P219a - Rough & Shakey @ all cruise speed

All right. So in normal operation all 4 cylinders operate equally, and the engine computer uses the signal from the upstream air fuel sensor (AFS) to measure and correct the fuel mixture. This is Fuel Trim and is applied to all cylinders. Short Term trim (STFT) is an immediate response and its goal is to keep the AFS centered on whatever the target mixture happens to be. Positive trim is fuel being added to the "base injector on-time" for those operating conditions. Negative trim is fuel being removed.

Next is Long Term trim (LTFT) . Its goal is to keep STFT centered near 0. If STFT moves away up or down for too long then LTFT makes a correction and keeps it in memory. (example- if there is a slight vacuum leak and STFT goes to +10% at idle, eventually LTFT will go to +10% while STFT returns to near 0). STFT plus LTFT equals TOTAL FUEL TRIM, which is the full correction to fuel injector on time.

What does this have to do with your code? Well, fuel trim is applied to all 4 cylinders. The feds decided that car makers had to keep tabs on individual cylinders. So the the computer can look at the AFS signal and figure out what the mixture is from which cylinder and can adjust the fuel trim for that specific cylinder. If the fuel trim for any one cylinder varies by more than 15% from the others, the ecu will set a P219a code.

SO, really ANYTHING that affects what comes out of a cylinder's exhaust port (ie, the exhaust gas mixture that is sampled by the AFS) can cause this. Compression, valve function, ignition, injectors, exhaust leak at the manifold, etc. Injector problems near the top of the list BUT THAT WOULD BE A GUESS! It will be difficult to diagnose a problem that is symptomatic only at cruise.

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How can it adjust per cylinder when there aren't 4 separate sets of sensors for each cylinder? Wouldn't you need 4 O2 sensors minimum - one per cylinder?

 

do the intake and get back to us

 

My 2010 manual doesn't list a P219A. Has it been added in later editions of Gen 3 manual?

If so, the page with the detection condition and monitor strategy ought to clear up how the system detects it.

 

Yeah intake manifold has small Exhaust Gas Recirculation passages, one per port. You can clean the Exhaust Gas Recirculation components till they shine, but until you deal with intake it’s all for naught.

When cleaning the intake, DON’T disconnect throttle body coolant hoses: there’s more than enough slack that you can lift the throttle body off and just tie to the inverter conduit or whatever.

 

This strategy is called Individual Cylinder Fuel Control and was initially rolled out on (some?) 2011 cars, with full implementation a year or two later.

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As I understand it, each exhaust gas pulse goes past the AF sensor in order. The ecu makes a deliberate shift in trim to one injector and watches for the response from the AFS. Once it has "synced up" that pulse with a particular cylinder, it just keeps track of the pulses (computers are pretty good at counting and math stuff - Oh, that pulse for cylinder 2 is a bit lean, let's give it +5% and keep an eye on it) I imagine it periodically rechecks its "sync".

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Yes, from 2011.

In two ways, according to the Repair Manual (more info):

“Air Fuel Ratio Sensor Monitoring Method

“When the ECM detects an air fuel ratio imbalance between the cylinders based on the fluctuations in air fuel ratio sensor output through all 4 strokes of one cycle (2 rotations of the crankshaft), the ECM determines that there is a malfunction.

“Crankshaft Position Sensor Monitoring Method

“The ECM monitors engine speed fluctuations. When the engine speed fluctuates significantly, the ECM judges this as an air fuel ratio imbalance and then determines this as a malfunction.”​

 

If you unplug the EGR valve, the ECM dials the ignition timing way back to protect the engine, and that by itself can change your sense of how the engine is running, so it might not tell you as much as you think.

Do you have compression or leakdown test results?

 

Hope that includes removing the intake manifold and physically cleaning the EGR passages. As I understand, the gen3 uses individual passages branching out from the EGR valve and cooler to each cylinder. If one or more passages are completely blocked, any chemical system (such as BG) cannot reach that blockage.

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I'm not seeing how such a chemical would get into the EGR passages even if they weren't blocked.

I kind of doubt it's the same chemical by the time it turns up in the exhaust.