trouble shooting P0300 errors

trouble shooting P0300 errors

Postby dchan51360 [OP] » November 4th 2016, 6:11am


In the process of trying to work out several errors I started to really dig into the dreaded P030x errors.

I thought it might help some people to understand a little more about these dreaded errors.

While our first inclination is to start throwing parts at the problem A lot can be said for knowing how the computer determines this error.

We often think the computer has sensed a misfire because of feed back from the plug or some other magical sensor it really doesn't happen that way.

There are RPM and speed sensors that tell position of the crank and cam at a particular time. When the computer senses an incorrect amount of time from one sensor to the next, the computer presumes that there was a slight slow down of the rotational speed of the engine. This could be caused by someone letting off the gas or by a misfire. The computer then tries to determine if the slow down or variation of timing happens right after a spark plug is supposed to fire. If the firing of a spark is immediately followed by a slight pause or slow down in rotational speed (didn't see the next sensor in the expected time) the computer calculates which plug might have misfired. It then makes adjustments and sees if it is fixed if not, it will continue to adjust mixture, spark, gas etc, to try to resolve. When it can't resolve the slow down/misfire, then it calculates which plug might be the problem and then throws up the error we see.. There is a formula to how many misses it can see before it posts the error. Random misfires or P0300's can often be one or two cylinders and if you are going real fast, or speeding up and slowing down a lot, it's possible the computer is getting confused and posting the wrong cylinders as misfiring.

Knowing this, you might be able to understand why a P030x is often not the only symptom. You need to dig deeper. If you continue to run the vehicle, and keep reseting the P0305 error you may never get to the meat of the problem where if you left it for a little longer you might find that a P0305 is accompanied by a p0306 and maybe an O2 sensor error. Where the problem could be a lazy or bad O2 sensor causing the computer to compensate by leaning out or dumping more fuel causing the misfire. If you keep chasing the error by throwing ignition parts at the problem, then resetting the computer, you may never get to the O2 sensor until your cat is damaged.

Looking at all the symptoms and condition of the parts is a good start. Check for arcing (watching the ignition loom at night) Looking at the condition of your plugs (look at if running rich or lean, oil soaked, etc). Do some plug wire checks (try cranking the engine with a plug out to see if there is a clean arc across the plug) etc.. Check the wear on your distributor, Cap/Rotor. Look for arc's on your coil, etc..

Spend a little money and get a good scan tool or get some time on your local mechanic's machine to see what the O2 sensors and MAF/MAP sensor are reading.

By doing all this you may avoid replacing lots of parts that don't need replacing.

Of course if you know you are in need of a good tuneup do that first. That's not throwing good parts at bad, That's just a good practice.

Just throwing parts at a P030x error in hopes of finding the correct piece of the engine system might get replaced is just a waste of time and money. It's also very frustrating.

There will always be that one problem that was fixed by luckily replacing the actual failing part but it's not the same for every person. Doing a good job of troubleshooting will save you many headaches and you will be rewarded.
Original Poster [OP]
Firing on 7 Cylinders (L1)
Firing on 7 Cylinders (L1)
Years of Membership: dchan51360 has been a member for 4 full yearsdchan51360 has been a member for 4 full yearsdchan51360 has been a member for 4 full yearsdchan51360 has been a member for 4 full years
Posts: 750
Topics: 6
Joined: August 2016
Location: San Francisco, CA
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Van Model Year: 1999
Van Make/Model: GMC Safari
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