Help with overheat/head gasket diagnosis

[rgonz8]

Looking for help confirming diagnosis on 1991 7.3 IDI N/A turboed.

Background:
June 2026: Recently (<1200miles) turboed 7.3 IDI non-turbo with OEM 093 turbo setup. Engine had been running great for a few thousand miles N/A. Only running <8psi boost max.

September 2025:
Engine was something of a thrown together deal. I dropped a valve on the passenger side of the original engine, lost the block and passenger side head. Found a rebuilt 0.030" over block and good passenger side head, drivers side head on this "new" engine was cracked (a match made in heaven) on craigslist. Put my one good head from the 1991 dropped valve original on the "new" block. Didn't do any head rebuilding or inspecting other than valve lapping and sealing. Assembled just to get it going, sort of a gamble. Ran great before the turbo this summer.

Problem/symptoms:
Had a pressurized coolant system symptom and overheating issue that seemed to start small and escalate quickly. Pressurized coolant burst out of reservoir. Jumped to the conclusion that it was a head gasket (didn't do a combustion gas check to confirm). Tore the top end apart and found the attached picture. No clear/catastrophic head gasket failure indications. BUT, there is light surface rust in the number 8 cylinder combustion chamber, the picture is actually after I've handled it a bit and some of the lighter rust flaked off. In the attached photo, number 1 Pass-side cylinder is on bottom left, drivers side number 2 is on bottom right. Rear of vehicle at top of pic.

The question:
Does this story point toward a bad head gasket, somewhere around #8 cylinder. Conclusive?

My plan is to take the heads in for machining, inspection, full treatment and go from there. Perhaps head stud for extra clamping, piece of mind.

Am I missing something simple/easy? Check ball on T-stat housing stuck, t-stat stuck, bad radiator cap? Cavitation? Air not being bled from coolant system? Cylinder bores are beautiful.

Looking for a gut check on what else this could be.

 

[Jesus Freak]

I hate to say it, but it sounds like cavitation. Look really close at that cylinder and you'll probably find pinholes.

 

[IDIBRONCO]

I will agree with checking the cylinder. Rotate the engine so that piston is at the bottom of it's travel first. If you can't see anything definite, then have the heads checked out. If the heads check out, I'd look at the radiator next. I say that because there's rust in all of the coolant passages in your heads. It may have been a case of overheating due to an internally clogged radiator.

 

[Old Goat]

Incase you don`t know what cavitation is, here is a good read.

Cavitation Article

Goat

 

[rgonz8]

Thanks for the input thus far! I'll look in detail through the Cavitation article. Also will look at #8 very closely for pin holes.

When I had this engine in the stand last fall, i looked into as much of the water jacket side of the cylinders as I could access with a borescope. The outer surface of each cylinder was immaculate, no pitting, not even any surface rust.

Is it a dumb idea to home brew a leak down tester? I can machine a plate with pressure gauge, o-rings, etc. Attach it to the top of each cylinder and bolt it down with 1/2-13s to head bolt locations. With the each piston at bottom dead center, I can pressurize the cylinders to XXpsi (up to compressor pressure), valve off from the source and compare leak rates. Leak down test w/o heads. I'd guess this would give some indication of cavitation? I suppose also of ring seal integrity. If Cyl 8 - the suspect cylinder - has a leak rate greater than others.

What would be a reasonable pressure to test at?

 

[IDIBRONCO]

Is it a dumb idea to home brew a leak down tester?

Nope. Not a dumb idea at all.

I suppose also of ring seal integrity.

Not really. Our engines were designed with large ring gaps so they will always have a lot of blow by unless someone uses gapless rings when they rebuild one.

What would be a reasonable pressure to test at?

Feel free to use compressor pressure if you like. Unless you have one amazing compressor, cylinder compression is much higher than that. On a good, tight engine, you can see over 400 PSI when doing a compression check. Just for a little safety factor, I'd recommend coming up with a way to not push all of the pressure into a cylinder at once. Put it in slowly. such as using a shut off valve and open it gradually. It probably won't hurt anything to put it in at once, but I see no need to risk it.

With the each piston at bottom dead center, I can pressurize the cylinders to XXpsi (up to compressor pressure),

Rest assured, even if the piston's not at BDC, it will be by the time that the cylinder is pressurized. I've witnessed that myself.

 

[rgonz8]

Thanks IDIBRONCO.

Definitely a good call to creep up to max compressor pressure, just because. Can use the reg on the compressor.

Good call on not sweating getting to BDC. Didn't even think that all the way through. Piston is ~12+ square inches. With 100 psi - yea, BDC will be found whether I want it or not....haha.

 

[yARIC008]

This is 99% cavitation especially with it bored over and the spots you saw in the cylinder wall. My pictures in the article you can see the cylinder looks perfect except the water spraying through the wall, lol. Pressurize the cooling system and you’ll see it.

 

[rgonz8]

This is 99% cavitation especially with it bored over and the spots you saw in the cylinder wall. My pictures in the article you can see the cylinder looks perfect except the water spraying through the wall, lol. Pressurize the cooling system and you’ll see it.

I don't see any pinholes or spots on the cylinder walls. Though I understand there's no way to really visually confirm no pinholes, they can be too small to see.

The only thing amiss in the disassembly thus far is the light surface rust on #8. Before disassembly I did pressurize the coolant system with a tester to 0.7bar/12psi. Pressure drop was a couple psi over a course of several hours, which seemed acceptable to me.

I'm hoping my leak down, block/cyl only, test setup I described a couple posts ago will reveal cavitation in more measurable terms.

Other thoughts, yARIC008? I definitely appreciate the input, could dang well be cavitation. A bit hard to believe since the water jacket side of the cyls look so dang nice.

I have to get my heads inspected, may reveal something there also.