2011 Prius sounds horrible after cleaning EGR cooler

I recently removed the EGR cooler assembly from my 2011 to clean it. (It was 100% clogged.) I've put everything back together (except for the cowl and wipers), connected the coolant hoses, EGR valve circuit connector, and MAF circuit connector, and refilled the coolant (expecting it to need further topping off as it sucks in the new coolant and bleeds out the air).

Car starts, but sounds like total junk. Clattering and vibration everywhere. No misfire counts on the scanner, which was my first thought.

So now I'm wondering whether something happened to the synergy drive (MG1/MG2). There's a coolant splash shield over it (it looks like), so I simply used a pan under the car to catch the coolant when I pulled the hoses off the EGR cooler. But now I'm wondering if something got into it.

Any pointers on how to figure this out? The noise is awful enough that I don't want to run it very long at all.

 

I have disconnected the 12V battery overnight. Will see if starting with un-adapted engine parameters helps. If not, I'll pursue checking the EGR valve's function. I noticed the motor end of the valve was rather warm after running the engine for just 30s. Here's hoping I can work a wrench in there to get the valve off without removing the entire cooler again.

Will report back.

 

Reconnected 12V power. When the ICE started, it sounded normal for about 2-3 seconds then started the terrible noise again.

 

I'll try pulling the EGR valve later to see if it responds.

 

Yes, although it's still a bit involved. I removed the air box and front EGR tube (between the valve and intake manifold along with the wiring harness bracket attached to the EGR valve. I then removed the nuts, studs, and gasket connecting the EGR cooler to the exhaust manifold tube. I loosened the bottom nut (behind the lower coolant hose going to the EGR cooler) and then rotated the EGR assembly forward enough to get a socket on the upper rear-facing nut.

If there's a time savings, it's in not having to disconnect coolant lines (with a possible refill) and not having to replace that difficult lower nut by the coolant hose.

Conceivably, a really thin 12mm offset wrench might be able to deal with the upper EGR valve bolt without this effort, but nothing I had worked.

 

A quick summary of where I am: EGR is working much better, but there's a persistent misfire on cylinder 4 on deceleration after being at speed AND with EGR connected (plugged in).

What has been done:
1. EGR cooler cleaned and reinstalled. (It's clear end to end.)
2. EGR valve replaced. The old one (probably factory) was sticking. I considered cleaning it, but decided to replace it just to get on with it.
3. New coolant in the engine. (Why not?)

What I observe:
1. The original problem before the EGR service was that the engine would vibrate and stutter on deceleration with EGR connected. I disconnected the EGR wiring harness to get around the problem until I could clean the EGR as above.

2. NOW, after a bit of driving at speed, varying it up and down to mix acceleration and deceleration, the engine is not stuttering nearly as badly as it was. I credit this to better EGR flow and the re-learning of EGR valve positions after the variable driving test.

3. NOW, however, cylinder 4 also has a consistent, heavy misfire on deceleration, which causes the stuttering effect I observed before. It's not as bad as before the EGR service, but it's still noticeable and concerning.

• It HAPPENS when decelerating (but not coasting), with the EGR counts dropping from a high of 75 or so down to something like 25. Thus, exhaust gases are still being introduced into the intake.
• It DOES NOT HAPPEN under hard acceleration (or any acceleration, for that matter).
• It DOES NOT HAPPEN when coasting, with the engine spinning 1000-1100 rpm.
• It DOES NOT HAPPEN when I disconnect the EGR valve again. The misfire counts stay at 0 across the board in this case.

It is this last point that I think holds a clue. I'm trying to figure out what would be different in cylinder 4 (which I assume is closest to the EGR pipe going into the intake) with versus without EGR to cause a misfire. I can check ignition coils, but I'd expect an ignition coil misfire to occur regardless of EGR. The plugs are all fairly new NGKs installed about six months ago. This leads me to think it's a misfire dependent on air or fuel.

I did not clean the intake during all of this, so that's a possibility, but I'm really at a loss to explain how the intake would affect things UNLESS it's simply because cylinder 4's intake passage, being closer to the EGR pipe, has carbon buildup sufficient to mess with the air/fuel induction.

Any thoughts here?