Hello ChevyChevelle

I found this info on another site.
Hope this gives you some extra help.

A P0300 OBD code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn't tell you specifically which cylinder(s) is/are mis-firing, nor why.

Symptoms
Symptoms may include:

* the engine may be harder to start
* the engine may stumble / stumble, and/or hesitate
* other symptoms may also be present

Causes

A code P0300 may mean that one or more of the following has happened:

* Faulty spark plugs or wires
* Faulty coil (pack)
* Faulty oxygen sensor(s)
* Faulty fuel injector(s)
* Burned exhaust valve
* Faulty catalytic converter(s)
* Stuck/blocked EGR valve / passages
* Faulty camshaft position sensor
* Defective computer

Possible Solutions
If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open.

Hope this helps.

Mark

 

Hmm... I knew this sounded awful familiar... Think it could all be related? Now you may see why keeping everything in one thread is a good idea CC.

Has a compression test been done? Has the timing been verified to be anywhere in the correct ballpark with a timing light? It is quite possible that the timing chain has jumped.

The P0300 code details can be found in the DTC section with more details and the diagnostic chart.

 

Sorry about the other posting I just thought the 2nd gen was a more specific location for
the posting. It is definitely related, it started with the truck running really bad and the PO300 DTC.
Just getting very frustrated with this truck!

I have verified the timing with #1 cylinder at TDC and the distributor pointing at the #1 mark
on the distributor casing. I forgot to mention earlier that I changed the gas filter when I did
the plugs and wires. Also when I changed the fuel pump I drained the gas and put in 5 gallons
of fresh gas.

I pulled the following info from my scanner in monitor mode.

Engine RPM - 725 to 740
Ignition timing advance - 14 to 16 degrees
Long term fuel trim1 - -18%
Long term fuel trim2 - -18%
Short term fuel trim2 - -8 to -17%
Intake air temp - 80 degrees
Air flow rate - .76 lb
Throttle position - 0%
Oxygen sensor1, Bank1 - .2 to .85v
Oxygen sensor1, Bank2 - .1 to .8v
Oxygen sensor3, Bank2 - .06 to .07v
Load value - 3.1to 3.5%
Coolant temp - 194 degrees
MAP sensor - 10 in hg

 

I rechecked the timing via the procedures here and it is right on.
OK, I've thrown the last part at the vehicle, I replaced the crankshaft sensor with no luck.
How do I tell if the Catalytic converter is clogged. I put my hand on end of the tail pipe and
the exhaust is so hot it almost burnt my hand. Still missing seems to be worse around 2500 - 3000 rpm.
I cleared the Po300 code, but of course it returns.

ok, here is a clue, the truck was absolutely running better when the secondary air pump was running.
It still had a slight miss but was 90% improved. when the pump stopped it went back to running really bad.

 

I know this sounds easy but i had almost the same problems on my 95 it was a clogged catilytic converter. Sounds bad but to test my theory i simply cut it off. Lol i know not the best idea but i was right it started running perfect after this. Put a new on and been good since. Oh yeah i did alot of the same things you have done also before this. Good luck

 

You don't have to cut out the cat to test it - simply remove the O2 sensor in front of it. If the truck runs better, the cat is more than likely plugged.

 

I have a 1999 gmc jimmy4x4 and It also has the check engine light on. I have scanned it with the obd scanner and it has the same problem and code. random multiple misfire. I also have a 1997 chevy blazer and im rebuilding the engine. I have been trying to figure out the misfire deal and had heard that theinjection system has been known to leak in the upper intake. theres these spyder fuel lines that are under the enjectors that are plastic they have been known to leak. I havent had a chance to take apart my jimmy cause I drive it every day but this is an idea that u could try to look at.

 

also my jimmy does idle rough and at times doesent start right away but it has gotten a little worse as I drive it back and forth between austin and sanantonio tx.

 

The nylon lines running from the electronic injectors to the poppet nozzles are a pretty good design and I have not heard of them leaking without someone first messing around with them. The FPR in the newer SCFI motors (96+) also is a much more robust design and will typically bypass pressure back to the tank instead of holding it, rather than leak into the plenum like the older CPI FPR would.

As far as your P0300 code, 3hondacrx, please search the forum and then post up a new thread concerning your issues. If you could, post links to the threads which you have read and expand on things which you tried as a result of that reading. Give details. No one here can read your mind to see what you have done to attempt to cure your problem. Also note that everything starts with a fuel pressure test on these motors. It is just the most common failure point.

 

Thank you Leeann, I am going to pull the O2 sensor as soon as the weather here gets above 10 degrees.