P0300 engine misfire problem

Been looking up what this could be but I honestly don’t know where to start. I drove my Tacoma to work yesterday morning which was a 30 minute drive and it drove perfectly fine. After work, I went to crank it and it started up idled to 800 rpms for half a second then almost instantly started dying. Went down to 200-400 rpms and was struggling to remain running. We left it there over night and went back with an engine diagnostic and it came back with two engine codes. P0171 and P0300. Ain’t really worried about the P0171 code since I’m pretty that’s to a 02 sensor that went out a while back just haven’t had the chance to fix it. The P0300 code is what is worrying me. It will idle fine though after warming up but every now and then it’ll start dying or when you take off it’ll start having a bad hesitation. I was told it could be a bad fuel filter which I’m thinking could be it as well but not for sure. I want to run by y’all cause most of y’all know the inside out of these Tacoma’s. If need info it is a 2002 Toyota Tacoma with a 3.4L v6 engine. I have a video of it idling if needed as well. Thanks

 

P0171 is "System too Lean" which could be flagged from multiple trouble spots O2 sensor being one of them the others being fuel pressure, mass air flow meter, injector blockage, or ECM. P0300 also lists these trouble spots along with EGR system. Given the heat component I would suspect the EGR valve and there are vacuum test procedures for that check or just buy a new one and put it on for $120. If you don't know the age of your fuel filter it should be replaced too. Others may may have more helpful advice.

 

my suspicion is she needs a fuel pump.

The P0300 code is a general system misfire, something that effects all cylinders as opposed to a single specific cylinder and as such the culprit is some that effects all cylinders such as a MAF or low fuel pressure. The incorrect reading from the front AF sensor AT THIS POINT IN TIME is a symptom. Check fuel pressure and flow and eliminate that. There are electrical checks that can be done on the MAF but these seem to be to determine if it is bad or not and do not check for "flakeyness" and as mentioned, they dont last for forever. Installing a new Toyota part for tests isnt cheap but it just might be a time saver.

 

Last month about 250 miles after I had replaced all of my injectors to fix another problem, I got a P0171 code. Oh gawd, I thought, another problem to fix. And probably another one after that.

After reading the suggestions to my post as to what it could be, I sprayed some Napa Quick Start all around the ends of every single vacuum hose that I could see. The engine would idle up if I sprayed in a certain area, so I narrowed it down to a hose that went to the throttle body. Put a clamp on it and retested. Problem solved!

Don't know what a can of Quick Start costs nowadays, but I understand the same thing can be done with brake cleaner. Why don't you try spraying around your engine? Would be a lot cheaper than .

 

Thanks for all the replies i went ahead ruled out all basic stuff like oil change, fuel filter and spark plugs. Basic tune up and started to idle and drive good. I went through an engine diagnostic and erased all the codes to see if they came back up. The p0300 code flashed back up but it was still driving fine so I wasn’t too worried but now two other codes popped up. P1130( manufacture control) and P0141(02 sensor heater circuit bank 1 sensor 2) any idea what that could be? Thanks yall

 

P1130 is the ECU did not receive the correct value from the front AF sensor(O2 some call it, incorrectly). And the P0141 also deals with that front AF sensor. Double check the plug and wiring and that it's intact. If that checks out you're likely looking at replacing that air/fuel sensor

 

You'd be surprised at how much info (some pretty good, some not so good) you would find by googling the codes. Just enter OBD Codes in the URL box .

 

your air fuel sensor is dying, replace it. that can cause lean condition when it's (anything) heater circuit starts to fail
(been through it before)

the misfires are something else, have that diagnosed at the cylinders. a weepy head gasket
can cause misfires and also foul air fuel sensor...so that could be single cause for both issues

I'd check for coolant in oil and oil in coolant right away (research how to do those checks properly) and compression tests


if the air fuel sensor is original and you have over 130,000 miles on it just replace that, they do not last forever and around these many miles they start to croak anyway.....and use OEM only !!! (there are two possible part numbers, the original part number has been upgraded since 2002, but use OEM...non-oem just will give you problems been there done that as well....not worth trying to save coin)

air fuel sensor is the one in front of the cats, and affects air fuel ratio, costs more
oxygen sensor is the one behind the cats, this simply sniffs for cat efficiency

very similar devices but the front is specifically called air/fuel sensor not o2 sensor

 

Alright I’m definitely gonna check out the air fuel sensor. I want to add last night I drove the truck all around town and it drove fine and started up perfectly fine, but this morning I started it up and it started idling at 200 rpms. I let it run for bout 30 seconds to see if it’d get any better but I noticed after about 5-10 seconds it would start vibrating really bad and the oil temperature light would quickly flash and then it’d idle at about 1200 rpms then slowly go back to 200. Any thoughts on that?

 

I had similar problem which within 2 weeks progressed to not going past 3K rpm with missfire and major loss of power. 200 - 500 rpm idle and when you hit the throttle it would missfire and i could hear missfire in the intake manifold. (not good).

Ended up being the fuel pump. Instead of failing instantly it was just getting weaker and weaker.

I would check fuel rail pressure

 

Ok I might just do that. Whenever it first started happening I had major blowback as well. It literally blew the top of my air box open.

 

Ight so I have replaced the air fuel sensor, spark plugs, fuel filter and the fuel pump. Nothing has changed it still has hesitation in power. The engine light came on as well and is reading P0141 ( 02 sensor heater circuit bank 1 sensor 2). Correct if I’m wrong but that’s the air fuel sensor that I just replaced isn’t it? If so, what else do y’all think I need to look into?