2010 Prius; Whole-vehicle shudder, P0304, seems unrelated

Definitely confirms that uneven blockage of those EGR ports on the intake manifold can lead to misfire codes (if there was any doubt).

And good thing your test drive was hard enough to activate use of EGR, if you just drove it around the block that wouldn't have warmed it enough to meet the EGR pre-requisites.

Also, you're not at all the first guy on here to have "cleaned everything" but missed those little EGR ports. It's good catch @Mendel Leisk !

 

I first noticed that idea in a video by @Ragingfit. He was dealing with EGR on these forums like 5 years ago, and we're still learning...

 

Every piece of the system is important.

 

Everybody likes a quick fix. Rough running can be “cured” by a simple disconnect, why not...

the Exhaust Gas Recirculation disconnect is tantamount to “completely plugged”, and everything that comes with that.

 

That's a slightly-too-hasty conclusion, because of the fact that the engine control module can tell that you have unplugged the valve, and that causes it to also change other engine parameters like ignition timing. A good illustration of the way "I did X and that tells me Y" reasoning can fail for someone who hasn't first made sure to learn everything X does.

... with, again, the difference that the disconnect is detectable by the ECM, which can do things to compensate.

 

I think Mendel's response was more to the idea of a long-term fix.

My response was simply to the short-term diagnostic interpretation. You have to remember not to think "now I know the engine runs better without EGR", because it's really "now I know the engine runs better without EGR, with the ignition timing retarded, and the other changes the ECM makes to compensate for my unplugging the EGR, which it knows I did".

 

@Ragingfit left his Exhaust Gas Recirculation system (on the Prius v in video series) disconnected for around 50k miles IIRC. I really appreciate his videos and transplant info, but think that disconnect is what fragged the engine.

 

Which one of us are you asking, and what result are you asking about?

If you have a valve that appears electrically 'present', but doesn't open, that'll have the same effect as a completely clogged system and without the other engine-protecting precautions that the ECM takes if it knows that the valve is unplugged.

 

Have we lost sight of earth? The quick EGR unplug test isn't going to hurt your engine in the real world, and in the vast majority of cases if it "solves" your problem, then there really is an EGR related issue. Likewise, the solution is (90+%) of the time a full EGR clean. In practice it seems a small percentage of time the valve does need replacing. This cigar really is a cigar, folks!

 

Well, part of that is why I'm not completely persuaded by Mendel that the disconnected valve "fragged" that engine. The ECM can tell right away that the valve is electrically unplugged, and the fail-safe adjustments it resorts to then are precisely to protect the engine from adverse effects of not having EGR.

If you do something sneakier, and sabotage the EGR flow without making the valve look unplugged, then you'll just have no EGR flow and no protective measures.

If you block the flow completely enough, the ECM will eventually catch on anyway, when it sees the manifold pressure not rising when it does its routine add-EGR-while-decelerating test. Once it catches on and reports P0401, it will be applying the same protective measures from then on. But it only does that test on some kind of schedule and only when you're decelerating under certain conditions, and it has to see the failure in two separate trips, so that might not happen for a while. (Then if somebody clears the code, it could be a while again before it gets re-detected.)