Question about Cadillac DeVille
P0300 is a random misfire code. It is always a cylinder specific misfire, One of your cylinders has a slight mis and your computer registered it at a certian RPM. In time if it gets worse you will get a number between 301 to 308. Then you will know exactly what cylinder is the problem
All the data will be stored at the time of the misfire to help determine if you have a idle issue or a higher rpm issue.
This has nothing to do with the egr unless you can determine the fault occured at idle.
If the egr was open at idle you would have a code for it and it would idle real rough and smooth out at higher RPM's Its suppose to be open cruising only.
Most likely your problem is a spark plug, injector, coil, or ignition module or intake manifold gasket vacuum leak.
This is providing you have no other issue like a sticking valve or small head gasket leak.
A data scanner will tell whether or not the egr is stuck open and a data scanner will tell you if all your sensors are ok or not.
You can throw parts at this all day put if you don't know what the criteria is for setting a code you will never be able to get to the source without spending lots of money.
The longer this misfire occurs the more damage that will be done to the catalytic converter then you will set a p0420 cat ineffenciency code.
On a car this old its possible the cat is borderline getting ready to be replaced and a misfire can set it over the limit.
Posted on Jan 31, 2010
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Posted on Jan 02, 2017
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