Suburban P0300 Misfire, Need Help Understanding Live Data

1999 GMC Suburban K1500 5.7L

Truck started running pretty rough all of a sudden and check engine came on. I splurged and got a good scan tool (been wanting one for awhile).

The code came back P0300 which is “Random Misfire”.

So in the past I would just start doing all the usual suspects, vacuum leak, plugs, wires etc. but I want to see if the live data from the scanner can offer some insight to someone with more experience reading the data before I throw parts at it.

So according to the data it looks like Bank 1 is running rich and Bank 2 is running lean. I believe it’s running rich cause I can smell the gas.

Now what I’m thinking is it might be a faulty O2 sensor, about 2 months ago the check engine came on for an O2 sensor and then like 2 days later it went away on it’s own and never came back. The live data for the O2 sensors seems to be consistent for both banks, front and back sensors, so I’m not too sure.

So anyways, I would appreciate some input here on what to look for in the live data to try to narrow this down before I just start throwing money and parts at it.


This probably won’t be on the list of things to check, but with one side lean and the other rich, my first suspect would be a vacuum leak, probably the intake manifold gasket. However first check any vacuum lines on the lean side first. Look for anything that would upset the balance between the sides.

To check the gasket, use a spray of some kind on the gasket. You can use WD40, carb cleaner, even propane. What you are looking for is a change in how the engine is running as you spray an area. It may speed up, slow down or just run smoother. If there is a change, the gasket has to be leaking there or the spray would not be getting inside the intake manifold or sealing the leak in the gasket.

Propane has a slight advantage on a V engine because it will get underneath the manifold where some sprays may not reach. One more thing that can upset the balance is the supply air line from the intake duct between the air filter and the throttle body to one valve cover. The PVC valve is on the other valve cover.

@keith has good ideas, try them. As for the live data… if you beleive one bank is rich the only way the scanner would measure that is from that bank’s O2 sensor. I’d suggest the O2 sensor is working. The scanner can only tell you what it can measure. If there is no sensor, it can’t measure it.

The trucks computer calculates some things like fuel trims. If bank 1 is rich the short and long term fuel trims should be negative, more than 25% negative. The computer can only report what it reads, it is still on you to diagnose it using the basics.

Slap a vacuum gauge on the truck and see what it reads. Should read low and unsteady if there is a leak.

Just hooked the vac gauge up and she’s reading ~18 in, steady no fluctuations.

So don’t think it’s a vac leak.

The motor and most of the accessories have about 30k on them, (new GM crate motor) so I have hard time believing it’s the plugs, injectors or distributor unless one them just happened to fail early.

Im thinking of doing the upstream O2 sensors first, I haven’t replaced them sense I’ve owned the truck so there is no telling how old they are.

P0300–Random Cylinder Misfire needs to be addressed first. GM cars of that era generally do not provide a specific code for the misfiring cylinder(s). To see which cyls are affected you will need to go to live data, then misfire counters, and see which cyls are failing and then do specific tests there.

If you’re smelling gas in the exhaust it could be unburned fuel from the misfire. Cars that old were not smart enough to turn off the injector on a misfiring cylinder.


If you are sure it isn’t a vacuum leak, then the most common cause is the crankshaft position sensor.

A properly operating crankshaft position sensor is necessary for the misfire monitor to function. Since you have the symptoms of a misfire, I would believe the misfire fault is accurate.

The most common cause of misfires are failing spark plugs and/or ignition wire.

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True to indicate the specific cylinder, P0301 through P0308. P0300 is for a random misfire and a faulty crankshaft position or camshaft position sensor can cause this code. The computer can’t figure out which cylinder is the issue without these working.