Random misfire under load pulling trailer

have a po300 code stored intermittently when pulling a trailer up hill under load. Have new plugs,wires,cap, and rotor. Fuel pump replaced and new fuel filter. had a crank shaft relearn performed. new air filter and cleaning of mass air flow sensor. Does not happen all the time but seems to be only when pulling a trailer up hill with air on under load. Any ideas of where to start would be helpful.

One thing…is your “overdrive” in the off position? The random misfire could be caused by momentarily overtaxing your engine. Most owners manuals recommend turning off the OD when towing and especially in the hills and mountains.


More information needed

Model year
what engine

I know of a bulletin that matches your symptoms, but I need that info first

Can YOU detect the misfire (does it actually exist) or is it just a computer glitch throwing a false code?

I went through this, a come and go misfire, a set of new injectors (found on e-Bay) cured it…But mine did it all the time, not just under a heavy load…

Verify the plugs have the correct gap. A spark plug with a too-wide gap can cause this.

Which cylinders are misfiring? GM products of that era will not give you a specific cylinder misfire code. Your mechanic will need to use his scan tool to access the live data and read both current and history misfires. Finding out which cylinders are misfiring is the first step in a misfire diagnosis. For instance on your vehicle if the misfire only happens under a heavy load and is on all the even numbered cylinders I’d suspect a restricted exhaust.

If your shop does not have this ability find a mechanic who has more than a stethoscope and an Etch-A-Sketch in his toybox.

How often do u pull trailer up hill? Weekly? Daily? If u do it rarely, than maybe u can live with it?