1999 Suburban 2500 454 misfire

[gearheads78]

OK my new to me daily driver / tow rig is killing me. A couple days after I started driving it I randomly started getting a misfire. Over a few weeks it started getting worse and I watched it while driving on a scanner and its coming from #4. To get a base line I did a tune up with plugs, wires, cap , rotor and new 02 sensors. Fixed!! Drove like a dream for 3-4 days then started to get a occasional miss again Over the coarse of about 3 weeks its gone from occasional to 15000 misfires on a 20 miles commute home. It's still #4.

Start looking deeper. Misted water on the wires in the dark no arcing. Pulled #4 plug looks great but replaced anyway with no change. Replace wire with another new wire.. no change. Compression test on 2, 4, 6, both hot and cold all 135-138 psi. Injector balance test... all 8 with in 2psi.

It makes no sense to me! What the h##l am I missing? Anyone have any idea? I have an injector on order and hope to have it in over the weekend but if that does not fix it my next step might be a match and a can of gasoline.

[68c10airstream]

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.

[gearheads78]

Originally Posted by 68c10airstream

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.

One of my thoughs also but would think there would be a variation in the compression test.

[95 S_Trucker]

Originally Posted by 68c10airstream

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:

Originally Posted by GM TSB

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.

[68c10airstream]

God i hope the Alzheimer isn't starting already. Thanks for finding the tsb 95 s trucker!

[gearheads78]

For anyone following this I am 98% certain I have found the problem. The light blue wire with black stripe is the #4 injector wire. Yes that is copper you are seeing.

[manimal]

Ive been dealing with Random misfires on my 98 K1500 5.0. I ended up pulling the heads for a valve job, dropped them off Friday and last night I went out and just for sh*s and giggles I flipped the intake over and saw the intake gasket was leaking in #6...where the misfire was!