Code PO300 – Check Engine Light Coming On

Hello, my vehicle is having issues. It threw a po300 code. Brought it into a local shop that I have dealt with for years and they ran diagnostics and came up with that code. After the diagnostic, I was told that the only thing that would cause it was the timing chain since all the electronics checked out. Previously I replaced the plugs,wires cap,rotor,fuel filter,air filter.

I had a friend replace the timing chain who is a retired gm mechanic.(He is not up to date on the computer stuff, but understands them somewhat). It does it most when you are driving on a flat surface or going down a slight incline when the engine isn’t under load.

It runs extremely bad when it is raining or damp outside. I also ran two tanks of fuel with Seafoam in them.It was doing this before replacing the parts listed.(Also replaced the camshaft position sensor).

I am not one who likes throwing parts at it. Could not find any vacuum leaks. It starts fine and seems to idle fine. I am at a loss of what else it could be. I’ve even looked at it in pitch black to see if I see any arcing and squirted some water around to see if it starts running rough to no avail. It has the vortech 4.3 V6 w/csefi. Thanks for your time.

Hi there,

That code is for multiple misfire issues, like spark plug wires etc. etc. so I would still be suspect of them. if you did not have this issue prior to the timing chain replacement…I would be highly suspect of a problem with its installation.

You can install a timing belt or chain incorrectly, but the engine will still start and run but you will have a loss of power issue, poor fuel mileage, check engine light on, rough idle speed and hard to start problems…but it will run.

We refer to this type of installation as “off a tooth” since the metal gears and the timing chain or belt are physically off just one tooth on the gear, causing the engine to be “out of time”.

Would be nice to have a qualified dealership mechanic at least diagnose this problem, anyone can repair it once you know for sure what the problem is. Might be worth a $100 check out charge at your dealership before you spend more money on this.

Sorry wish I could be more of assistance to you, but need more information to go farther.

Please share this with your friends,
Austin Davis

6 Comments on "Code PO300 – Check Engine Light Coming On"

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  1. vira says:

    Hello Austin!
    hope you had a wonderful vacation with family! we had the cylinder head checked but no cracks, a but we did have a blown head gasket. that was all fixed including as well as other things mentioned in my previous post. we replaced the spark plugs and wires, coil, throttle position sensor, distributor cap but now we keep getting the 6th cylinder misfire code.spark plugs and wires were rechecked and just fine but that code keeps coming up and it misfires and rides rough and idles rough as well and uses way more gas than usual. could the cylinder head be the culprit again?! you suggest a compression test to see if its an internal engine problem? i hope not, feels like we are running around in circles!! feel like trashing the jeep! lol
    i would appreciate your assistance,

    • Austin Davis says:

      Was a wonderful vacation….and still trying to catch up from being off 🙂

      The compression test will rule out an internal engine problem in a particular cylinder. If 1 or more cylinders are much lower in compression than the others, then there is some kind of internal wear….could be a valve problem in the cylinder head, or could be a more serious problem with a piston in that cylinder.

      Unfortunately the first thing to do in that situation is to remove the cylinder head for inspection again. If the cylinder walls and the pistons and rings look ok, I would send the cylinder head off to a machine shop and have a valve job done.

      Before you spend any more time and money, get the compression test from a qualified mechanic.

  2. vira says:

    Hello Austin,
    Wondering if you could help me out with an issue we’ve been having with our 99 jeep Cherokee sport. Recently, we had some repairs done and parts replaced…new radiator, thermostat, cylinder head, and other things related to it over heating. Of course, with car repair it never ends, one thing after another is needing repair on that darn thing!! Currently, the jeep is using way too much gas, it rides and idles rough. the spark plus and wires and a new throttle position sensor were replaced and still the same thing! the check engine light went on again and the code came out to be that the throttle sensor had low voltage and the 1st and 2nd spark plugs were misfiring. But how can that be is we replaced those parts. It ran good for an hour or so but it want back to the rough driving, idling ,etc. WE looked at it again, and we didn’t see anything out of the ordinary. Now we find ourselves wondering what is going on. Our Jeep is using up all the gas way too quickly and the rough ride and idling still exist. would appreciate your assistance! we are soo frustrated with this darn jeep.
    thanks in advance,

    • Austin Davis says:

      Hi Vira, soooo sorry for my delay, have been on vacation a few days hiking in the Smokey Mountains…with no internet 🙂

      You had the cylinder head replaced???? I am assuming the engine severely overheated and damaged the headgasket? I think I would start with doing a COMPRESSION test on each cylinder and rule out the possibility of any internal engine damage….which would show up as low compression on a few cylinders. Low compression can cause the engine rough idle/miss feeling and poor fuel mileage.

      If compression is OK, then start with the distributor, check the cap and rotor installation and condition, check the ignition timing and the firing order of the spark plug wires. Even a seasoned mechanic can get the ignition timing off or the firing order wrong. Firing order is the placement of the spark plug wires on the distributor cap.

      Rule those things out and get back to me if you still have problems. Sorry for the delay again.


  3. Dan says:

    Yes that is a misfire code but the tech should have also looked and found out what cylinder or cylinders were misfiring to help narrow the search down for the fault it could be a bad injector bad coil or coil wire fouled plug coolant leaks from head gaskets or intake gaskets fuel pump not suppling enough fuel pressure causing multiple cylinders to miss under load could be water in the fuel rail or rust or dirt. Maybe its the MAS air flow sensor is faulty or just dirty and needs to be cleaned when the plugs were changed did they all look the same or were some white signs of coolant leaks or maybe there is a injector sticking on how is the exhaust back pressure is the converters or mufflers plugged the possibilities are endless without more information. The diagnosis skills of your local dealership or a trained trusted tech are required on this one for sure but if you had stated the year make model and mileage I could have narrowed the search a bit. Bet of luck

  4. Rob says:

    If your vehicle used Dexcool coolant, it could eat away at the intake gaskets resulting in coolant leaking into the cylinders causing multiple misfire code. This happened on my 99 Silhouette. Had to get an intake reseal job ($1300) and then found out about the class action about it ( It may not be the issue here but is one potential cause of the symptoms, especially if you are losing a bit of coolant over time and/or smell a little coolant when under the hood.
    I think timing chain is a red herring. GO somewhere else for a second opinion, GM dealer would know about the dexcool issues and should have seen their share.

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