Northstar p0300

[Tristan]

I have a 2003 cadillac deville with a p0300 code that i have been dealing with for the longest. I have done everything i know and still have the misfire. The engine misses under load and sometimes at idle as well. I have changed the spark plugs, both ignition coils, cleaned the throttle body, cleaned the egr, changed the intake plenum, new crankshaft position sensors, new cam sensor, and now i am very stumped. I have no idea what could be wrong. No oil in coolant or coolant in oil but very slow coolant leak somewhere ..

Topic

[Wayne]

Can start here:
http://ericthecarguy.com/faq/finding-and-fixing-the-causes-of-poor-mpg?start=4#Misfires

There’s more than a few things that could be causing this. If you have access to a scanner with live data and misfire data capablity this would likely be the quickest way to try and lock in on the issue. Otherwise you’re going to have to approach this methodically, leaks, fuel pressure, exhaust back-pressure, etc..

[John Hugon]

I assume you checked the spark plugs boots for signs of carbon tracking? Things to check that I found…chassis wiring cable between the PCM and the front bank coil cassette….chassis wiring cable between the PCM and the front bank fuel injectors.

Check the plenum duct which is the rubber bellows between the throttle body/crossover and the plastic intake manifold …under the engine cover. Spray it with brake cleaner while pushing it with your fingers or a screw driver. ….change in idle indicates it’s torn.

[EricTheCarGuy]

I’d start with a power balance test to find the offending cylinder(s). After that, it’s a matter of finding out what the deficiency is in those cylinders. More information on solving issues like this in this article.

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

Good luck and keep us posted.

[Author]

Replies