misfire on 4 and 5

[tripleD]

ok guys ive had the best of the best gm mechanics work on my wifes 99 4x4 tahoe!! need some answers PLEASE!! runs perfect when drivin normal but get down on it it will jump up and go then goes to hell shakin like crazy!if u hold it the check engine light comes on but at the same time it clears up! my scanner and everyone elses scanner says misfire on 4 and 5!sometimes u can get on it alittle it gets up to about 50 to 55mph the shakin begins but hold it steady and it will clear up! it has completely been tuned up all censors changed u name it ive changed it! guys im at a dead end i need help bad cause u no if mama aint happy well she makes me unhappy so help me please!!

[95 S_Trucker]

Had this happen on a couple of trucks

There is a TSB on it

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.

[starterman99]

Had this same problem on my 97 took it to the dealer a couple of times they could never find anything other than my money.finnaly pissed me off so when it did it one time to many I put the pedal to the floor,she coughed,spit, farted and bent the exhaust valve in #5.then I found the problem and the service bulletin.

[tripleD]

thanks guys well i guess i will take the top end off and take a look cause thats the only thing i havent done!!

[95 S_Trucker]

Mine was a 97 too. It made me when I was towing the trailer. But on the bright side, I got 2 more doors and a big block now.


How many miles are on the Tahoe? It may be worthwhile to pick up another set of heads and have them gone over, then just swap out the heads. You'd have less down time, And no surprises. (finding a crack in your existing heads)