you first need to check the fuelpressure,,these must have no less than 60psi at all times,,,..low fuel pressure could ause this code..
also,,,remove and clean the egr valve,,it may be dirty and not closing all the way,,thus,causing a vacuum leak from the exhaust
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.