Symptoms of a cracked head

[Moofus02]

Just after I bought my 99 suburban the wife was driving it and it blew the heater hose y fitting that feeds the rear heat. I'm sure she got it hot. I've put 200k miles on it since then but it burns a cap and rotor about every 3 months. I've got a center cylinder on each bank with 75psi compression. The other 6 are all between 185/195 psi. Will a cracked head kill compression? It's always had a slight miss since then but after I eliminated the cats with a true 2 1/2 inch duel exhaust the miss is more noticeable. It's a 5.7 vortec. At least the converter stays locked up. When I get something else running I need to fix this

 

[Schurkey]

it burns a cap and rotor about every 3 months.

1. Test plug wires and coil wire for resistance/opens
2. What is the spark plug gap? Replace plugs as needed to get a gap of .045-ish. Some plugs have a long side electrode that's intended for .060-ish gaps.
3. Buy a QUALITY cap and rotor, instead of Communist Crap.

I've got a center cylinder on each bank with 75psi compression. The other 6 are all between 185/195 psi. Will a cracked head kill compression?

A cracked head "can" reduce compression. More likely, you'd be seeing coolant in the chamber.

Connect a cylinder leakdown tester, find where the air goes. ANY leakage past intake or exhaust valves, or bubbles into the cooling system is not acceptable. There will always be some leakage past the rings. The gauges on the leakdown tester can help you determine whether the ring leakage is excessive or not.

It's always had a slight miss since then but after I eliminated the cats with a true 2 1/2 inch duel exhaust the miss is more noticeable. It's a 5.7 vortec. At least the converter stays locked up. When I get something else running I need to fix this

So the computer is unhappy due to rear O2 sensors not providing proper signal?

Connect a scan tool, verify all sensors and computer outputs. Look at misfire counts per cylinder to determine which are the "problem children".

Verify fuel pressure.

Consider an upgraded electronic spider rather than the poppet-style, if it hasn't already been upgraded.

 

[Erik the Awful]

When the engine's cool, take the radiator cap off. Fire it up and watch the coolant flow in the radiator. If there are any air pockets in the coolant they'll burp out when the engine gets up to running temp (you can tell when the upper radiator hose gets hot). Once the engine's warmed up, if the air bubbles in the radiator don't stop, you have a cracked head or block.

 

[Moofus02]

No coolant in oil and no compression gasses in coolant not using coolant. Acts like its got a couple bent intake valves contaminating the mixture on the next cylinder in the order. More noticeable with new exhaust. Not as bad as my tbi motor with blown gasket between cylinders. I'm going to do a compression test with oil in those two and see if it changes. And a leak down. Truck is too nice to run like this lol

 

[Schurkey]

Do as you like...but I have zero faith in "wet" (oil added) compression tests.

I suppose this might have worked on upright, in-line engines, but once the cylinders leaned-over--Slant 4, Slant 6, V-6 or V8, etc--how does the oil climb up the piston to seal the intake-manifold side of the rings?

OTOH, adding incompressible oil will tend to raise the compression pressure on it's own, potentially giving false results.

The cylinder leakdown test should tell you all you need to know about the valves.

 

[Moofus02]

Do as you like...but I have zero faith in "wet" (oil added) compression tests.

I suppose this might have worked on upright, in-line engines, but once the cylinders leaned-over--Slant 4, Slant 6, V-6 or V8, etc--how does the oil climb up the piston to seal the intake-manifold side of the rings?

OTOH, adding incompressible oil will tend to raise the compression pressure on it's own, potentially giving false results.

The cylinder leakdown test should tell you all you need to know about the valves.

Thank you a leak down sounds like the best method