The Prius has intermittent hesitation/surging on low-speed acceleration. No pending or stored codes. Fully depressing the accelerator pedal results in normal acceleration and stops the problem immediately. It feels like the car has bad fuel or is missing intermittently but with no misfire codes, this seems not to be the root of the problem. I'm wondering about the throttle position sensor or associated wiring/connections or corrosion. The car has 216K miles. 20K miles ago, the cleaned the EGR Cooler and replaced the input damper plate. The associated misfire condition had broken the plate springs. I don't know whether there is more internal engine damage.
The misfire is more like to be associated with blown head gasket. Which IMO was due to EGR being carbon clogged, likely for the last 100k, but that’s maybe for another discussion. Once the head gasket starts leaking (incompressible) coolant into cylinders, the pistons struggle to get by Top Dead Centre, and that damper takes the brunt. how long have you had it, since new?
I got it at 196K or so and I've put 20K on since replacement of input damper plate and cleaning EGR Cooler. EGR Cooler was completely clogged causing P0300, P0301, and P0302 codes. I'm not sure yet that the head gasket is broken since the roughness goes away when accelerator is fully depressed.
The damper takes the brunt of any rotation variation caused by any sort of misfiring at all for any reason. The accumulation of incompressible coolant in the cylinders makes for a much more dramatic condition, the brunt of which gets taken by the connecting rods, pistons, and sides of the engine block. You mentioned cleaning the EGR cooler. Was the intake manifold taken off to have its four small EGR passages thoroughly cleaned at the same time? If those passages are clogged by different amounts, the unbalanced EGR delivery across the cylinders will be made even worse by upstream cleaning of the cooler. A situation where the rough running is in the midrange, going away at idle and going away at full load, does tend to suggest an EGR-related issue, as it matches when EGR is used: in the midrange, not at idle, and not at full load.
It’s not causing those codes, directly. The codes almost certainly are caused by coolant in cylinders one and two, due in turn to head gasket failure (at the wall between those two cylinders), due in turn to uneven heat, due in turn to the beta-version EGR system Toyota doesn’t talk much about, that ideally needs cleaning every 50k miles. seller could have added a stop-leak product to the engine coolant too.
The P030x codes are for misfires, and coolant in the cylinders can be one way of causing misfires. The other classic things that can cause misfires are improper fuel/air ratio, faulty spark, or bad compression. EGR delivered through differently-clogged manifold passages also can cause misfires. When an engine is misfiring, there are a lot of possible causes to think about. Whole articles get written about misfire diagnosis (see starting on p. 15 in that link). Sometimes there are clues that help. If there is some misfiring right at start-up, and then it clears up quickly, that might tip the balance more toward coolant dripping into a cylinder overnight while the car was off. If there is no misfiring at idle, no misfiring at full go, but misfiring in the middle, that can cast suspicion on the small EGR passages in the intake manifold, because that pattern fits so well with when EGR is and isn't in use.
I did my EGR system and intake, throttle body around 184k. EGR rad was pretty much 100% clogged any only got it about 90% unclogged. Intake ports were about 1/4 to 1/3rd the size they were supposed to be. I used to have to punch it just to accelerate normally. After doing all this, I would peel out the wheels trying to accelerate like that. Had to relearn how to drive the car. I'm not sure if it has that much power still, unfortunately.
No white smoke in exhaust but there was some hesitation on ICE start-up once today. Still no codes though as there were when I got the car and before I cleaned the EGR cooler and intake manifold. I did notice today quite a bit of exhaust pressure at the oil filler cap. Next step will be Leak Down Test to figure out whether it's piston rings/cylinder wear or head gasket (could be valves but I doubt it.) I'd probably fix a top end problem myself but most likely would get a used replacement motor if it was a block issue.
No white smoke in exhaust but there was some hesitation on ICE start-up once today. Still no codes though as there were when I got the car and before I cleaned the EGR cooler and intake manifold. I did notice today quite a bit of exhaust pressure at the oil filler cap. Next step will be Leak Down Test to figure out whether it's piston rings/cylinder wear or head gasket (could be valves but I doubt it.) I'd probably fix a top end problem myself but most likely would get a used replacement motor if it was a block issue.
I had white smoke, but my mechanic found that because I did not see it. I noticed my radiator fluid level dropped, and just filled it. I did think while my head gasket repair was being done that maybe If I had reacted quick enough to an overheat light that maybe my head gasket would not have messed up, and maybe I missed making sure that there was enough fluid in both the return bottle and the radiator HOWEVER, many mechanics that work on Prius know that head gasket failure happens alot. I wish that I had time and moxy to take apart the whole intake system like you did(there is a humerous utube video for a gen 3 take apart and overnight clean soak in brake cleaner) But my method has been intake, engine, and exhaust cleaner repeatedly to keep it at bay. I would suggest a good Bars/Leaks additive to the radiator fluid just to see if it makes a difference. I have stopped an oil leak at my oil pan gasket and oil pressure sending unit by adding a two part Bars/Leaks made to stop such leaks. 700 miles on the highway and the mobil 0W 40 oil level is holding. I still have a few misfire codes, but they are occasional, I cancel them and they sometimes come back. In general, I'm having more trouble with my hybrid battery not holding a charge for very long. It holds plenty of charge as long as the system is warm. And as long as I use it again while it's still warm after a few hours then it's still holding charges. That's normal also, and I plan to replace the battery with a nexcell LiFeP04 in a few months. Hope radiator sealers take care of your problem. If not try Dura Lube exhaust cleaner, CataClean, Bars/Leaks two part, Techron, Chemtool, and especially Regain that have a high amount of PEA. I have used them all over the last two years. HOWEVER, it does seem to have made much more difference to have put Bars/Leaks two part in my oil.....less blowby maybe?
At one point recently, I thought I might be losing coolant due to head gasket issues but the coolant was escaping past a clamp at the cylinder head near the EGR Cooler. After that was fixed, a bit more coolant was needed as the cooling system was eliminating air bubbles. Now, the coolant level is stable so if there's a head gasket issue, it's allowing exhaust gas into the crankcase. I think the excessive case pressure is more likely stuck piston rings or cylinder wear which will be indicated by a leak down test or a camera view inside the cylinders. Seafoam soak of rings I think might help but cylinder wall defects will eventually lead to an engine swap. I'm still getting like 42 mpg so it's the "ain't broke don't fix it" theory.
