Vortec 5.7 stumbles and backfires without giving and check engine codes

[LilWee247]

S10 with Vortec 5.7 from 1998 K2500. Truck has been running perfect for 2 years. Started it up one day and now it stumbles and backfires with low power. No check engine light or codes. Fuel pressure looks fine. New fuel. Put scan tool on and watched live data for O2 sensors and they appear fine. Motor has about 145k on it. Not sure what else to check.

Thanks

 

[LilWee247]

Could it possibly be the timing chain stretched? If so, how can I verify without taking the timing cover off?

 

[L31MaxExpress]

Could it possibly be the timing chain stretched? If so, how can I verify without taking the timing cover off?

Not likely! Start with a good cap and rotor. They are known to burn through internally and crossfire even though they look ok externally. I personally have had the best luck with MSD with brass terminals. The aluminum terminal factory ones are a terrible piece. Aluminum is a poor conductor and it overheats the insulating material, breaking it down. A bad coil can cause a similar issue as can a worn distributor gear. While you are changing the cap and rotor, grab
the old rotor and try to turn it both directions. If it has alot of slop its got a bad gear or the bushings are shot in the distributor shaft. Either way I use an aluminum distributor from RockAuto as a replacement.

 

[LilWee247]

Could this happen even though I'm not getting any codes for misfires?

 

[L31MaxExpress]

Could this happen even though I'm not getting any codes for misfires?

Yes takes these old computers alot of miles to figure out they are misfiring in most cases and generally it will throw a P0300 first.

If you have a good scanner, I would also compare commanded egr opening vs actual. The linear egr valves have a common problem where they open much too quickly. If the EGR opens too soon or hangs open it will run like hammered dog poo.

 

[LilWee247]

So, it I take the the cap off, will there be obvious signs of this occurring?

 

[LilWee247]

Also this cap and rotor are only 2 yrs old with about 5000 miles worth of driving.

 

[stutaeng]

Do you have ability to look at live data fuel trims? Rich or lean should be pretty to see by looking at fuel trims...

 

[LilWee247]

Best I can tell, trims look fine

 

[L31MaxExpress]

So, it I take the the cap off, will there be obvious signs of this occurring?

Yes, it will have alot of play in it. Up and down, side to side, or lots of play as you try to turn it.