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Old 05-12-2017, 08:30 AM   #4
95 S_Trucker
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Re: 1999 Suburban 2500 454 misfire

Quote:
Originally Posted by 68c10airstream View Post
A friend complained of the same thing on a 97 454 in a suburban and i remember something in a tsb (technical service bulletin) about exhaust valve guides either being too tight and valves dragging 9misfire) or too sloppy and the stems getting hard carbon and then dragging (misfires).

My friend also said it only happened after a hour or more pulling a trailer.
That TSB is only for the 4.3/5.0/5.7 engines. It could still happen to a 7.4, but I haven't personally seen it on a 7.4. I've seen it on dozens of small blocks.

The vortec 454 is known to have injector problems. Its a good idea to replace all of the injectors. I recommend replacing the fuel pressure regulator when you do the injectors.



TSB for reference:



Quote:
Originally Posted by GM TSB View Post
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 # PIP3081 - (07/23/2004)

Models:

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

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 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.

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. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.
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