Last time, I shared the story of a 2005 Dodge Stratus, 2.4 DOHC certified to California emissions standards and VIN designated "J". The car had a code P0491 stored...a fault with the secondary air injection system that sets when the ECM determines that too little or too much air is passing through. I explained how I used my scope to capture the information I needed to diagnose the problem and determined from that capture that there was a restriction to air flow. I had removed and inspected the one way valve, found it corroded badly and hard to pass air through, and thought I had found the problem. The part was replaced, along with the air pump (severe current fluctuation in the original) and relay (as recommended by the manufacturer when the air pump is replaced). The capture below is AFTER this repair:
This time, I added a channel to include battery voltage. This way I could tell on the capture when the key was turned on, the engine started, and then shut off again. The information shows all is working as it is supposed to, but still only 0.10 volt change in MAF with the pump running. There is still a restriction....
The air pump on this system is controlled by a relay, which in turn is powered by the ECM. The scope capture confirms that is happening. Air is drawn in through a filter located just below the intake manifold, then through a dedicated MAF sensor to the pump. The MAF signal was confirmed by disconnecting the line at the one way valve and seeing MAF voltage increasing to 5.0 volts. From the pump, the air flows through a nylon line up to the rear of the cylinder head to a one way valve located on the exhaust manifold. This is where I originally thought the restriction was occurring, but as you can see...I was wrong.
This time, I removed the valve with the line attached and ran the engine again. Air flowed just fine, and that is how I learned that...even though the valve was very hard to blow through...the spring pressure was normal and the pump was more than capable of opening the valve. The restriction HAS to be on the exhaust side, and that means removing the manifold.
After passing the one way valve, the air enters a separate chamber cast into the cylinder head. From there, individual ports are drilled into each exhaust port...similar to individual EGR ports you would find on the intake side of some engines. I didn't think these ports would be clogged, considering the engine only had 35,000 miles on it, but all the evidence said they were. With the manifold removed, I saw this:
As it turns out, I was kind of right...the ports were not clogged because THERE WERE NO PORTS TO CLOG! Somewhere in this car's past, the head was replaced with a Federal emissions head, and the ports were never drilled through. The tech was "smart" enough to know that this wouldn't work, and double gasketed the exhaust manifold after cutting passages of his own in the gaskets themselves.
How to test next time? I should have removed the valve with the line the first time around to confirm the fault, but let the low mileage on the vehicle cloud my judgement. But I don't feel too bad...after all, I didn't put the wrong cylinder head on!