Misfiring problem


#1

i got a 1997 sonoma which has a p0300 code. i changed the plugs and wires, cap and rotor and the pcv value, and it still misses. what could this be? please help.


#2

The problem might be with a dirty/defective Mass Air Flow sensor. This can cause a multiple cylinder misfire code.

You might first try obtaining a can of aerosol MAF sensor cleaner and give the MAF sensor a good cleaning. If that shows no improvement then what should be done is connect a scan tool that can do real time monitoring. Take the vehicle for a drive, and then monitor the relationship of throttle position to the amount of air the MAF sensor is detecting. If at 100% open throttle the MAF sensor detects only 85% mass air flow there’s a problem with the MAF sensor.

Tester


#3

could i use an air blower on that mass air flow sensor


#4

if it turns out not to be that mass air flow sensor, what else could cause that?


#5

“could i use an air blower on that mass air flow sensor”

No that would damage it. There is a cleaner made special for it.


#6

could a thermostat cause a misfire?


#7

Cleaning the MAF sensor may very well clear the trouble. Check for any vacuum leaks passed the MAF sensor also. I don’t think the thermostat will have much influence on this trouble. A faulty cam sensor or knock sensor could also cause the trouble I think.


#8

how do i clean that sensor properly


#9

Some auto parts stores will actually carry a specific air flow cleaner. I have successfullly used a more general “electric motor and parts cleaner” as well.


#10

ok thanks but will that turn the p0300 code off


#11

After whatever is causing the trouble is fixed it may take a number of drive cycles for the CEL light to go out. Some codes clear as soon as the trouble is fixed and some require a number of drive cycles before they go away. Clearing the codes will make it go away immediately.


#12

I have not seen a thermostat cause a misfire, but I don’t know that it couldn’t. I just don’t see how.

The P0300 is a misfire that is random, the engine can’t figure out which cylinder is misfiring. If one cylinder had a steady misfire, then you would get a P030x with x telling you which cylinder.

Have you modified the air cleaner assembly by putting on a cold air intake or use a K&N air filter? This can be the source of your P0300 and MAF sensor problems. If you did, and you want to keep it, you need to keep a can of CRC Mass Air Flow Sensor Cleaner handy.


#13

no it is all factory so the possiblity would be that mass air flow sensor dirty


#14

well here is what the truck is doing you drive it it runs nice and when u stop at a stop sign it runs rough and shakes


#15

You could have a dirty maf sensor, a vacuum leak, a burned valve, a fuel problem like a dirty filter or fuel pump issue, a stuck egr valve, or a number of other things.


#16

thank for the advice i will check the fuel filter first then go from there.


#17

With a random misfire code I’d check the crankshaft position sensor.


#18

Out of curiosity, why do you ask about the thermostat? If there is reason to suspect a problem you should deal with it. If it is stuck open the car will run too cool & that means it will continually run rich. Its isn’t outside of the realm of possibility that this could cause a misfire.

In addition to cleaning the MAF & checking for vacuum / intake leaks I’d also want to put a fuel pressure gauge on it, though that is unrelated to possible concerns about the thermostat.


#19

I missed the post about running fine and shaking at idle only so retract the crank sensor comment.