Misfire Cylinder 4 P0304

Well --- on the Gen 3 Prius the EGR system is a source for various knocking and misfire codes and rough running. The design allows too much soot to flow from the exhaust into the system. This soot coats everything and eventually blocks both the cooler and EGR valve .. additionally the small ports in the intake manifold feeding gas from the EGR valve are compromised. That's why it's important to clean or replace the valve and cooler -- and fully clean the intake manifold and the pipe to the manifold from the EGR valve. The whole system needs to be cleaned.

I was getting the EGR system code as well as various misfire codes ... it's important to understand what is happening. The EGR valve may be working ... but, if the cooler is clogged --- the gas cannot flow through the cooler into the valve and onto the cylinders. Additionally -- the cooler could be allowing gas to flow and the valve could be sticking. So - no flow can be caused by two problems. Naturally, various in-between situations can also occur. It's important to understand -- as soot covers the cooler -- Gas temp is increasing. Higher temps affect all the parts and the operation of the recirculation into the heads.

Misfire: The system works by directing the exhaust gas into the intake manifold -- the final ports are very small as they enter the heads. As soot builds up through the connecting pipe (metal) and into the small ports of the plastic manifold the gasses are not equal to each cylinder. This unequal gas introduction causes a particular cylinder to be different vs the other -- the injection point is past the computers control. Not all cylinders are getting the same air/fuel mixture ... misfires occur. In my case only one port was open .. #2 ... I was getting #2 misfire.

As the system gets fully clogged the misfires may actually go away as now the engine is not getting any gas recirculation and all cylinders are again having same air/fuel. That was my situation --- later in the process.

The problem throughout all of this is the engine is not getting the cooling effects of the EGR system as the cooler becomes coated and eventually soots up -- engine runs hotter, miss fires ..... hotter temps increase oil usage .... overheating of heads increases likelihood of gasket problems.

PVC valves are simple devises and are reliable -- there have been reported issues with those in Prius due to oil. Replacing with manifold cleaning is a valid procedure -- they are cheap and very easy to replace when the manifold is off and not at all easy to replace if one becomes unlucky and needs it done independent of any other work.

Not wanting to risk having double labor costs -- I bought a new OE EGR valve before work started VS trying to clean once opened up. When we opened up the system -- the cooler was so clogged I had to replace as well. This caused some delay as I buy parts online for best price. Toyota has updated the Spark Plugs to a hotter plug .. so I replaced mine for the second time a bit early. Once all cleaned and back together -- no more misfires and the oil usage dropped to almost a new car. Now at 215k. I was not having any coolant usage -- so we hopefully escaped that related issue.