2000 GMC Sierra P0303 trouble code

check the intake manifold for vacuum leaks,this will cause a missfire and unstable idle.before you put more parts on,spray it with carb cleaner and see if the idle picks up or become smooth if it does the intake gaskets are leaking.

OK, sprayed the intake and all over upper end to look for obious vacuum leaks. Used ether spray. That will usually pick it up really fast. Carefull using this stuff really flammable. No luck. Did change number 1 and number 3 coils to see if the misfire will move. Must drive today to see.

No change in moving coil. Misses unsteadily at idle. Not all the time. Sometimes it idles perfect, sometimes is misses intermittently at a stop light and when I first crank it up. Runs fine when I take off and get going cant feel any miss and does not cut out. ???

which cylinder is it missing on?is it one cylinder or all?also check the ground wire on the back of the cylinder head,starting to see trouble in that area on these vehicles,see if the ground is corroded or rusty.

OK, I think it is #3. That was my assumption. When I did the coil swap from #1 to #3 the interriment miss seemed identical and this didn't make any difference. I will check the ground strap. You may have hit on something. Recently the oil sending unit began leaking and I had to replace that. I had to go over the back of the engine to replace (you know where they are) and that or me messing in that area maybe caused the ground to become unstable.

Its missing on maybe 1 cylinder or possibly 2 sometimes. It is not all. When I take off seems to run good. Got a fuel pressure tester and checked the pressure. Holds steady about 48psi when running. When I rap the engine goes up to about 5 or 6 psi. Did the disconnect on the check valve and it went up as well. My gas gage in the cabin is starting to jump around a little and I am beginning to think that I have a fuel pump trying to die on me. Any other suggestions.? Miss seems to be persistent and not getting any worse.

Posted reply on August 22. Any feedback would help. I made a small donation as well.

sorry,missed your reply been busy.fuel pressure should be 55-62psi, 48 is to low,is just enough to make it run.check the fuel filter for a restriction.post back with what you find.

OK great call. Changed the fuel filter (again, 4th one) and she started to idle better. Had black particles come out of the filter. The miss is sometimes still there but not as pronounced. Put the fuel pressure gage back on and this is what I get. Key on engine not running 50 psi, engine running at idle 48 psi, check valve line off engine running 58 psi, rap engine check valve connected 54 psi then back down to 48 psi. Its a puzzle to me.

fuel pressure should be 55-62psi at idle,was the fuel filter plugged?if you have black particles could be possible fuel contamination.fuel pressure is still to low at idle.also with the fuel guage on it,turn the key on let the fuel pressure build,then shut the key off and see how long it takes for the pressure to drop,shouldnt drop any more than 5psi every 20 minutes.if it dops fast could have an injector leaking,but I would go after the fuel pump.

Did have engine on and shut off and watched pressure drop. Wasn't much and took a while. I am beginning to think the old fuel pump is about to go. At 133000 miles might be time for a change. Everyone that I talk to recommends to buy the GM pump because they say Airtex pumps don't last. Any suggestions? I really appreciate the feedback that you guys have given me so far. Its been a big help and its good to talk to someone who knows about automobiles...

I personally would go with the OEM pump,because it has a better warrenty and if it does fail its covered at any GM dealer,with the aftermarket its only warrantyed at those chains and aftermarket electrical parts are low quality and just pure junk.this is what I do dayin and day out at a GM dealer.

I finally got it fixed ! The fuel pump was not the issue. What I did: Removed the throttle body (TB)from the engine (not simple). I removed the Idle Air Control (IAC) from the throttle body. I cleaned the back side of the throttle body and the "seat" where the IAC plunger seats on the TB with brake cleaner. The seat and back side of the TB were covered with black soot, and it was heavy coverage...I then put it all back together...High idle at first, then adjusted the TB cable setting and engine idle settled down. I took it out for a drive and the intermittent miss was gone! I always cleaned the front of the TB but never thought about the back side and the IAC being the problem, but looks like it was. Have had no codes since (approx 1000 miles). Throttle response is better now as well. The engine reacts now as if it were new again ! Hope this helps someone....