The gear has been already replaced, sometime last week, when the checked the distrb I was told it was in good shape, and shouldn't need to be replaced.

 

Alright Hook, this is a current, as of 5-30 of what has been done to the truck. I have put delco cap, rotor, plugs in it as you suggested, as well as the autolite wires. I have also put in the distrb gear along with the camshaft sensor. The previous owner never had the p0300 problem, and had replaced all of the secondary ignition parts all delco and the whole fuel system including tank, lines, pump, regulator, spiders, injectors. The only thing I could think it may be is something stupid like the fuel filter(I don't even know if that would effect it, but what else could I try) and or the distrb it self. The tech that did the gear said that there wasn't any play in the shaft. I've got 3 garage's stumped and many blazer techs. I'm going to do a full fuel system clean and filter replacement, and have the camshaft retard checked, and have a compression test done. I'm complelty stumped, and just wish the truck would run like garbage to atleast give me an idea of where to go next. I greatly appreciate all the help, and if you have anymore suggestions and or ideas I would appreciate hearing them.

 

Have you checked fuel pressure and leakdown yet? Again, just because all that stuff is new, doesn't guarantee that's it's performing properly.

Save your money on a compression test: Anything that would show up during the test would be a mechanical problem and cause a constant misfire on a specific cylinder, not random like the DTC indicates.

The guys that checked the distributor and replaced the gear should know that gears don't just go bad. There's a reason, and the most probable reason is worn distributor shaft bushings. As I mentioned earlier, the only 100% reliable way to diagnose worn distributor shaft bushings is to view the ignition pattern, (waveform) in the raster format.



All cylinders should be lined up perfectly with each other, (indicating that ignition timing is the same for each cylinder). When the bushings are worn, the waveform looks more like a backwards "C" type of arc, immediately recognized by a trained technician. When the bushings are worn, ignition timing is different for each cylinder causing a misfire. Without checking it like this, you'll never know if this is the problem. Some of the more advanced scan tools are capable of this test.

 

Well, I still don't know what's wrong with my Jimmy. The SES light flashes at 55mph and on. The code still comes up the same as the p0300. I've done the plugs, wires, cap, rotor, distrb gear, and camshaft sensor. I'm friends with the person who I bought it off of and he told me he did all the secondary ignition parts along with the whole fuel system(lines, pump, tank, and the spiders.) Also come to find out the 4.3 in my jimmy is out of a safari. My next step is to have the camshaft retard checked. Any new thoughts from anyone or Captain Hook on this update?

 

You have the same problem as me. You can throw the best electrical parts at it all you want. Its not mentioned very often on the forums. I have been chasing the same thing with mine for many years.
I did a little test the other day up a hill. Noticed that cylinder #5 all of a sudden starting misfiring like crazy. At no other time does it do this. I get the flashing check engine light but no stored codes.The problem lies with the valves (exhaust) sticking under load.
I phoned a local machine shop and the mechanic knew all about this issue. He has a running account with a few dealerships just for this problem.
So I am looking at getting my heads rebuilt, to address this GM fault.


Here is the TSB:

Subject: SES Light and P0300 When Towing , Cruising Uphill or on Hard Acceleration - 4.3 5.0 5.7 accelerate cruise cylinderhead RPO L30 L31 L35 LF6 LU3. DTC P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0307 P0308 PIP3081 - (07/23/2004)

Models: (96-02 Chevrolet Astro) and (96-02 GMC Safari) and (96-00 Old Body Style Chevrolet Suburban) and (96-00 Old Body Style Chevrolet Tahoe) and (96-02 Chevrolet Express) and (96-00 GMC Yukon) and (96-00 Chevrolet Old Body Style C/K Truck) and (96-00 GMC Old Body Style C/K Truck) and (96-02 GMC Savana) and (96-99 P32 Cab and Chassis) and (96-02 Chevrolet S-10) and (99-02 Chevrolet Silverado with 4.3L V-6 Engines (96-02 GMC Sonoma) and (96-02 GMC Sierra with 4.3L V-6 Engines).

The following diagnosis might be helpful if the vehicle exhibits the symptom described.

Condition/Concern:
The vehicle may exhibit a SES Light due to a P0300 and misfire. If the misfire is related to the information below, it will typically happen while cruising uphill, pulling a trailer or on hard acceleration and then stop misfiring shortly after returning to an idle. Typically, cylinders 3,4,5 or 6 will be the ones to experience this.

Recommendation/Instructions:
If the P0300 diagnostics did not isolate a concern, the following may help:
Remove the valve cover, valve springs and valve seals on the effected cylinder or cylinders. A small wire tie or rubber bands can be placed in the valve stem keeper groove to prevent the valve from falling into the cylinder. Rotate the valve while moving it up and down in the guide to see if it binds. If a binding valve is found, remove both cylinder heads and use either of the following methods to increase the stem to guide clearance:

• Send the head out to a machine shop and advise them to increase the valve stem to guide clearance to 0.002 inch per guide.
• Hone the guide with a 9 mm hone, such as Snap On BCG249, until a clearance of 0.002 inch is obtained. This will usually take about 4 strokes of the hone.

Reassemble the cylinder heads using new valve seals.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

NOTE: GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.

 

Hook, what about the camshaft retard check?

 

Good call scottytink

When the distributor bushings are worn excessively, it causes the distributor gear to wear. Replacing the gear without replacing the distributor is a total waste of time and money. The gear is going to wear again, guaranteed. Root cause for the misfire AND the worn gear is worn distributor bushings. After replacing the distributor, then check & adjust camshaft retard.

 

The distrb gear wasn't worn, not enough to cause any sort of misfire, it was replaced just because it had already been pulled out. The bushing in the distrb are fine. No play

 

A worn distributor gear will not cause a misfire, nor will it cause the bushings to wear. Worn bushings cause the misfire due to the shaft rotating in an egg shaped pattern, and the gear follows the same pattern, and the camshaft gear wears out the distributor gear. The distributor gear is softer than the camshaft gear making it sacrificial, (much cheaper to replace the distributor gear than the camshaft). Watching the pattern on a scope, you can spot the worn bushings long before it causes a driveability issue. If you can see or feel movement in the shaft, it's way past junk. Think about where all of the worn off gear material went.... yup, time for an oil & filter change.

 

I'm thinking my next step is going to be to have the retard checked. and see where that leads me. what should it be at?