P0300

GOT_NITRO? · October 19, 2004

Here is what is going on. Engine misses when starting out and under light throttle. Seems to disappear when passing on the highway during acceleration. Bad gas? I'm in the process of running the tank down to empty. I used some dry gas, no help. Stabil, no help. Eventually throws a P0300 (multiple / random cylinder misfire). Whatever it is, it's affecting all of the cylinders or at least several. Any ideas? I would like to check the fuel pump and regulator. I have a new fuel filter to install (that is where I am going to start). The truck ran fine then all of a sudden the next morning it ran like s**t. I did get about 7 gallons of fuel the previous day but it ran fine the rest of that day and evening (45 miles or so). What should I check for???? Also, about 2 months ago I changed my intake gaskets and flushed and filled the cooling system. Since then I have this distinct hissing sound coming from under the center of the dash or so it seems. I cannot find any vac. lines unplugged. I thought it was water noise from the heater core but I'm not sure. Seems to vary directly with engine rpm. The truck has been like this for the past 2 months and has run fine. Could this noise be related to the engine misfire that is now occurring eventhough there was no misfire previously????????

GOT_NITRO? · October 19, 2004

Has anyone out there ever cured a P0300 code. Come on GM people, help me out. What was done that finally got the code and misfire to quit? Someone out there has had to of dealt with this code and eventually fixed the misfire.

Peter_C · October 20, 2004

Uhhh..I got that your have a truck but that is about it??? Maybe my newbie butt doesn't know where to look. 1996 on...The code is thrown when the sensors see a change in RPM over a long period of time. Did you reset your cam angle sensor after removing the distributer?

GOT_NITRO? · October 20, 2004

Uhhh..I got that your have a truck but that is about it??? Maybe my newbie butt doesn't know where to look. 1996 on...The code is thrown when the sensors see a change in RPM over a long period of time. Did you reset your cam angle sensor after removing the distributer?

<{POST_SNAPBACK}>

1997 GMC YUKON. 350 VORTEC. AFTER CHANGING THE INTAKE GASKETS THE TRUCK RAN GREAT. I DROVE IT OVER 2500 MILES SINCE THEN. IT ONLY THROWS A P0300. I DID HAVE TO CHANGE THE CRANK SENSOR ABOUT A MONTH AGO BECAUSE IT WAS NOT RETURNING ENOUGH VOLTAGE BUT IT RAN FINE AFTER THAT. THIS MISFIRE JUST CAME ABOUT ONE DAY. ALSO I HAVE DETERMINED THE HISSING NOISE UNDER THE DASH IS COMING FROM THE HEATER CORE. SOMETIMES WHEN I BACK UP AND STOP THE NOISE DISAPPEARS BUT THE SECOND THE TRUCK MOVES IT COMES BACK. CONSTANT SOUND AT IDLE AND CRUSING. SOUNDS LIKE WATER TRYING TO GET THROUGH A CONSTRICTED SPACE. I DID THROW IN A FEW OF THOSE SEALING TABS AFTER CHANGING INTAKE GASKETS. MAYBE THEY ARE THE CULPRIT???

Peter_C · October 20, 2004

The crank and cam sensor are what determine the misfire data. Did you reset the cam angle sensor yet? The hissing is most likely a vacuum leak. Disconnect the vacuum hose leading into the cab and see if it stops.

GOT_NITRO? · October 20, 2004

The crank and cam sensor are what determine the misfire data. Did you reset the cam angle sensor yet? The hissing is most likely a vacuum leak. Disconnect the vacuum hose leading into the cab and see if it stops.

<{POST_SNAPBACK}>

I JUST MARKED THE DISTRIBUTOR AND PUT IT BACK WHERE IT WAS. THE MANUAL SAID THERE IS NO ADJUSTMENT NEEDED AS LONG AS THE DIST. WAS MARKED AND REPLACED IN THE SAME POSITION. THE CAM SENSOR HAS TO BE RIGHT SINCE IT RAN GREAT WHEN I WAS DONE WITH THE JOB. HOW DO YOU SET THE CAM ANGLE SENSOR ANYWAY?????

GOT_NITRO? · October 21, 2004

MAN, I THOUGHT I HAD IT.......RAN THE TANK NEAR EMPTY......PUT IT SOME STABIL AND 3 GALLONS OF AMOCO PREMIUN.....RAN GREAT ALL THE WAY HOME(25 MILES).......RAN GREAT THAT NIGHT TOO SO I FILLED UP WITH ABOUT 11 GALLONS A PREMIUM FUEL AND SOME STABIL ADDITIVE.......RAN GREAT. THIS MORNING...........THE MISFIRE IS BACK!!!!!!!!!!!!!! CAME HOME.......CHANGED FUEL FILTER..........DROVE 25 MILES TO WORK.....RAN GREAT!!!!!!!!! WTF????????

LikeaRock14 · October 22, 2004

WELL...IDEA....DID YOU YELL AT IT...?????

LOL sorry, couldn't resists...

As far as the fuel filter, the filter might have never been replaced and been so clogged that there wasn't enough fuel entering each cylinder to ignite, causing a misfire

PunchT37 · October 25, 2004

You need to start by first checking fuel pressure. Then do a injector balance test.

This will be hard to do without a scanner of some sort. Gm went to a different

injector set on these engines as the origional poppet style injectors suck. Don`t

forget the easy stuff like spark plugs,wires,vacumn leaks etc.

GOT_NITRO? · October 26, 2004

I'M READY TO YANK THE WHOLE F**KIN SYSTEM AND INSTALL THE TUNEPORT SYSTEM I HAVE IN THE OLD SPARE PARTS BOX. OR MAYBE AN EDELBROCK INJECTION SYSTEM. HEY, HOW ABOUT A HOLLEY 750???? I NEED THE SNAP ON SCAN TOOL BUT JUST DON'T HAVE ONE.

budhayes3 · October 26, 2004

Get back to basics and check the plugs and wires. Put a vacuum guage on it and make sure that it's nice and steady at idle with between 17-22 hg, definitely check fuel pressure with specs...I'm assuming you have some sort of manual to reference. Retrace your steps with the intake repair, check for vacuum leaks and/or broken electrical connectors. Since the crank sensor was replaced, check the teeth on the ring that it references and make sure it's clean and not chipped.

Hopefully some of these tests can get you on the right track....although I do like the Tuned Port idea.

GOT_NITRO? · October 28, 2004

PLUGS ARE CLEAN, FUEL PRESSURE IS 60 - 62 BEFORE START UP AND 58 - 60 WHILE RUNNING. FUEL PRESSURE GOES UP TO 64 OR SO WHEN THE ENGINE IS REVVED UP. SYSTEM HOLDS FUEL PRESSUE. TPS VOLTAGE IS GOOD. THE MISS IN MORE CONSISTANT AT LOWER RPM CRUSING UNDER 30 MPH........LESS NOTICIBLE ON THE HIGHWAY AT SAME RPM (1500 - 2000).

millerwd · October 28, 2004

Have you tried NEW spark plugs? If you're on a budget buy 4. Install on one side. If it doesn't help put the 4 new in the other side.. Sounds more like plug wires though. As they can be a beeeatch because moisture/realtive humidity can effect the problem.

I understand your frustration with how intermittent this problem is.

Good luck.

GOT_NITRO? · October 28, 2004

THE PLUG WIRES AND PLUGS WERE CHANGED IN EARLY JUNE. WIRES ARE THE "BEST" NAPA HAD TO OFFER (7 MM) AND THE PLUGS ARE DELCO RAPID FIRE #12's. CAP AND ROTOR WAS CHANGED IN EARLY JULY. SHOULD I BUY NEW PLUGS, WIRES, CAP AND ROTOR AGAIN JUST TO RULE THEM OUT?

budhayes3 · October 28, 2004

Hmm, that's alot of cash to blow...did you save the old stuff? Maybe you can swap em back in to rule them out. Kinda sounds like wires...or a possible vacuum leak. I'd think the next step would be to check manifold vacuum at idle and while cracking the throttle, revving, etc.

You can test the wires with on ohm meter, and visually inspect cap, rotor, etc. Could also be a hairline crack in the porcelain of one of the plugs.