2010 Prius random misfire

Hello, my girlfriend's 2010 Prius (116k miles) is running very rough. I know this is a long post, but any help is appreciated.

It initially was throwing a misfire in cylinder 4 and an EGR fault (unfortunately I did not note down the exact code). So I swapped the ignition coil and spark plug with cylinder 1 and the issue did not follow. I then concluded it was the EGR system. So I replaced the EGR valve with a new one, and cleaned out the manifold, connecting pipe, and the EGR cooler. After this, the car was now throwing codes for Cylinders 1 & 3 and "Open EGR Circuit". So I checked the resistance of the wiring from the ECM to the EGR and all were in spec. I did the same for voltage and the same story. The CEL would return every time I reset and the car would run rough so I decided to try the very unlikely chance it was the ECM. Got a used one off eBay and installed it in the car. The EGR code did not return, but now it is throwing the random cylinder misfire P0300 and cylinders 1 & 2, but it changes randomly.

I have checked the insides of the Cylinders with a borescope and from what I could tell they looked fine. The car is not burning oil or coolant, but I checked the coolant for exhaust gases using an AutoZone tester and the color did not change from dark blue. So I don't believe it is a blown head gasket. I blocked the downstream side of the EGR valve as the Car Care Cut mentions in his video and the car is still running rough for the most part (at low speeds it will run fine for short bursts). There is a slight rotten egg smell to the car after running it, which makes me believe it could be the catalytic converter, but I thought that it would throw a specific code? Any help would be greatly appreciated as I am kind of at a loss.

 

Yes, the plugs looked good, but I did also replace them just to be sure.

 

I had cleaned it out at the start, but I did not disconnect the battery at all. I will give that a try though!

 

Cleaned the MAF and still got the same issue.

 

Sorry for the delay, but I believe I have finally figured out what was wrong. I was not getting a code for the catalytic converter, but since the car started to misfire quite suddenly I think the O2 sensors never got enough data to throw a code. To test this theory, I drove the car around my block so that it would heat up and took the temperature of the exhaust manifold outlet and the outlet of the catalytic converter. The upstream temperature was just below 600°F while the outlet of the Cat was ~100°F. This leads me to believe that the Cat is all but completely clogged. This makes sense as the car was slightly curbed right before it began to misfire.

I will be ordering a cheap Ebay Cat since I am in Tennesse and I plan on selling the car soon anyways. Unless anyone has a reason why I should opt for a more expensive Cat? Any tips are appreciated.

 

If clogged enough, would it not cause back pressure in the cylinders? Which would make it run ultra-lean causing a misfire?

 

I will definitely pull the cat before ordering a new one. Unfortunately, I will not be able to do this until this weekend as I will be traveling.

The intake manifold I have taken off and cleaned twice now. The first time it was rather dirty, but not fully blocked for any of the ports. After cleaning it there was no build-up at all even after some test drives.

For the Coil packs, I bought a new one as well as a new spark plug and swapped cylinders which did not change anything. I was hoping the issue would follow, but it threw the same codes.

I do not have Tech Stream, but am trying to get it. Even the lite version seems a bit pricy, do you know if any of the free versions work? I am quite skeptical of them.

 

Thanks for the info! I didn't even see the daily activation fees. I am just DIYer and highly doubt I would use it again so the two-day key would be perfect.

Right now my plan is to remove the CAT and confirm that it is indeed blocked. I will likely start the car and see if performance improves with the exhaust system removed. If it does not then I will reinstall the exhaust system and use TechStream to see what's going on. I would do this before but am quite confident the CAT is clogged as mentioned above.

 

Oh, I am sure I actually would use it again quite quickly, but right now the plan is to sell the car in less than 4 months lol.