Cavitation or head gasket

SuperDave

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I just bought an 89 7.3 service truck that has been sitting 18 years due to a bad torque converter. I got the thing to run fairly easily and all was good until I started trying to trace a misfire, i decided to run a compression test and one cylinder came up slightly lower than the rest and was making a spitting sound through the intake. Next I decided to do a leakdown test and that particular cylinder would after several seconds start making bubbles in the radiator. I'm assuming it's a gasket issue or a head issue because it bubbles air out slowly no matter where in the cylinder is in the bore, the first time I applied air tho the cylinder the pressure pushed the piston back to BDC so I rolled it back to TDC and held it with a breaker bar, same slow bubbling. It couldn't be a huge leak because it's only about 30% leakage. I'm just trying to prepare myself before I tear it down, it will be a shame if the engine is junk because it starts easy, runs good has great oil pressure and doesn't smoke. Also the oil is clean, no water/coolant in it.
 

gnathv

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Retorque the head and see if it changes. May or may not give you a better idea of what’s going on, but it’s a place to start.
 

IDIBRONCO

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I haven't heard of cavitation causing a miss while the engine's running. Your "spitting' sound through the intake usually means that a valve isn't sealing right. Did the sound happen during the compression test or during the leak down test?
 

ih8minimumwage

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Loosen the rocker bolts on the offending cylinder, roll it to TDC and apply the air. See if it changes as you move the cylinder down. It sounds like yours stays the same all the way through, so you're most likely looking at a headgasket. If it had gone up as the cylinder dropped you might have been looking at cavitation through the sleeve.

About the best you can do outside of pulling the head and pressurizing the cooling system.
 

Big Bart

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To your point about cavitation. Sound more like a head gasket. From the few pics I have seen on this site of cavitation, (Pin holes in the cylinder wall.) it seems to happen mid cylinder and lower. So to your point if the cylinder was at TDC the rings would prevent most of the compressed air from going past the rings. What air does would be lightly compressed. Thus hard and unlikely that secondary air from the leak down test would find its way into the cooling jacket via a pin hole. (But would via the head gasket or crack in the head.)

How does the coolant look? Rusty or good? If good likely not cavitation, it is not very common, but very feared. You have probably researched and know, but is not caused by rust in the cooling system. But rust speeds up the process. (Rust thins the cylinder wall too.) Thus if the coolant looks good likely the coolant was maintained.

I would pull the engine and fix before you perhaps hydro lock that cylinder and cause a bent rod or worse. Cavitation holes are pin holes, but usually leave a mark around them that is noticeable on the cylinder wall. Will give you a chance to visit that valve leak too. Also redo the oil cooler seals, valve cover gaskets, main seal, and front seal while you have easy access.

Fresh head gaskets and ARB studs will bring more reliability to your project. Note the oil valley pan gasket is on back order for months. So treat yours nice taking it off, you likely will need to re-use it.

Let us know what your plan is. Post some pics if you get a chance.
 
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SuperDave

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I did pull torque on all the head bolts with an old beam type torque wrench and they were all at spec. I'm going to start pulling the head off this afternoon when I get off work and see what the damage is. The spitting thru the intake is the intake valve and I figured while it's apart I will hit all the valves with a little grinding compound to make sure they seal nice and tight since probably every single one that was open while sitting is probably a little rusty. Thanks for all the info guys, I feel better that I'm not going to find a catastrophe when I pull the head and I'm guessing the gasket failure will probably be somewhat obvious. Thanks again!
 

TNBrett

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Check the valve guides while you’re in there. These motors like to wallow them out. I had some so bad you could wobble the stem with the springs still on.
 

SuperDave

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Is the 7.3 one you have to remove the head bolts in sequence/steps or is it old enough to where you just pull them all out in any order? It's been so long since I had to tear into one I don't remember.
 

SuperDave

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SuperDave

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Crack above intake valve on 2 middle cylinders, head looks like both of those cylinders ate a bolt at some point yet the pistons are not affected. Head gasket was fine and cylinder walls seem fine. I don't know if I want to look for another head or pull the good 7.3 out of my friends '94 that had a cab fire and use it. Thoughts? Burnt 7.3 with less than 100k that seems ok or find a good head and run this one?
 

ih8minimumwage

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If you can get a complete engine that was running and swap your outside parts over, I'd go that route to get it going and then keep eyes peeled for a cheap set of heads.
 

IDIBRONCO

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I'll bet that most of what is on the heads in the combustion area is carbon buildup and will scrape right off. At least you know why it was pushing the air back out of the intake.
 

Big Bart

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Crack above intake valve on 2 middle cylinders, head looks like both of those cylinders ate a bolt at some point yet the pistons are not affected. Head gasket was fine and cylinder walls seem fine. I don't know if I want to look for another head or pull the good 7.3 out of my friends '94 that had a cab fire and use it. Thoughts? Burnt 7.3 with less than 100k that seems ok or find a good head and run this one?
Might have eaten a glow plug tip.

Cylinders look good.

Head needed to come off regardless.

Look at TNBrett’s recent thread, he found a good deal on some rebuilt heads as I recall. If not pull your buddy’s head and let the machine shop make the call if it is any good.
 

TNBrett

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Looks more like corrosion to me than a glow plug tip or some other foreign object. I would think the aluminum piston would have taken the brunt off the damage if that were the case.
 

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