1999 C1500 Suburban 350 Vortec - 86K - Started misfiring - Code P0300
- Engine misses throughout driving speed range
It first started with code P0307 misfire and went away. Next time
engine started code P0301 misfire and went away. The next time the
engine was started code P0300 multiple engine misfire and is always
there now. Checked plugs ( AC 41-932 ) the gap was about .056 . We
have only put 3K on the car and have had it for about 2yrs. The car
always ran supper. I have not done anything to the car except change
the oil. I have and AUTOXRAY EZ-SCAN 5000 that I read the codes with.
The gap is not necessarily the issue with a plug misfire, if they are
original at 86k I would consider plugs and wires here, other issues can
cause this such as dirty injectors but at 86k the tune up is the first place
Shep's advice is good, Warren. Just FYI, I just bought a '97 Tahoe with
350 vortec. It had a slight miss at all speed ranges. Replaced plugs
only--they really looked too good to discard, IF their replacement did not
fix the miss. It continued to miss, still just slightly at all speeds as
before. I replaced the fuel filter and that made it run like new. Has
99000 miles. Those are nice engines. My mechanic-friend has one in a
king-cab with 180,000 miles that runs equally as well as mine. HTH, s
I do not know when the plugs - wires - filter - have last been
changed, but it sure sounds like a good place to start. I think the
fuel filter will be first. Thanks - I will keep you updated.
The reason we purchesed the car was because of the very dependable
Vortec engine. We had a 1964 Suburban and we really liked it.
The newer Vortec engines have changed. Do the new Vortec engines have
the same reliability.
You must be kidding! The old style Vortec's were not what I'd consider
"dependable". The new style GEN III chev small block (new Vortec) engines
are far more reliable then the old one's.
If you change everything and still have problems, you may want to check
the intake manifold gasket.....I've seen and repaired many of these for
warpage, causing a vacuum leak.
Changed the fuel filter and it still ran bad. Started checking for
loose connection, had my hand around in back of the throttle body and
suddenly the engine started running better. Removed the air tubes and
found that the electrical connector to ( I think it is for the
injectors ) but not 100% sure, was loose. The 2 seals were jamed all
the way into the cover so that it would not clip together. Cleaned
both seals and positioned them in the right places, clipped it back
together and it runs great.
The fuel filter really needed to be replaced, I could barely blow air
Thank you for the help.
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