99 350 vortec problems

[coleslaw]

Ok long shot, two weeks ago I did a once over and checked fluids etc. All were good. Last weekend I noticed a ticking in the top end....checked oil, was bone dry. No visible leaks, little wetness around the front crank seal but not dripping. Yesterday my overflow bottle was empty and coolant levels down. Checked plugs and couple were crusty but not the worst I have seen. No coolant leaks either.

Yesterday started misfiring at idle and accel until 2500rpm. Can hear it through exhaust(straight pipe) and truck shakes. Gas mileage changed yesterday as well for the worse. Yet to preform any diagnostic tests but will be today. No misfires until I pulled plugs.

Suggestions?

[00Silverado4x4]

Pressure test for a possible head gasket or cyl head problem. Also have to check for intake/coolant leaks. It sounds like you may have coolant getting into your cylinders from somewhere. This wouldn't show as an external leak necessarily so you're going to have to test for internal leakage.

[coleslaw]

Pressure testing after lunch. Got misfire in cylinder 8. Plugs fine haven't got around to wires and dist cap/rotor. Busy at the shop today.

[1997SierraSLT]

These first generation Vortec engines were notorious for lower intake manifold gaskets failing. If I recall the gaskets went through 3 or 4 revisions as there were design issues as well as compatibility issues with the GM Dex-Cool coolant. Coolant is either lost through an external leak or coolant leaks into the combustion chamber and is burned off.

 

It also does not help matters any that there are only 8 total lower intake manifold bolts (2 at each corner only) nor that the ECM isn't intelligent enough to diagnose any issue of any complexity. All it will ever throw is a P0300 code (random misfire) no matter what appears to be causing the problem.

 

I can say that of the random sensors I've changed out over the years, the MAP sensor seemed to make the most difference in driveability. Also changing out the fuel pump made a dramatic difference. They can wear out just like any other pump. You can perform a fuel pressure test and everything will appear to be fine, but that is just half the equation. You also need to have adequate fuel flow rate or you'll starve the injector of fuel.