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Super Mechanic
Super Mechanic, Master technician
Category: Chevy
Satisfied Customers: 16817
Experience:  29 yrs. exp., ASE master, cert. in G.M. electrical, a/c, susspension, brakes, electronic steering
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why would my 99 Tahoe throw a random misfire code only when

This answer was rated:

why would my 99 Tahoe throw a random misfire code only when under a load like pulling a slight grade?

Hello and welcome, Super mechanic here. Thank you for trusting us with your concern. I will do my best to help you with your question.



****You have the hit happy face or a high rating for me to get paid for the answer.*****



If you need any more on it just let me know. Thanks


About your vehicle.



There has been a service bulletin for that type of concern. Here is what it states.


Engine - SES Lamp ON/Misfire DTC' Set Towing Uphill

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


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


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.


If the P0300 SI 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.

Super Mechanic and 3 other Chevy Specialists are ready to help you
Customer: replied 4 years ago.

Does the service bulletin obligate the manufacturer to research and repair the issue?

Customer: replied 4 years ago.

Does the service bulletin obligate the manufacturer to research and repair the issue? I changed the fuel pump 2 years ago, had the intake manifold gasket and 02 censors changed about 4 years ago... Did the service bulletin provide any insight into what needs to be replaced?

It is actually for valves binding in the guides and the dealerships don't have to take any resposibility for the repair. The head(s) would have to be sent to a machine shop, basically a valve job and any good reputable repair shop can do it, thanks