Hi. Need help on a misfire. 2003, 136k miles, original battery. Getting a rough misfiring on a warm engine. P0304 and P0303, kinda alternates between the two, 04 recorded then 03 live, then back. New-ish (<10k, OEM) coils, plugs. Swapping doesn't show anything. Compression test fine. No signs or smells, etc of coolant leaks. Never need to add any fluids. MAF/throttle body done a couple years ago. Techron run through 6 months ago. I've had it for about 4 years. I know the original owners so it was treated well. Note, I have a P1437 from a stolen cat that's been there for ~6 months. Misfire just started ~2 weeks ago, mild shimmy ("did I just feel something") then on a road trip 30-35 miles in came on for real. Limped to a mechanic I knew, not a Prius whisperer, tho. He's stumped. Injectors? Trying a swap or test. Vacuum leaks, manifold or lines... TIA.
Do a search for "2003 Prius DTC P1437" and all kinds of help will come up. You will need to replace the cat, for starters.
You probably got an aftermarket cat after the OEM got stolen, unless you're in a CARB state, or it was long ago and insurance paid for an OEM. If recently, insurance co would have totaled the car for a stolen cat. If so, it doesn't have the VAV vacuum reservoir. I just explained to the another post here the easy fix. If yours is OEM, and you live in snowy area, that VAV is probably rusted and not operating. It throws the 1346 or 7 depending on whether if rusts open or closed. Easiest fix is to spray it down and clean it up to operate again, or just use bolt cutters to cut the rod that VAV pushes so the ECU thinks it's operating. If aftermarket, just trick it with another reservoir of the same approx volume. I would be amazed if you really have the OEM battery on a 25-year old car. 10 years is typical, and that's on cars that are used a lot so the the battery is kept charged. 136k mileage is around 400 miles a month - I just don't see it happening. But, that's OK, whatever battery you got is working. Just probably not the original. Otherwise, the codes are typical and straightforward to diagnose. Swap the plugs and coils from the 3 and 4 to 1 and 2 and see if the codes follow to identify which is the issue.
Wild guess.... did it rain just before the problem started, or did you drive through some puddles? Misfires tend to occur when there's water in the fuel supply... that's been my experience.
Yep, that's a strong indicator of the rusted fuel filler tube issue on these cars when they've lived in snowy areas for awhile. Snow off the rear passenger is thrown in the wheel well and packs on top of the fuel tube. Typical road salts eat throught the top of the fuel tube - may not be visible, but feel up there to find the rusty hole. Then, as you say, when it rains, or the snow melts, water gets in the fuel and messes with your combustion.
Yeah, I didn't notice his location. Still, these cars are old, no telling where they spent some time. But, in general, most of these were sold in CA, so makes sense that it's been there. Yep, just takes 15 seconds to reach under there to check.
Short of draining and refilling the tank, the cheapest test is ROAD TRIP! Drive until the Low Fuel (1 bar) chime sounds, then put in half a tank of PREMIUM ethanol-free fuel (it's California, so I expect >$6/gallon for premium) and then drive some more, and repeat. A repeat of the Techron treatment may be in order, as well. OP says it started 2 weeks before his post (6 May), which marks mid-April, and I seem to recall that even SoCal got some rain about that time. OP says hardware is new-ish, so it's unlikely (but not impossible) to be at fault. Depending, OP might have gotten bad fuel at a mid-April refuelling. Does OP have records of an unfamiliar fuel stop? Or, it might just be that he filled up and got crud from the bottom of the station's tank. These things happen....
