Misfire Under Load

kdm101 · June 6, 2009

My truck just developed this the other day (Thursday), so it's not like it was getting worse over time. It's very noticeable around 1500 RPMs and I apply slightly more throttle. Until it kicks the tranny down a gear, it misfires. Just yesterday while on the highway, I heard it backfire under similar conditions - steady RPMs and slightly more throttle and a slight grade to induce a load. I had filled the tank the night before this happened, so there's a possibility it's bad gas. My travels yesterday burned up nearly all of that tank and I filled up and added a can of seafoam just in case. MPGs on that tank were consistent with what I've been running recently. Power seems to be OK as well when it's running smooth.

So, I'm planning on replacing the plugs and wires as a first step, but thoughts of a manifold gasket leak or clogged cats have been circulating in my head. I should also indicate that I have CEL that has been lit for about 2 months due to a P0452 code. Could something with the purge system be causing a vacuum leak that's causing bad timing? I also won't discount the possibility of a bad fuel pump - which I just replaced last year.

Suggestions on where to go next will be highly appreciated. If I don't get this sorted out too soon, I'll be heading to the dealer to properly diagnose the code I have anyway. This is my daily driver and I'd rather fork out some money to have someone fix it right away than screw with it for weeks trying to save a few bucks.

kdm101 · June 8, 2009

Well, amazingly enough, it appears to be fixed with just replacing the plugs and wires. It's strange that it went from running fine one day to crappy the next. No matter, I'm happy to have it running right again!

WaterGuy · June 20, 2009

During a heavy load, the combustion in the cylinders are at the max. This condition makes it more difficult for the spark plug to "spark". A weak wire can not deliver the extra "punch" to make the plug "spark".