so it was a upper intake leak? i found that intake leaks, bad distibutors (teeth) or sticikng valves from dirty injectors seem very common on these engines

Intake leaks are very common on the 5.7 vortec. My dads was leaking at the lower gaskets between the intake manifold and the heads

I have a 99 5.7 with 170k miles that has a major problem with misfires when cold and under load. The DTC code is always #4 cylinder. The vehicle actually bucks when this happens and it takes a few seconds for it to calm down. I have changed everything and checked the cam retard angle only to find it at 0-degrees. This does not happen once it is up to operating temp.

po303 misfire, need help
 

Hey guys, I haven't posted anything lately but I got another Tahoe and yes I'm glutting for punishment but what can I say I love the Tahoe. I just replaced my intake gasket due to antifreeze pouring into my oil and I knew I needed to do the spider update. I was trying to hold off until after Christmas but it didn't hold up. About a week before Christmas, I was on my way home from work when I noticed that my temp hand ran up to 210 and I was thinking maybe I need to stop and check my antifreeze. I did and it just wouldn't fill up to the top of the radiator and I limp it home. It got up to 230 and wanted to cut off so it did and coasted home to my driveway. After it cooled down I had to get it up my driveway I added water to it. I then pulled the oil dip stick and OMG milk up to the top of the stick. Well, I have another tahoe that I had put the new updated spider on and never got it inspected for the emission requirement for NC. So it was just sitting in my side yard. Good news it that the other Tahoe engine had started knocking the week before when I had moved it. Now it's become the parts vehicle, so there is my new spider that I had put on with all the new parts. So, I went ahead and yanked the intake off of both Tahoes and fixed it to the 99. The 99 started a few months ago with po303 and 174, 171,131,151 and the all famous po300. Well since I was doing the intake and I was watching it as I had it all back together I knew that the O2 sensors were shot. I just replaced the 2 pre converter o2 sensors I found a fuel line with a hole in it. I did smell fuel for a couple of months but I was thinking it was the spider. So, parts Tahoe got robbed again. Now I have been getting the po303 since the spider change. Tonight I ran the seafoam trick and it didn't fix my problem. I connected my timing light and noticed that number 3 was firing and then it would stop hence my misfire condition. Now I'm looking for any new ideas, I'm going to check the dist cap tomorrow which the cap came from the parts Tahoe that only ran for maybe 120 miles. I am also going to change out the control module and ignition coil. Any other ideas I might be missing. Thanks in advance. Jerry

Update
 

Hey guys, just a quick update to my issue with my 1999 Tahoe and my po303 misfire. About 2 weeks ago I took it into my local Chevy dealer and they gave me some of the worst news I've ever had with the Tahoe. They took it into the bay only to come to me that I had a burned valve on the cylinder number 3 that was the cause for po303. I wasn't going to except that for my cause for a misfire and I was running out of time due to vehicle inspection being due. I did another tune up with cap, rotor and finally wires and plugs. I kept checking with my ob2 scanner to see if my ready codes were set and ready for inspection but never ready and kept getting po303. Well, I finally checked this evening with my scanner and all my ready codes and now I can get my inspection tomorrow. I just wanted to share with you guys. All of this started after I did my spider upgrade and intake gasket. I found during the update on the spider I found a fuel line leaking, that had been leaking for a couple of months. I hope this helps....