1999 Suburban Hot Misfire
 

Ive got a '99 Suburban k1500 4x4 with the 5.7 Vortec motor in it. Ive got an issue where the truck will start, idle and run perfectly for as long as I drive it. Ive taken this truck for 2 hour+ trips now without issue. No hesitation, and no hint of a misfire. If I shut the truck off and let it sit for 15-20 minutes, it starts misfiring. It will miss as soon as I start it up, and all through any driving I do for a while. Eventually (after 30+ minutes of driving), it will stop and run fine again. It seems to be temperature related as It doesn't happen if the truck is cold or cooled off a good amount. I can get in, drive the 20 miles to work, no issue. Get in at the end of the day, start it up and drive home, no issue. However if I try to get back in the truck soon after I get to work, to go somewhere it will miss terribly.

Now, I have a PO307 cylinder 7 misfire code. Ive changed the spark plug, the wire, the cap and rotor along with the cam signal sensor located inside the distributor all with no success in solving this problem. Ive got a spark checker and the spark seems to be weak on #7 wire. The plug looks good, not wet, no pitting and has a nice light brown coloring to it. Im at a loss. I cant figure what else it could be. Ive swapped ignition wires to see if I get a different cylinder misfiring and nothing turns up, same #7 cylinder misfire code. Tried new plugs, same code, same issue. Ive tried a new cap, and a new rotor, same issue and same code. Its getting really frustrating and if anyone has any ideas, I would be eternally grateful or their help. Thanks
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on mine it was a stuck poppet i had good spark and good compression but the poppet style injector was stuck closed, i replaced the hole spider assy with the upgraded one,found it on amazon for about $215 it was worth every penny,hows the spark on the other cylinders look?


Sean

97 4x4 burb

the spark on all the other cyls are good and is good on #7 when cold the spark only gets weak when the engine has heat soak for 20-30 min then I get code P0307 and when it cools off the misfire is gone till it gets hot again.

Thanks Sean for the info not looking forward to pulling the manifold but it is what it is. just drove it today and it runs GREAT till I shut it down and restart.

maybe the ingition module or coil is starting to fail when the engine get hot,is that weaker spark you see no all the plugs or just #7,the upper manifold looks worse than it really is,did you try a compression check when hot long shot but might be worth it


Sean

97 4x4 burb

Hey Sean yea I cked the comp both hot and cold its good but the spark is only weak on #7 when hot. I can try a coil will look in to the manifold r/r and replaceing the spider.
Thanks for your help.

check your crank sensor too....had the same issue on a jeep i used to have...same symptoms....would drive for a few hours then shut it off, try to start it a few minutes later and would have a misfire...good luck

check the wire routing on the #7. It may be touching the block and grounding out along the way. Or arcking with another wire?

Even still a misfire in 1 cylinder in an 8 wouldnt make it stumble like that. You probably got bad wires all around. Check them out.

Ive got a '99 Suburban k1500 4x4 with the 5.7 motor in it. Ive got an issue where the truck want start all the time it misfire while trying to start but once it start , idle and run perfectly for as long as I drive it. . No hesitation, and no hint of a misfire. If I shut the truck off and let it sit for 15-20 minutes, it starts misfiring. It will miss as soon as I start it up. I have no code I have change about every thing but the distributor

i emailed OP if he ever found out the problem: "Yes I did it was the main fuel injector connector on top of the intake manifold. Take it off an apply some dielectric grease to the connector."