How common is a head gasket failure at 155k miles that doesn't breach a coolant passage? The engine's running rough (so much so that the vehicle is undrivable) and throwing P0303 and P0304 codes. These are misfire codes for cylinders 3 and 4. There's no antifreeze in the oil, and no disappearing coolant, but the engine burns about 1 quart per 800 miles. I'm going to examine the plugs and swap fuel injectors to chase the problem. How do you do a compression check on a Gen 3?
Well... Your profile says you have a 2009 Prius which is Gen2 in US that year, not Gen3. Which one is it? Because if it's Gen 3 than you're about due for headgasket failure because of the stupidity of Gen3 Prius engine designers and the loss of coolant is symptom that takes a while to show up sometimes. But if it's really a 2009 Gen2 Prius then this is a very rare mechanical failure and I'd place my money on it being more than two quarts low on oil for too long causing the crankshaft bearing to fail, which creates engine misfire error codes. But that diagnosis is based on general experience not looking at your car specifically. After you look at the plugs for symptoms doing a compression test and leak down test would be wise... You can google how to execute a function you can put the car in called "maintenance mode" which will spin the engine for you so you can get the compression test data.
High quality dual camera borescope inspection with the cooling system under pressure. Signs are steam cleaned piston(s), possible active drips from the head gasket and sometimes a scored cylinder wall. Compression, leakdown and gas in coolant tests are not definitive in the early stages. Car Care Nut HG Borescope at 10:57 A normal piston with miles will have black carbon on the entire top.
I would swap coils, much easier. Most likely these efforts are “arranging deck chairs on the titanic” though. Ever cleaned the EGR? with your oil consumption, consider a salvage engine or new short block. Do you know it’s full oil change history (have you had it since new)?
Thanks for the replies, PriusCamper and rjparker. The vehicle in question is a Gen 3 (2010) I bought used for my grown daughter 3 years ago. Even at 120K it had an EGR cooler that was packed with carbon deposits to the point where you could not see its honeycomb construction. The motor has always burned a lot of oil. The question about coolant-free head gasket leakage arose from a conversation with someone who suggested that a dry leak between adjacent cylinders (in my case, cylinders 3 & 4) allows exchange of compressed gases between them, causing P0303 and P0304 misfires. The Gen 3 head gasket failures I've witnessed always seem to involve coolant/oil mixing. But I suppose that coolant/oil mixing is the final, easily-identified stage of head gasket failure. PriusCamper: an interesting suggestion re failed crankshaft bearing causing misfires. Can you elaborate on the connection between those maladies? Seems like something so severe would create an audible sound. Mendel: thanks for the suggestion. Yes, my brother and I have collaborated on Gen 3 engine swap-outs and we have a rebuilt Gen 3 motor queued up for this misbehaving Prius if we decide the problem is head gasket failure.
Now that you confirmed its a 2010 and not 2009 and I don't get the impression you let oil levels get that low so it's probably true misfire codes. And I think your friend is correct that it's a dry leak between cylinders because that's definitely where the headgasket failure is in gen3, but usually between cylinder 1 & 2... If you want a long term fix that will be good for hundreds of thousands of miles you'll want to find a used Gen4 engine... And there's a guy in Oklahoma who will swap the engine in your driveway in one day for $1500. Or for more short term, just replace the head gasket and then get ready for the bad piston rings to start burning oil because Toyota decided that a 1 mile boost in MPG was more important than worrying about the car burning too much oil before 200K miles.
