02 tahoe p0300

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hey guys i have an 02 tahoe 5.3 flex. when driving above 50 or over 2500 rpm, the check engine light flashes and i get p0300(random misfire). i have changed the plugs, fuel filter, both upstream 02 sensors, cleaned the maf sensor and cleaned the fuel system. i sprayed the intake manifold with maf cleaner to check for leaks but didnt notice a change. and other ideas? thanks guys!!
 
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they looked like they where in great condition so i havent changed them. they arent factory wires. i just got the car a few months ago
 
Been a few years.. but thay code sounds like what I had on my 05 silverado turned out to be evap canister problem.. dont quote me though I could be mistaken
 
Sometimes the Chevys will throw misfire codes without a misfire and the problem will go away if you do a cam/crank variation relearn with a computer. I don't know what causes the problem to begin with but would guess a slightly longish timing chain.
 
All that and you did not replace the wires? Does the vehicle misfire under load? such as hard acceleration, possibly going away around 3000 rpm? that would be secondary ignition. Secondary ignition is plugs, wires, coil's, the high voltage stuff.
A Plugged injector is possible if if misfires all the time.
Bad Cat and Cam/Crank relearn are also possible.
how did the plugs look?
 
I used to get that every so often on the Camry. Are you using the correct type of plug for that engine? Have you tried swapping the coils around? What fuel system cleaner did you use?
 
Originally Posted By: macdole99
hey guys i have an 02 tahoe 5.3 flex. when driving above 50 or over 2500 rpm, the check engine light flashes and i get p0300(random misfire). i have changed the plugs, fuel filter, both upstream 02 sensors, cleaned the maf sensor and cleaned the fuel system. i sprayed the intake manifold with maf cleaner to check for leaks but didnt notice a change. and other ideas? thanks guys!!


If this code comes up more than once after a reset then it should eventually settle down to one specific cylinder. Swap coils around to eliminate them, the wires are suspect if not GM OEM, then the triggers (cam and crank) are next. Then check valve springs and such.

This can be a bit vexing, but it is very serious. It can be some really bad and expensive things, like a bad lifter eating the cam, etc.
 
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i changed the plugs also ngk iridum plugs im really stumped. tahoe board said it likely is a leak in the intake manifold but service records show it was changed in 2005.
 
It starts and runs perfectly up to 2500 RPM right? It then begins to fire badly the more gas you give it above that?

Its a random misfire so that basically eliminates a host of items like a plugged injector or a bad coil.
Low fuel pressure would be one thing that would effect all cylinders but usually has other symptoms associated with it like harder starting than usual.
Do a pressure test with the pressure regulator connected and disconnected.
The ignition module could be a fault but it too usually has different symptoms.

It is rare for a sensor to go wonky out of the blue and cause something like this, usually when a sensor goes out its gone and more than likely will result in at a minimum a sensor code or no start condition.

Get a cheap vacuum gauge and connect it. Run the engine up to higher RPM until the misfiring occurs and watch what the gauge does, if it begins dropping you have a bad cat.
It can tell you a lot of other things also that might be wrong with the engine, it is one of the most used tools i own.

A partially clogged or melted cat will flow enough to allow the gasses escape up to a point but may not be flowing well enough to allow enough through at higher RPM. If this is the case it will only get worse.

Time to get into more diagnosing rather than throwing more parts and money at it i think.
Stuff like this is sometimes difficult to get to the bottom of and can be a real pain.
Please follow up on this, its going to be interesting.
 
lately it will start then die but the catch and run like normal. going to pressure test the system and go from there but i will keep you informed. thanks for all your help
 
After a process of elimination combustion chamber fouling from cumulative oil consumption, a small vacuum leak in the air box, and weak coil packs were causing my random misfire issue. The fuel system cleaner at the top of the list if you consume oil. What fuel system cleaner did you use? Which one matters, some do nothing.
 
Trav, good call on the plugged exhaust. When that happens the engines run fine until high rpm or load is put on them. Very tricky to diagnose!
 
Just giving an update I took the car to the Chevy dealer and they did the crank relearn and so far all is well with it.but he didn't rule out a small intake leak.
 
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