9 my uncle bought this truck and I fell IN LOVE with it, it was so cool. Lowered, sounded awesome, comfortable, big…

28 years later, I was gifted it from my mom after my uncle passed so I’m super nostalgic with it, I’ve been doing a 90’s retro build on it and it’s turning out great but this P0300 code is wrecking my motivation big time. I’ve spent so much money and time trying to figure it out, right when I’m like alright I’m not going to replace that because it could be this, by the end of the week I bought both parts, replaced them both and then I’m sitting in this vibrating, choppy cab thinking WTF?!???

I’m replacing old original parts because my uncle wasn’t a mechanic by any means so it sat in the garage forever so it’s not that bad that it’s a new distributor, plugs etc but it’s super frustrating.

1965 Mustang, misfire, tweak the distributor, replace a spark plug BOOM! I’m roaring down the street… this truck is like

“Nope, it’s not that either but thanks…”

I’m really thinking it could be the cat??? The exhaust sounds like it has a bunch of cotton in it and after the distributor swap on the first start up it smelled like rotten eggs so idk anymore… Can a cat really cause a P0300?

 

[Schurkey]

FIRST, you need to find out if the tool you're using is capable of displaying individual cylinder misfire.

SOME (cheap-junk, consumer-grade) code readers and scan tools are NOT capable of showing the REAL code--which might be P0301 through P0308 or some combination. They can only display P0300. This puts you at a serious disadvantage, because you're looking for "random" misfire instead of knowing which cylinder(s) are the evil child.

A proper scan tool will show current misfire, and misfire history for EACH CYLINDER. Then you can fix the problem without being distracted by the other, good cylinders.

A blinking/flashing MIL light means the catalyst is in deadly danger from overheating and melting. Maybe it's too late, the catalyst is already damaged. Fix that LAST, after the engine is otherwise running properly--because if the engine is still screwed-up, you may cook the new catalyst(s).

Verify fuel pressure when accelerating, and see how long it takes for pressure to bleed-off.

Connect a REAL scan tool, look at the sensor signals, (O2, coolant temp, cam and crank sensors, knock sensor, and all the rest) and the output signals (IAC, EGR, ignition advance, and all the rest.)

You can check for "codes", while you're in there looking at the real data.

The small-block Vortec engines have known problems with intake gaskets and the "spider" injection system. The intake gaskets have been updated at least once; the spider can be replaced with electronic injectors as an updated assembly.

And #3 seems to be the weak spot on the distributor cap; I've heard this from several sources but never experienced it first-hand.

 

[Ron88]

Can you narrow it down to one cylinder by pulling one plug wire at a time ?

 

[1998OBS]

FIRST, you need to find out if the tool you're using is capable of displaying individual cylinder misfire.

SOME (cheap-junk, consumer-grade) code readers and scan tools are NOT capable of showing the REAL code--which might be P0301 through P0308 or some combination. They can only display P0300. This puts you at a serious disadvantage, because you're looking for "random" misfire instead of knowing which cylinder(s) are the evil child.

A proper scan tool will show current misfire, and misfire history for EACH CYLINDER. Then you can fix the problem without being distracted by the other, good cylinders.

A blinking/flashing MIL light means the catalyst is in deadly danger from overheating and melting. Maybe it's too late, the catalyst is already damaged. Fix that LAST, after the engine is otherwise running properly--because if the engine is still screwed-up, you may cook the new catalyst(s).

Verify fuel pressure when accelerating, and see how long it takes for pressure to bleed-off.

Connect a REAL scan tool, look at the sensor signals, (O2, coolant temp, cam and crank sensors, knock sensor, and all the rest) and the output signals (IAC, EGR, ignition advance, and all the rest.)

You can check for "codes", while you're in there looking at the real data.

The small-block Vortec engines have known problems with intake gaskets and the "spider" injection system. The intake gaskets have been updated at least once; the spider can be replaced with electronic injectors as an updated assembly.

And #3 seems to be the weak spot on the distributor cap; I've heard this from several sources but never experienced it first-hand.

I’m really leaning towards a bad cat I’m looking at O2 simulators so I can delete them all together, does anyone have any experience with those? How does a guy in California go cat delete with a truck that’s been on non-op for 8 years? I’m just trying to get this this running right and back on the road…

 

[1998OBS]

Can you narrow it down to one cylinder by pulling one plug wire at a time ?

I’ll need to ask someone with a scan tool for help, you’re saying pull each wire and see which one doesn’t make the engine go out of wack to find the dead cylinder right?

 

[Schurkey]

PULLING each plug wire in turn is a fine way to destroy the ignition coil. Pulling plug wires creates an open circuit in the coil secondary circuit.

Better to GROUND each plugwire in turn. I just posted that photo and instructions yesterday; I'm too lazy to do that again right now. Maybe later.

It's ten times easier and more accurate to just look at the scan tool for misfire info.

 

[wrenchjockey23]

While there are many things that may cause this… a couple things come to mind knowing how these trucks operate. You can spray the exhaust manifolds to see which, if any individual, cylinders are misfiring if it’s actually misfiring at idle. I once encountered one that ran HORRIBLY. It sounded rich, smelled rich but idled very poorly. After testing, I finally found number 5 exhaust valve stuck open. These ‘96-‘99 Vortecs (and some vans etc extend in later years) have issues with valve guide/ valve stem clearance and will eventually stick. It starts with misfire on half throttle or more up hills etc when it’s hot outside. You will see random and specific cylinder misfire codes. If you stop or slow down it goes away. It’s very difficult to diagnose as it leads most to tune up stuff as well as fuel injection parts as they are all problematic. If fuel pressure is verified, tune up stuff ok, etc, you may want to take a look at any bank that has cold manifolds when sprayed with water bottle. I pulled the valve cover and it was painfully obvious which valve was stuck. Trying to pry it with medium prybar and it would not move. I’ve always wondered if the valve seals leak and over time/ high mileage the residual goo builds up in the guide and eventually the valve sticks, or it happens in the exhaust port, etc. perhaps the clearance was incorrect from the factory. The TSB from GM mentions higher mileage Vortecs. I suppose worn / tired valvesprings ( light pressures anyway) are also partly to blame? The truck I mentioned also melted the spider injector tip as exhaust gases made their way into the intake due to the stuck valve. Also, due to the failed cylinder, map sensor was whacking out the fuel management, creating a more difficult to diagnose poor running vehicle. The guy that owned that truck took the heads off and the local machine shop addressed the guide issue. After that, it ran great. I hope this helps!

 

[SUBURBAN5]

X2 you need to hook up a scan tool before you throw away more money. Also keep in mind if you dont use a professional scan tool your playing the guessing game in a way. I'm for trying everything like spray the manifold and testing different test but you need the ecm to communicate with you so you can see if the ecm is even working properly or if that's causing issues. Get a scan tool