Misfire under HEAVY load

[mrobertson]

I know this has been covered before but my situation is a little different.

I tow a 23 foot boat with my 1997 4 door tahoe. Weight estimation roughly 5000 - 5500 pounds. When hooked to the boat, i get a cylinder 2 misfire code P0302.

Now - as soon as i unhook the boat and cruise down the road, light goes off and no more code.

Also - i tow a smaller 19 foot boat, weight is probably 2000 - 2200 pounds. The truck will not misfire or throw a code when towing this boat.

So - i guess i only get the misfire towing a really heavy load. Truck runs great otherwise. Truck has new head gaskets and intake gasket. Not sure how many miles on the current plugs/wires/cap.


So - where would you start? Do a fresh tune-up?

 

[SunlitComet]

Direct your attention to that cylinder itself and inspect its plug and wire first and go from there. For the wire look for any burn, chaffing or flash marks that might indicate electrical leakage.

 

[Gzes]

You might be pushing it too hard. When I tow I never have had problems but I use a 16 foot dual axle flatbed.

 

[SunlitComet]

He is within the the weight rating even with a 23 footer based on his weight estimation. Even with a 3.42 rear end if that is what he has.

There is another issue that may involve a valve binding but usually not on #2.

 

[Gzes]

Do you have overdrive? If so what does it do when you have it engaged?

 

[gpracer1]

If it is not spark leakage, it may be running lean on that cylinder as in the spider. May try running some techron through the tank.

 

[blueflamed03]

weird only under load........

 

[SunlitComet]

Trust me I love my chevron products but two bottles of tech ron in a half tank won't help the spider when it gets bad. The other valve related issue was this:

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.

 

[ande6009]

Seems like nobody has found the answer to the P0303 problem. Im not afraid to take a looksee into the valvetrain, just wanted some ideas before I went diving head first into merky waters.

 

[gpracer1]

Under a load vs normal I would suspect the map, clean the maf and check O2 operation. This is assuming the fuel pump is supplying enough flow/pressure and the plugs wires etc are good.