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Doug, ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 6945
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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I have a 2001 Mitsubishi Montero Limited 3.5l with a P0300

Customer Question

I have a 2001 Mitsubishi Montero Limited 3.5l with a P0300 code.
New: Fuel pump, crank sensor, Iridium spark plugs (NGK), spark plug wires (NAPA), fuel pressure regulator. I performed smoke test with a cigar (Nasty) and smoke came out of the air filter box.
Submitted: 1 year ago.
Category: Mitsubishi
Expert:  camerontech replied 1 year ago.

hi there,

sorry for the late reply. having some internet issues tonight..

I might need a little more info as this can be a number of things.

1. how is the vehicle running

2. if you clear the code, how long before it comes back on

3. what are long and short term fuel trims

4. are the o2 sensors giving a nice switching signal.

5. can you access freeze frame data to pinpoint what the computer was seeing when it activated the code



Customer: replied 1 year ago.
Hi, I'm Gabe.
The idle is a little rough, not too rough but noticeable, and the P0300 pops up at 60-65 mph feeling a stumble but it smooths out when I decelerate and accelerate.
I'll hook up my scanner tomorrow and tell you the live data reading if that's cool.
Expert:  camerontech replied 1 year ago.

no problem,

if you can get freeze frame data on global OBD2, that can be a big help to point in the right direction.

we have to figure out if this is a lean mixture misfire, this will be noticed in the o2 sensor activity and lt and st fuel trims, or a ignition misfire.

im at work tommorow, but will check messages and i will reply before the end of the day.

Customer: replied 1 year ago.
I just hooked up my scanner.
SHRTFT2=-1.6 TO -2.3
O2B1S1=.100 TO .860 (FLUCTUATING)
O2B1S2=.080 TO .160 (FLUCTUATING)
O2B2S1=.100 TO .860 (FLUCTUATING)
O2B2S2=.060 TO .120 (FLUCTUATING)
Customer: replied 1 year ago.
Hey Cam, any idea on these readings?
Expert:  camerontech replied 1 year ago.

long term is definatly lean. ST im suspecting would be too if you looked at freeze frame data.

right now im suspecting a vacuum leak or mass airflow sensor

Expert:  camerontech replied 1 year ago.

can you possibly pull up the freeze frame data?

Customer: replied 1 year ago.
I already checked for vacuum leaks and the smoke came out of the air box.
I'm suspecting the maf
Expert:  camerontech replied 1 year ago.

run the engine, and poor water over intake maniflold while monitoring fuel trim and 02 activity.

smoke test is very low pressure.

while an idle engine is is 2o inches of vacuum minimum, and if gthere is a leak, you will usually see engine rpm drop and rich when water test..

Also.... inspect and clean the throttle body too

Expert:  camerontech replied 1 year ago.

ive seen intake leaks at the lower intake, ive used intake/carrb cleaner for checking leaks here

Customer: replied 1 year ago.
OK, do that. I was thinking about taking a trip to the salvage yard and pick a maf
Expert:  camerontech replied 1 year ago.

let me know


Customer: replied 1 year ago.
I used carb cleaner to check for leaks too and no leak was apparent
Expert:  camerontech replied 1 year ago.

maf could definatly cause this, so if you can find a good used one cheap to try , could be worth the trip.

i would drive the vehicle there if possible and record the same data and try and take a record snapshot when the stumble happens.

and record the live data at the speed it acts up

Customer: replied 1 year ago.
OK, I'll do that
Customer: replied 1 year ago.
OK, I bought and installed another maf and no change, it felt a little rough and the stumble came back at 65mph.
I recorded the data on my scanner but I don't know how to upload it.
Expert:  camerontech replied 1 year ago.

if it stored a code, there will be a saved data stream in the freeze frame menu. this is the info needed.

Customer: replied 1 year ago.
Freeze frame:
Customer: replied 1 year ago.
These were recorded while driving and with the misfire taking effect at 63mph
Expert:  camerontech replied 1 year ago.

what were the upstream o2's when it was stumbling and on the freeze frame.

sorry for the questions , but im trying to determine if its a secondary ignition problem or lean condition.

from looking at those numbers, it looks more secondary than lean issue.

o2 readings from freeze frame and the ones you recorded should help


Expert:  camerontech replied 1 year ago.


if you have a missfire counter on your scan tool, monitor that while the misfire is happening and see if its an entire bank or 2 mating cylinders that would lead to a defective coil pack.

one misfiring coil pack can cause a 0300 misfire code. this is what im beginning to think too.

unplug the coils and check the connector for terminal issues and contacts.

along the the o2 sensor data requested, see what the injector pulse width for both banks are

Customer: replied 1 year ago.
B1S1 Freezing at .020 when misfire is in affect. And starts working once I let go of throttle and accelerate.
Expert:  camerontech replied 1 year ago.

sorry, what do you mean by let go of throttle and accellerate? do you mean let off the throttle for a second and than hit the throttle again?

Customer: replied 1 year ago.
Expert:  camerontech replied 1 year ago.

do you have a misfire counter screen on your scanner?

this can pinpoint which cylinders are misfiring. mainly to see if its a whole side of the engine thats misfiring or just a couple of mating cylinders.

The o2 sensor should go full rich when accelerating.

Customer: replied 1 year ago.
Sorry but my scanner doesn't have the misfire feature.
Expert:  camerontech replied 1 year ago.


p0300 misfires can be a little tricky to pinpoint as its a random misfire the computer is noticing.

this is where a bit of trial and error and process of elimination when you are lacking a few peices of equipment. (which is fine, as you cant own every peice of equipment out there).

you are on the right track for testing for intake leaks, mass air flow sensor and plugs and wires. thats the first place i would have hit first too.


at this point i would be watching the missfire counters to pinpoint excactly which cylinders are acting up.

since we cant do this , we can only recheck and try a few things without having to pass the job off to someone with the equipment needed.

1. recheck for vacuum leaks.

do this by watching o2 sensors at idle and spraying cleaner while watching. if it goes rich, than you have a vacuum leak.

vacuum leaks are not always detectable by listening and rpm increase. the most accurate way is to watch o2 activity while spraying.

2. faulty o2 sensor

swap the front 02 sensors left to right and see if the .020 freeze changes to the other side.

if it does than the o2 is lazy and will need to be replaced.

3. check coils and make sure they can all jump a 1/2 inch gap with a nice blue spark. if any of them dont than replace them.

4. if 1 entire bank is missing, check for timing marks on that side and see if the timing belt jumped.

5. swap injectors from 1 side to the other if possible and see if the 02 freeze changes side. if it does than there is a bad injector.

6.clean and recheck throttle body and make sure there are no cracks in the air intake snorkel tube.

7. compression test

Customer: replied 1 year ago.
I'll work on it throughout the week and post updates if that's ok
Expert:  camerontech replied 1 year ago.

of course, ill do my best to help you out


Customer: replied 1 year ago.
Update: I got a P0421 now with P0300, could be a bad cat (bank 1)?
Expert:  camerontech replied 1 year ago.

its a cat efficiency code.

this will only cause a misfire if the cat is plugged.

if you have a back pressure sensor, pull the front o2 sensors out and rev the engine to 2000 rom. there should be no more that 1.5 psi before the cat.

monitor the o2 sensors at 2000 rpm too.

the front sensors (pre cat) should fluctuate from lean to rich (.1v to .8v) reasonably quick, while the rear o2 sensors should be a steady constant voltage.

i noticed the previous post whith your os reading and that the o2 sensors looked ok, this was at idle.

it will have to be checked at 2000 rpm too. the rear o2 sensors should not copy the front.

the misfire is at a higher rpm so testing at idle will probably not help.

monitor all 4 o2s at a high rpm and see which banks are acting up, as well as the injector p/w, both banks should be equal m/s

Customer: replied 1 year ago.
2,500 RPM
B1S1=.120 TO .860
B1S2=.080 TO .860
B2S1=.100 TO .820
B2S2=.820 TO .900