1999 Suburban random misfires.... yup I'm going there

[joesnextoy]

just picked up this truck for a decent price , got it because previous owner couldn't fix it and gave up
its a 1999 suburban 137k 5.7 vortec
ok did all the basics ....... all cyn compression is at 170 , new distribitor, new wires, plugs, new updated spider, new MAF, new coil, new ign mod, new IAC motor
previous owner did intake gasket, replaced ecm and had it programed
I checked with noid light at the harness to spider all good, checked for spark with spark tester all good, fuel pressure 60 psi

right now have p0300 , with dead misfires being at cyn 5 & 7 and it almost feels like when the truck warms up it gets worst

I have googled and binged everything and im at a loss ..... thinking im overly frustrated and cant think straight anymore so any help i would be very thankful

 

[Cameron]

Have you pulled the plugs from 5 and 7 and checked the conditions of the electrodes for fouling from a lean or rich mixture?

 

[joesnextoy]

rich

 

[Diamondwulf]

Try swapping your plug wires for those two cylinders at the distributor.

 

[1BADI5]

Leak down test time

 

[SUBURBIAN]

Yup, I suspect valve sealing issues. Maybe not necessarily enough to show up in a typical compression test.

 

[1BADI5]

Yeah the early Chevy Colorados had issues with heads that had bad castings; a leak down test was the only way to diagnose it correctly

People would get the P0300 and freak........doesn't help that code is very generic

Good luck, sounds like your a bit of a gear head too, so I'm sure you'll get it sorted out

 

[David A my 99 hoe]

just picked up this truck for a decent price , got it because previous owner couldn't fix it and gave up
its a 1999 suburban 137k 5.7 vortec
ok did all the basics ....... all cyn compression is at 170 , new distribitor, new wires, plugs, new updated spider, new MAF, new coil, new ign mod, new IAC motor
previous owner did intake gasket, replaced ecm and had it programed
I checked with noid light at the harness to spider all good, checked for spark with spark tester all good, fuel pressure 60 psi

right now have p0300 , with dead misfires being at cyn 5 & 7 and it almost feels like when the truck warms up it gets worst

I have googled and binged everything and im at a loss ..... thinking im overly frustrated and cant think straight anymore so any help i would be very thankful

Not sure if you have fixed the issue yet. Mine just did the same thing. I put a new spider in it (it needed one but didn't fix the problem) did another complete tune-up even tho it was only 5k miles since I did it when i rebuilt the engine (didn't fix the problem) fuel pressure was at spec. New coil and crack sensor (same issue) everyone was telling me that the valves was sticking. When to pull off the intake which you have to pull the distributer first. Well after I removed the distributer I found the problem. The gear on it was all screwed up. Cam was fine. Dropped a new distributer in and its as good as gold now.

 

[DallasTahoe]

just picked up this truck for a decent price , got it because previous owner couldn't fix it and gave up
its a 1999 suburban 137k 5.7 vortec
ok did all the basics ....... all cyn compression is at 170 , new distribitor, new wires, plugs, new updated spider, new MAF, new coil, new ign mod, new IAC motor
previous owner did intake gasket, replaced ecm and had it programed
I checked with noid light at the harness to spider all good, checked for spark with spark tester all good, fuel pressure 60 psi

right now have p0300 , with dead misfires being at cyn 5 & 7 and it almost feels like when the truck warms up it gets worst

I have googled and binged everything and im at a loss ..... thinking im overly frustrated and cant think straight anymore so any help i would be very thankful

If everything already mentioned doesn't fix it, try checking your valve lash on those two cylinders, I had the same issue, after resetting the valve lash my misfires stopped.


Sent from my iPhone using Tapatalk

 

[bartonz]

Not sure if you have fixed the issue yet. Mine just did the same thing. I put a new spider in it (it needed one but didn't fix the problem) did another complete tune-up even tho it was only 5k miles since I did it when i rebuilt the engine (didn't fix the problem) fuel pressure was at spec. New coil and crack sensor (same issue) everyone was telling me that the valves was sticking. When to pull off the intake which you have to pull the distributer first. Well after I removed the distributer I found the problem. The gear on it was all screwed up. Cam was fine. Dropped a new distributer in and its as good as gold now.

+1
And check that pesky cam retard +/- 2 degrees. In park at 1,000-1,200 rpm