CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Misfiring problem

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.

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

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

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

“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.

could a thermostat cause a misfire?

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.

how do i clean that sensor properly

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.

ok thanks but will that turn the p0300 code off

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.

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.

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

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

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.

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

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

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.

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