Wife has a 2012 Prius and was starting out on a trip this morning. Made it 10 miles and car started to shudder all the sudden so she pulled over. Codes are showing misfire on all cylinders randomly. Starts fine will idol but with constant misfires and shudders harder if you hit the gas. No coolant loss. Head gasket replaced 20k ago as well as EGR cleaned, new pvc valve, new plugs etc. the only thing I can think of is I removed the very back heat shield that’s underneath the car last night as it was coming loose but looking under the car I don’t see any dripping fuel or anything damaged wires etc. any thoughts on what it might be?
Check for vacuum leaks if a hose is off or cracked? Those symptoms may resemble those of a malfunctioning fuel vapor-recovery valve.
This may be a little off topic, but if the misfire was just on one cylinder is it possible to disable just that cylinder? If the misfire was due to something that would normally require a replacement engine, maybe this could be an emergency fix. When I was younger I once owned an '83 Tercel that was running on 3 out of 4 cylinders, and I drove it like that for about a year. I never worked on my cars in those days so I don't know why that one cylinder wasn't working.
Was it something I said? Or that I sort of hijacked the thread? I have a 2010 that could really benefit from the answer to this question!
Sometimes stuff just gets lost in the shuffle. If you want to disable a cylinder, disconnect the electrical connector on its associated fuel injector.
You need to give more information. How many miles on the engine? Which cylinder? When did it start? How old are the spark plugs? If you replaced them, did you use cheap chinese copies from ebay or amazon or did you use quality ones? Why would you think you would need to replace the engine just because of a misfire? Is the coolant level down? If so, it's likely you have a failed head gasket. If not, it could be just the spark plug, coil, and/or injector. You should start with with a code reader to tell you which one is misfiring. If the #1 and/or 2 cylinders, maybe a head gasket. Switch the coils of the known bad cylinder with a known good one. Clear the code. If the misfire doesn't move, switch the spark plug, and switch back the coils. If the misfire changes, you could suspect the plug. While the plugs are out, shine a light inside and see if the top of the piston(s) look clean from the coolant cleaning them off. Only running on 3 of 4 cylinders certainly will likely cause engine damage. You could also have gotten bad fuel...
You guys are the best. Thanks for the replies. I'll try to make a long, boring story a lot shorter. February 2025 I bought a 2010 with 177k. More rust than I have ever seen on a Gen 3, I sort of knew it was going to be a nightmare, but the price was right. Right after I got it I did the EGR cooler cleaning. But it was too late, the tiny (at first suspected, then confirmed) head gasket leak soon got to be not so tiny. I watched a few HG videos, but I know my limits. I also know most people hate HG stop leak products. I put too much of two different ones in, and sure enough it clogged the radiator and heater core. I replaced those and got about 65k and almost a year of no coolant leaking. A few months ago, the coolant level in the reservoir began going down slowly. Then quickly. So I got some more stop leak, but this time I didn't put too much in. And I also bypassed the heater core just in case. The stop leak did help a little, but now the engine shakes at certain throttle positions (which vary with speed). The misfires are sometimes severe enough that the MIL flashes. Techstream just shows a misfire on cylinder 1 (Yes, Mendel, as you suspected.) No other DTCs. As I said, I used to have an '83 Tercel running on 3 out of 4 cylinders which I drove for about a year with no apparent issue other than reduced power. I'm waiting for my 2013 V which is still in the shop after a replacement Gen 4 engine and a replacement tranny. As soon as I get the V back I can put the 2010 out to pasture (it's at 255k now, 78k in 15 months). Until then it would be great to drive the 2010 without the engine shaking like it wants to break free of its mounts. And this won't lead to significant issues in the short-term? Also, thanks ASR for the suggestion to look at the plugs and coils.
You've ruined the engine, and coolant system. The fake stop leak is horrible and ruins everything it touches. There is no substitution for fixing something correctly.
Sorry been busy at work, a small update though. Still have not been able to pull the car in the garage yet (finishing a head gasket on another prius i bought) but heres what i can tell you. Car has 215k or so? Codes are p0302 cyl 2 misfire, P0300, random multiple misfire, and p304 cyl 4 misfire, sorry i thought it was all cylinders but appears to only be two. Wife had already done about 200+miles on this tank of gas so im guessing its not bad gas. Always use tier one gas and change up between adding seafoam, techron to the tank so dont think its injectors. I blocked off the EGR to see if it could be stuck open but no luck, misfires continued. I pulled the air box just to check my pcv hose and any vacuum hoses on the front of the engine and I dont see any leaks or hoses off. Have not pulled the cowl yet to remove the plugs but all the wiring to the coils look fine. Coils are original to the car. Head gasket was done by me aprox 20k ago. No coolant loss and no rattle on startup so i have little concern about the head gasket, i just finished my 7th with 5 of these cars still in our family. I purchase everything such as plugs though toyota. Water pump, pcv, and plugs were all replaced 20k ago. EGR cooler and Egr were cleaned 20k ago, and again egr valve does not appear to be stuck open. Misfires started all the sudden going down the road so I cant image it would be two coils or plugs going bad at the same time either but do plan to remove the plugs and swap coils to rule it out. Until I get the cowl off cant see much else on the back of the motor but cant think of many vacuum lines back there. Still sort of puzzled. Any thoughts on what i could monitor in techstream or on scan tool that might shed some light?
Why couldn't you have more than 2 spark plug and/or coil go bad at the same time? It's happened. Coils, plugs, that's what is misfiring... "if" neither are bad, then wiring. Check for wet plugs, and clean piston tops.....
